solar system classification essay

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What is the solar system?

The solar system comprises 8 planets , approximately 170 natural planetary satellites (moons), and countless asteroids , meteorites , and comets .

There are eight planets in the solar system. The four inner terrestrial planets are Mercury , Venus , Earth , and Mars , all of which consist mainly of rock. The four outer planets are Jupiter , Saturn , Neptune , and Uranus , giant planets that consist mainly of either gases or ice. Pluto was considered the ninth planet until 2006, when the International Astronomical Union voted to classify Pluto as a dwarf planet instead.

Where is the solar system?

The solar system is situated within the Orion-Cygnus Arm of the Milky Way Galaxy . Alpha Centauri , made up of the stars Proxima Centauri, Alpha Centauri A, and Alpha Centauri B, is the closest star system to the solar system.

Scientists have multiple theories that explain how the solar system formed. The favoured theory proposes that the solar system formed from a solar nebula , where the Sun was born out of a concentration of kinetic energy and heat at the centre, while debris rotating the nebula collided to create the planets .

Is there life in the solar system aside from on Earth?

Europa and Enceladus , moons of Jupiter and Saturn respectively, are ice-covered rocky objects that scientists think may harbour life in the water beneath the surface. Some geological evidence points to the possibility of microorganisms on Mars .

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solar system , assemblage consisting of the Sun —an average star in the Milky Way Galaxy —and those bodies orbiting around it: 8 (formerly 9) planets with more than 210 known planetary satellites (moons); many asteroids , some with their own satellites; comets and other icy bodies; and vast reaches of highly tenuous gas and dust known as the interplanetary medium . The solar system is part of the " observable universe ," the region of space that humans can actually or theoretically observe with the aid of technology. Unlike the observable universe, the universe is possibly infinite .

The Sun, Moon , and brightest planets were visible to the naked eyes of ancient astronomers, and their observations and calculations of the movements of these bodies gave rise to the science of astronomy . Today the amount of information on the motions, properties, and compositions of the planets and smaller bodies has grown to immense proportions, and the range of observational instruments has extended far beyond the solar system to other galaxies and the edge of the known universe. Yet the solar system and its immediate outer boundary still represent the limit of our physical reach, and they remain the core of our theoretical understanding of the cosmos as well. Earth -launched space probes and landers have gathered data on planets, moons, asteroids, and other bodies, and this data has been added to the measurements collected with telescopes and other instruments from below and above Earth’s atmosphere and to the information extracted from meteorites and from Moon rocks returned by astronauts. All this information is scrutinized in attempts to understand in detail the origin and evolution of the solar system—a goal toward which astronomers continue to make great strides.

Composition of the solar system

Located at the centre of the solar system and influencing the motion of all the other bodies through its gravitational force is the Sun , which in itself contains more than 99 percent of the mass of the system. The planets, in order of their distance outward from the Sun, are Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , and Neptune . Four planets—Jupiter through Neptune—have ring systems, and all but Mercury and Venus have one or more moons. Pluto had been officially listed among the planets since it was discovered in 1930 orbiting beyond Neptune, but in 1992 an icy object was discovered still farther from the Sun than Pluto. Many other such discoveries followed, including an object named Eris that appears to be at least as large as Pluto. It became apparent that Pluto was simply one of the larger members of this new group of objects, collectively known as the Kuiper belt . Accordingly, in August 2006 the International Astronomical Union (IAU), the organization charged by the scientific community with classifying astronomical objects, voted to revoke Pluto’s planetary status and place it under a new classification called dwarf planet . For a discussion of that action and of the definition of planet approved by the IAU, see planet .

Any natural solar system object other than the Sun, a planet, a dwarf planet, or a moon is called a small body ; these include asteroids , meteoroids , and comets . Most of the more than one million asteroids, or minor planets, orbit between Mars and Jupiter in a nearly flat ring called the asteroid belt. The myriad fragments of asteroids and other small pieces of solid matter (smaller than a few tens of metres across) that populate interplanetary space are often termed meteoroids to distinguish them from the larger asteroidal bodies.

The solar system’s several billion comets are found mainly in two distinct reservoirs. The more-distant one, called the Oort cloud , is a spherical shell surrounding the solar system at a distance of approximately 50,000 astronomical units (AU)—more than 1,000 times the distance of Pluto’s orbit. The other reservoir, the Kuiper belt , is a thick disk-shaped zone whose main concentration extends 30–50 AU from the Sun, beyond the orbit of Neptune but including a portion of the orbit of Pluto. (One astronomical unit is the average distance from Earth to the Sun—about 150 million km [93 million miles].) Just as asteroids can be regarded as rocky debris left over from the formation of the inner planets, Pluto, its moon Charon , Eris, and the myriad other Kuiper belt objects can be seen as surviving representatives of the icy bodies that accreted to form the cores of Neptune and Uranus. As such, Pluto and Charon may also be considered to be very large comet nuclei. The Centaur objects , a population of comet nuclei having diameters as large as 200 km (125 miles), orbit the Sun between Jupiter and Neptune, probably having been gravitationally perturbed inward from the Kuiper belt. The interplanetary medium —an exceedingly tenuous plasma (ionized gas) laced with concentrations of dust particles —extends outward from the Sun to about 123 AU.

The solar system even contains objects from interstellar space that are just passing through. Two such interstellar objects have been observed. ‘Oumuamua had an unusual cigarlike or pancakelike shape and was possibly composed of nitrogen ice. Comet Borisov was much like the comets of the solar system but with a much higher abundance of carbon monoxide .

All the planets and dwarf planets, the rocky asteroids, and the icy bodies in the Kuiper belt move around the Sun in elliptical orbits in the same direction that the Sun rotates. This motion is termed prograde, or direct, motion. Looking down on the system from a vantage point above Earth’s North Pole , an observer would find that all these orbital motions are in a counterclockwise direction. In striking contrast, the comet nuclei in the Oort cloud are in orbits having random directions, corresponding to their spherical distribution around the plane of the planets.

The shape of an object’s orbit is defined in terms of its eccentricity . For a perfectly circular orbit, the eccentricity is 0; with increasing elongation of the orbit’s shape, the eccentricity increases toward a value of 1, the eccentricity of a parabola. Of the eight major planets, Venus and Neptune have the most circular orbits around the Sun, with eccentricities of 0.007 and 0.009, respectively. Mercury, the closest planet, has the highest eccentricity, with 0.21; the dwarf planet Pluto, with 0.25, is even more eccentric . Another defining attribute of an object’s orbit around the Sun is its inclination , which is the angle that it makes with the plane of Earth’s orbit—the ecliptic plane. Again, of the planets, Mercury’s has the greatest inclination, its orbit lying at 7° to the ecliptic; Pluto’s orbit, by comparison, is much more steeply inclined, at 17.1°. The orbits of the small bodies generally have both higher eccentricities and higher inclinations than those of the planets. Some comets from the Oort cloud have inclinations greater than 90°; their motion around the Sun is thus opposite that of the Sun’s rotation, or retrograde.

Solar System Essay for Students and Children

500+ words essay on solar system.

Our solar system consists of eight planets that revolve around the Sun, which is central to our solar system . These planets have broadly been classified into two categories that are inner planets and outer planets. Mercury, Venus, Earth, and Mars are called inner planets. The inner planets are closer to the Sun and they are smaller in size as compared to the outer planets. These are also referred to as the Terrestrial planets. And the other four Jupiter, Saturn, Uranus, and Neptune are termed as the outer planets. These four are massive in size and are often referred to as Giant planets.

The smallest planet in our solar system is Mercury, which is also closest to the Sun. The geological features of Mercury consist of lobed ridges and impact craters. Being closest to the Sun the Mercury’s temperature sores extremely high during the day time. Mercury can go as high as 450 degree Celsius but surprisingly the nights here are freezing cold. Mercury has a diameter of 4,878 km and Mercury does not have any natural satellite like Earth.

Venus is also said to be the hottest planet of our solar system. It has a toxic atmosphere that always traps heat. Venus is also the brightest planet and it is visible to the naked eye. Venus has a thick silicate layer around an iron core which is also similar to that of Earth. Astronomers have seen traces of internal geological activity on Venus planet. Venus has a diameter of 12,104 km and it is just like Mars. Venus also does not have any natural satellite like Earth.

Earth is the largest inner planet. It is covered two-third with water. Earth is the only planet in our solar system where life is possible. Earth’s atmosphere which is rich in nitrogen and oxygen makes it fit for the survival of various species of flora and fauna. However human activities are negatively impacting its atmosphere. Earth has a diameter of 12,760 km and Earth has one natural satellite that is the moon.

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Mars is the fourth planet from the Sun and it is often referred to as the Red Planet. This planet has a reddish appeal because of the iron oxide present on this planet. Mars planet is a cold planet and it has geological features similar to that of Earth. This is the only reason why it has captured the interest of astronomers like no other planet. This planet has traces of frozen ice caps and it has been found on the planet. Mars has a diameter of 6,787 km and it has two natural satellites.

It is the largest planet in our solar system. Jupiter has a strong magnetic field . Jupiter largely consists of helium and hydrogen. It has a Great Red Spot and cloud bands. The giant storm is believed to have raged here for hundreds of years. Jupiter has a diameter of 139,822 km and it has as many as 79 natural satellites which are much more than of Earth and Mars.

Saturn is the sixth planet from the Sun. It is also known for its ring system and these rings are made of tiny particles of ice and rock. Saturn’s atmosphere is quite like that of Jupiter because it is also largely composed of hydrogen and helium. Saturn has a diameter of 120,500 km and It has 62 natural satellites that are mainly composed of ice. As compare with Jupiter it has less satellite.

Uranus is the seventh planet from the Sun. It is the lightest of all the giant and outer planets. Presence of Methane in the atmosphere this Uranus planet has a blue tint. Uranus core is colder than the other giant planets and the planet orbits on its side. Uranus has a diameter of 51,120 km and it has 27 natural satellites.

Neptune is the last planet in our solar system. It is also the coldest of all the planets. Neptune is around the same size as the Uranus. And it is much more massive and dense. Neptune’s atmosphere is composed of helium, hydrogen, methane, and ammonia and it experiences extremely strong winds. It is the only planet in our solar system which is found by mathematical prediction. Neptune has a diameter of 49,530 km and it has 14 natural satellites which are more than of Earth and Mars.

Scientists and astronomers have been studying our solar system for centuries and then after they will findings are quite interesting. Various planets that form a part of our solar system have their own unique geological features and all are different from each other in several ways.

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The solar system, explained

Our solar system is made up of the sun and all the amazing objects that travel around it.

The universe is filled with billions of star systems. Located inside galaxies, these cosmic arrangements are made up of at least one star and all the objects that travel around it, including planets, dwarf planets, moons, asteroids, comets, and meteoroids. The star system we’re most familiar with, of course, is our own.

Home sweet home

If you were to look at a giant picture of space, zoom in on the Milky Way galaxy , and then zoom in again on one of its outer spiral arms, you’d find the solar system. Astronomers believe it formed about 4.5 billion years ago, when a massive interstellar cloud of gas and dust collapsed on itself, giving rise to the star that anchors our solar system—that big ball of warmth known as the sun.

Along with the sun, our cosmic neighborhood includes the eight major planets. The closest to the sun is Mercury , followed by Venus , Earth, and Mars . These are known as terrestrial planets, because they’re solid and rocky. Beyond the orbit of Mars, you’ll find the main asteroid belt , a region of space rocks left over from the formation of the planets. Next come the much bigger gas giants Jupiter and Saturn , which is known for its large ring systems made of ice, rock, or both. Farther out are the ice giants Uranus and Neptune . Beyond that, a host of smaller icy worlds congregate in an enormous stretch of space called the Kuiper Belt. Perhaps the most famous resident there is Pluto . Once considered the ninth planet, Pluto is now officially classified as a dwarf planet , along with three other Kuiper Belt objects and Ceres in the asteroid belt.

Moons and other matter

More than 150 moons orbit worlds in our solar system. Known as natural satellites, they orbit planets, dwarf planets, asteroids, and other debris. Among the planets, moons are more common in the outer reaches of the solar system. Mercury and Venus are moon-free, Mars has two small moons, and Earth has just one. Meanwhile, Jupiter and Saturn have dozens, and Uranus and Neptune each have more than 10. Even though it’s relatively small, Pluto has five moons, one of which is so close to Pluto in size that some astronomers argue Pluto and this moon, Charon, are a binary system.

Too small to be called planets, asteroids are rocky chunks that also orbit our sun along with the space rocks known as meteoroids. Tens of thousands of asteroids are gathered in the belt that lies between the orbits of Mars and Jupiter. Comets, on the other hand, live inside the Kuiper Belt and even farther out in our solar system in a distant region called the Oort cloud .

Atmospheric conditions

The solar system is enveloped by a huge bubble called the heliosphere . Made of charged particles generated by the sun, the heliosphere shields planets and other objects from high-speed interstellar particles known as cosmic rays. Within the heliosphere, some of the planets are wrapped in their own bubbles—called magnetospheres —that protect them from the most harmful forms of solar radiation. Earth has a very strong magnetosphere, while Mars and Venus have none at all.

Most of the major planets also have atmospheres . Earth’s is composed mainly of nitrogen and oxygen—key for sustaining life. The atmospheres on terrestrial Venus and Mars are mostly carbon dioxide, while the thick atmospheres of Jupiter, Saturn, Uranus, and Neptune are made primarily of hydrogen and helium. Mercury doesn’t have an atmosphere at all. Instead scientists refer to its extremely thin covering of oxygen, hydrogen, sodium, helium, and potassium as an exosphere.

Moons can have atmospheres, too, but Saturn’s largest moon, Titan, is the only one known to have a thick atmosphere, which is made mostly of nitrogen.

Life beyond?

For centuries astronomers believed that Earth was the center of the universe, with the sun and all the other stars revolving around it. But in the 16th century, German mathematician and astronomer Nicolaus Copernicus upended that theory by providing strong evidence that Earth and the other planets travel around the sun.

Today, astronomers are studying other stars in our galaxy that host planets, including some star systems like our own that have multiple planetary companions. Based on the thousands of known worlds spotted so far, scientists estimate that billions of planetary systems must exist in the Milky Way galaxy alone.

So does Earth have a twin somewhere in the universe? With ever-advancing telescopes, robots, and other tools, astronomers of the future are sure to find out.

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solar system

Introduction.

The solar system itself is only a small part of a huge system of stars and other objects called the Milky Way galaxy . The solar system orbits around the center of the galaxy about once every 225 million years. The Milky Way galaxy is just one of billions of galaxies that in turn make up the universe .

At the center of the solar system is a star called the Sun . It is the largest object in the solar system. Its diameter, or distance through its center, is 865,000 miles (1,392,000 kilometers). In addition, the Sun contains more than 99 percent of all the material in the solar system. The Sun is a very hot ball of hydrogen and helium gases. It has a temperature, at its core, of more than 28,080,000° F (15,600,000° C). It constantly changes the hydrogen in its core into helium. This process gives out huge amounts of radiation, or energy. Living things on Earth depend on this energy, in the form of light and heat.

The Solar Wind

The gases that surround the Sun shoot out a stream of tiny particles called the solar wind. It flows outward through the whole solar system. The solar wind is what causes auroras, or displays of colored light in the night sky in parts of Earth. In the Northern Hemisphere these auroras are called the northern lights.

The Planets

Scientists used to call Pluto the ninth planet. But in 2006 scientists decided that several objects in the solar system, including Pluto, should be called dwarf planets.

Millions of small chunks of metal and rock called asteroids also orbit the Sun. Most asteroids are found in a ring between Mars and Jupiter. They are believed to be debris, or bits of material, left over from collisions between other bodies in the solar system. The largest asteroids are hundreds of miles in diameter, but most are much smaller. Small asteroids regularly fall to Earth or burn up in the sky as glowing meteors .

Comets are small chunks of dirt and ice. Billions of them orbit the Sun in very long paths shaped like ovals. When they are closest to the Sun, the Sun’s radiation causes them to glow. Most comets are too small or too distant ever to be seen from Earth. Comets come from two parts of the outer solar system: the Kuiper Belt and the Oort Cloud.

Outer Regions

Beyond Neptune lies the Kuiper Belt, a flat ring of millions of small, icy objects. These objects orbit the Sun at a very great distance. They are mostly 30 to 50 times farther from the Sun than Earth is.

At the outer reaches of the solar system is the Oort Cloud. It is a huge cloud of countless small, icy objects. The Oort Cloud surrounds the rest of the solar system.

How the Solar System Was Formed

The solar system was formed about 4.7 billion years ago. It probably started as a loose cloud of gas and dust. Scientists think that a force called gravity pulled parts of the cloud together into clumps. The largest clump was squeezed together so tightly that it got very hot. This clump eventually became the Sun. Over millions of years the other clumps became the planets. The Sun’s strong gravity eventually pulled the planets into their orbits. Over time some of the leftover clumps became asteroids, comets, and other small, icy objects.

Exploring the Solar System

In 1957, the Soviet satellite Sputnik 1 became the first human-made object to orbit Earth. Since then, scientists have sent many spacecraft to explore various parts of the solar system. Spacecraft have carried astronauts into orbit around Earth, to the moon , and to human-made space stations. Other spacecraft, called probes, have carried cameras and scientific equipment but no astronauts. Space probes have landed on the planets Mars and Venus, on asteroids, and on Titan, which is one of Saturn’s moons. In addition, space probes have flown past all the planets in the solar system. They have taken many photographs and collected much valuable information.

Other Planetary Systems

The solar system is also known as a planetary system. Since the 1990s scientists have found many planetary systems beyond our solar system. In these systems, one or more planets orbit a star—just as the eight planets in our solar system orbit the Sun. These planets are called extrasolar planets. Finding other planetary systems is not easy, however, because extrasolar planets appear much dimmer than the stars they orbit. As space probes travel farther away from Earth, they are likely to discover more extrasolar planets.

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• The Solar System and its planets

The Solar System is made up of the Sun and all of the smaller objects that move around it. Apart from the Sun, the largest members of the Solar System are the eight major planets. Nearest the Sun are four fairly small, rocky planets - Mercury, Venus, Earth and Mars.

Beyond Mars is the asteroid belt – a region populated by millions of rocky objects. These are left-overs from the formation of the planets, 4.5 billion years ago.

On the far side of the asteroid belt are the four gas giants - Jupiter, Saturn, Uranus and Neptune. These planets are much bigger than Earth, but very lightweight for their size. They are mostly made of hydrogen and helium.

Until recently, the furthest known planet was an icy world called Pluto. However, Pluto is dwarfed by Earth’s Moon and many astronomers think it is too small to be called a true planet.

An object named Eris, which is at least as big as Pluto, was discovered very far from the Sun in 2005. More than 1,000 icy worlds such as Eris have been discovered beyond Pluto in recent years. These are called Kuiper Belt Objects. In 2006, the International Astronomical Union decided that Pluto and Eris must be classed as “dwarf planets”.

Even further out are the comets of the Oort Cloud. These are so far away that they are invisible in even the largest telescopes. Every so often one of these comets is disturbed and heads towards the Sun. It then becomes visible in the night sky.

Related articles

• Earth – traveller in space
• Mars - the red planet
• Saturn the gas giant
• The Kuiper Belt

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Essay on Solar System

We see the sun every day shining in the sky and at night, we see the moon. Many other heavy bodies like satellites, meteoroids, and asteroids not visible to our naked eyes also make up the solar system. The sun and its planets together form the Solar System. The existence of the Solar System is about 4.6 billion years old.

100 Words Essay on The Solar System

200 words essay on the solar system, 500 words essay on the solar system.

The solar system comprises all the planets that revolve around the sun. The solar system also contains moons, asteroids, comets, minor planets, and different types of gases and dust.

The planets are categorised into two categories: internal planets and outer planets. Mercury, Venus, Earth, Mars, Jupyter, Saturn, Uranus, and Neptune are called inner planets . Earlier, there were nine planets considered till 2006, but now, Pluto does not lie in the list of planets, it does not meet the standard set for the planets.

It is now termed a dwarf planet. In our solar system, the earth is the only planet where life exists. There are many solar systems that exist in the universe, it is more than 500. Our solar system includes the Kuiper belt that lies past Neptune’s orbit.

The Sun is a star that is made up of massive hot gas that gives us heat and light . The Sun is the focal point of the solar system, every substance in the solar system revolves around the Sun. There are eight planets in the solar system, Mercury is the closest planet to the Sun and the smallest planet in the solar system whereas Neptune is the farthest one and Jupiter is the biggest planet in the solar system.

Only Earth has a supportive environment for living creatures. The Earth rotates around its own axis and revolves around the Sun, similarly the moon orbits around the Earth. For complete rotation the earth takes one day and for completing one cycle around the sun it takes 365 days. It is what we call one year and due to gravity we all are stuck to the surface of the Earth.

A Comet is a large body in space made of rocks, ice, and frozen gas. The centre of a comet is called the nucleus. Asteroids are also large bodies in space made of rocks and minerals, they mostly orbit the sun between Mars and Jupiter in an area called the Asteroid Belt.

The solar system comprises eight planets, about 170 natural planetary satellites, and uncountable asteroids, meteorites, and comets. The solar system is situated within the Orion-Cygnus arm of the Milky way galaxy . Alpha Centauri made up of the stars Proxima Centauri, Alpha Centauri A, and Alpha Centauri B are the closest star systems to the solar system. The sun which is located at the centre of the solar system affects the motion of the body through its gravitational force. It contains more than 99% mass of the system.

Planets and Their Moons

Mercury | Mercury is the closest and smallest plate in the solar system, it orbits around the Sun and takes 87.97 earth days, it spins around slowly compared to Earth and it is slightly bigger than earth. It has a solid surface that is covered with craters and has a thin surface.

Venus | Venus is the second closest planet to the Sun. Venus is very similar to the earth in shape and densityVenus is the hottest planet in the solar system, it has a thick and toxic atmosphere covered with carbon dioxide and sulfuric acid in the form of yellowish clouds, and trapped heat.

Earth | Earth is the only planet that has a livable environment that sustains life and the ecosystem. It is the third closest and fifth largest planet in the solar system. On earth, life is possible for various reasons, but the most essential thing is the availability of water and the presence of oxygen. Earth is also known as the ‘Blue Planet’ because 71% of the earth’s surface is covered with seas, oceans, and large rivers of water

Mars | Mars is the fourth planet from the sun in the solar system. It appears as a red, orange, and radish ball because of the presence of iron oxide which is why Mars is also known as the ‘Red Planet’. Mars is positioned just next to the Earth. The evidence of water and oxygen raised hopes about the possibility of life on Mars.

Jupiter | Jupiter is the largest planet in the solar system and the first of the four gas giants. It is the fifth planet from the Sun. Jupiter also has a ring system like all the large gas planets, although these rings are not famous or as visible as Saturn’s ring.

Saturn | Saturn is the second largest and least dense planet in the solar system. Saturn can float in water because Saturn is made of gases, it's a gas giant with an average radius of about nine and a half times that of earth. Saturn has rings that are made of gas and dust.

Uranus | Uranus is the coldest planet in the solar system, it revolves around the sun and takes 84 earth years to complete one rotation around the earth. Uranus is called an ‘Ice Giant’ planet because it is covered with ice and Hydrogen gas.

Neptune | Neptune is the eighth planet and farthest planet from the sun in the solar system, its atmosphere is made of hydrogen, helium, and methane gas. Neptune is a dark, cold, and very windy planet in the solar system.

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Solar System - Free Essay Examples and Topic Ideas

The solar system is a group of celestial bodies that are gravitationally bound together by the sun. It consists of eight planets, including Earth, and a variety of other objects, such as moons, asteroids, comets, and dwarf planets. The sun dominates the solar system, providing the light and energy necessary for life on Earth. Each planet in the solar system has unique characteristics and features, including its size, composition, and distance from the sun. The study of the solar system is an ongoing field of research and exploration, and continues to yield exciting new insights into the nature of our universe.

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The Solar System 101

Space is sometimes called “the final frontier,” a phrase popularized by the iconic Star Trek television series. But it is an apt description of humanity’s scant understanding of the planets, stars, and other celestial bodies beyond Earth. Although, we understand the parts of our own solar system better than those outside of it, we still have a lot to learn. Watch these National Geographic 101 videos to learn more about our cosmic neighborhood.

Earth Science, Astronomy

What is a Planet?

This seemingly simple question doesn't have a simple answer. Everyone knows that Earth , Mars and Jupiter are planets. But both Pluto and Ceres were once considered planets until new discoveries triggered scientific debate about how to best describe them—a vigorous debate that continues to this day. The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:

• It must orbit a star (in our cosmic neighborhood, the Sun ).
• It must be big enough to have enough gravity to force it into a spherical shape.
• It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.

Discussion—and debate—will continue as our view of the cosmos continues to expand.

The Scientific Process

Science is a dynamic process of questioning, hypothesizing, discovering, and changing previous ideas based on what is learned. Scientific ideas are developed through reasoning and tested against observations. Scientists assess and question each other's work in a critical process called peer review.

Our understanding about the universe and our place in it has changed over time. New information can cause us to rethink what we know and reevaluate how we classify objects in order to better understand them. New ideas and perspectives can come from questioning a theory or seeing where a classification breaks down.

An Evolving Definition

Defining the term planet is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system. Over historical time, objects categorized as planets have changed. The ancient Greeks counted the Earth's Moon and Sun as planets along with Mercury, Venus, Mars, Jupiter, and Saturn. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited. The first known model that placed the Sun at the center of the known universe with the Earth revolving around it was presented by Aristarchus of Samos in the third century BCE, but it was not generally accepted. It wasn't until the 16th century that the idea was revived by Nicolaus Copernicus.

By the 17th century, astronomers (aided by the invention of the telescope) realized that the Sun was the celestial object around which all the planets—including Earth—orbit, and that the moon is not a planet, but a satellite (moon) of Earth. Uranus was added as a planet in 1781 and Neptune was discovered in 1846.

Ceres was discovered between Mars and Jupiter in 1801 and originally classified as a planet. But as many more objects were subsequently found in the same region, it was realized that Ceres was the first of a class of similar objects that were eventually termed asteroids (star-like) or minor planets.

Pluto, discovered in 1930, was identified as the ninth planet. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. It is unlike the terrestrial planets (Mercury, Venus, Earth, Mars), or the gas giants (Jupiter, Saturn), or the ice giants (Uranus, Neptune). Charon, its huge satellite, is nearly half the size of Pluto and shares Pluto's orbit. Though Pluto kept its planetary status through the 1980s, things began to change in the 1990s with some new discoveries.

Technical advances in telescopes led to better observations and improved detection of very small, very distant objects. In the early 1990s, astronomers began finding numerous icy worlds orbiting the Sun in a doughnut-shaped region called the Kuiper Belt beyond the orbit of Neptune—out in Pluto's realm. With the discovery of the Kuiper Belt and its thousands of icy bodies (known as Kuiper Belt Objects, or KBOs; also called transneptunians), it was proposed that it is more useful to think of Pluto as the biggest KBO instead of a planet.

The Planet Debate

Then, in 2005, a team of astronomers announced that they had found a tenth planet— it was a KBO similar in size to Pluto. People began to wonder what planethood really means. Just what is a planet, anyway? Suddenly the answer to that question didn't seem so self-evident, and, as it turns out, there are plenty of disagreements about it.

The International Astronomical Union (IAU), a worldwide organization of astronomers, took on the challenge of classifying the newly found KBO (later named Eris). In 2006, the IAU passed a resolution that defined planet and established a new category, dwarf planet. Eris, Ceres, Pluto, and two more recently discovered KBOs named Haumea and Makemake, are the dwarf planets recognized by the IAU. There may be another 100 dwarf planets in the solar system and hundreds more in and just outside the Kuiper Belt.

The New Definition of Planet

Here is the text of the IAU’s Resolution B5: Definition of a Planet in the Solar System :

Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:

• A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
• A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
• All other objects,except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Debate—and Discoveries—Continue

Astronomers and planetary scientists did not unanimously agree with these definitions. To some it appeared that the classification scheme was designed to limit the number of planets; to others it was incomplete and the terms unclear. Some astronomers argued that location (context) is important, especially in understanding the formation and evolution of the solar system.

One idea is to simply define a planet as a natural object in space that is massive enough for gravity to make it approximately spherical. But some scientists objected that this simple definition does not take into account what degree of measurable roundness is needed for an object to be considered round. In fact, it is often difficult to accurately determine the shapes of some distant objects. Others argue that where an object is located or what it is made of do matter and there should not be a concern with dynamics; that is, whether or not an object sweeps up or scatters away its immediate neighbors, or holds them in stable orbits. The lively planethood debate continues.

As our knowledge deepens and expands, the more complex and intriguing the universe appears. Researchers have found hundreds of extrasolar planets, or exoplanets, that reside outside our solar system; there may be billions of exoplanets in the Milky Way Galaxy alone, and some may be habitable (have conditions favorable to life). Whether our definitions of planet can be applied to these newly found objects remains to be seen.

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• Solar System Essay

Introduction to Essay Writing on Solar System on Vedantu

An essay is a piece of writing where an author expresses in detail all the information on a particular topic. An essay differs from other writing because it is more structured and it provides the author with their own perspective. In this particular essay, we shall know in detail about the solar system. Use this essay as a reference essay and try writing an essay on the solar system.

Let us begin our learning!

Essay on Solar System

The solar system consists of the sun, eight planets, and sixty-seven satellites of the planets, and a large number of small bodies (comets and asteroids). Earlier, Pluto was considered the smallest planet but now Pluto is not recognized anymore as a planet. The inner solar system comprises Sun, Mercury, Venus, Earth, and Mars. Jupiter, Saturn, Uranus, and Neptune form the outer solar system. These four planets are massive in size; hence they are called Giant Planets. Each planet revolves around the sun in its own orbits at its own speed.

Let us explore all the celestial bodies present in the Solar system.

The Sun was born 4.6 billions of years ago and it was formed from a giant rotating cloud of gasses and dust known as solar Nebula. The sun is the biggest star present at the center of the solar system. It is a self-luminous sphere of gasses. Its gravitational force holds the entire solar system. It has a radius of 695,508 kilometers and is 150 million kilometers away from Earth.

Mercury is the smallest and closest planet to the sun. It is also called Swift planet because it completes its revolution in 88 earth days. Its diameter is only one third of Earth but its density is about the same. The temperature of this planet is as high as 450 degrees Celsius in the mornings and nights are freezing cold. The surface of this planet is filled with craters, mountains and valleys.

Venus is the second closest planet to the sun and the hottest. Venus is the brightest planet and hence called the morning star. Venus is named after the Roman Goddess of love and beauty. Venus completes one revolution around the sun in 255 earth days. Venus spins clockwise on its orbits unlike other planets. Its surface is covered with clouds, craters, mountains and lava plains.

The third planet in the solar system is Earth. This is the only planet that sustains life. It is called the Blue planet because 70% of the earth's surface is covered with water. Earth takes 365 days to complete one revolution around the sun. This planet has only one natural satellite, the Moon.

The fourth planet from the sun in the solar system is Mars. It appears as a red-orange ball because of the presence of iron oxide and so it is called the Red planet. It is the second smallest planet after Mercury. Mars is named after the Roman God of war. Its surface is covered with volcanoes, craters all over.

Jupiter is the largest planet in the solar system. Jupiter is rich in hydrogen and helium gas and so it is also called a Gas Giant planet. Jupiter takes 4333 earth days to complete one revolution around the sun. This planet has 79 satellites. Jupiter has four rings.

Saturn is the least dense planet in the solar system. It is the second-largest planet. Saturn can float in water because it is made up of gasses like helium. The beautiful rings around the planet are made up of bits of ice, rock, and dust. Saturn revolves very slowly around the sun. This planet is named after the Roman God of agriculture and wealth.

Uranus is the coldest planet in the solar system. It takes 84 earth years to complete one revolution around the sun. Uranus is called an ice giant planet because its layer is made of ice and hydrogen, helium and methane. Uranus looks blue in color because of the presence of methane. Uranus has 27 satellites.

Neptune is the eighth and the farthest planet from the sun in the solar system. Neptune is named after the Roman God of the sea. Its atmosphere is made up of hydrogen, helium and methane and the presence of methane gives the color blue to the planet. It takes 165 earth years to complete one revolution. Neptune has 6 rings.

Comets and Asteroids:

Comets and Asteroids are the small celestial bodies that rotate around the sun. Asteroids are made up of rocks, metals and water. Comets are made up of frozen ammonia, methane and small amounts of rocky material.

FAQs on Solar System Essay

1. How many planets are there in the solar system?

There are eight planets in the solar system.

2. Is the sun a planet or star?

The sun is a big star located at the centre of the solar system.

3. Which planet sustains life?

The Earth planet sustains life.

4. Which is the coldest planet in the solar system?

Uranus is the coldest planet in the solar system.

5. How to write well on any topic?

It is very important for the students to learn to write on their own. To write a good essay students should follow the following steps - 

Try to understand the topic you want to write about 

Read from multiple sources to get an idea of the topic 

Prepare a structure that is what all you want to cover in your writing 

Note down all the important points according to your structure 

Arrange the collected information in the pre-decided structure 

Remember to keep your readers engaged in your essay

Try to use idea and words which doesn't hurt anyone's emotions

Start writing and with time you would get better in the process

 You can also send us your essays or writing which will be evaluated by the faculty.

6. What should be the structure on which an essay can be written?

Like every writing, an essay also has three parts that are the introduction, body, and conclusion. Keep the introduction very interesting, get the attention of your reader by starting with a short story then gradually introduce your topic through that story. Secondly, make the audience aware of the keywords of the topic. In the body, write in detail about the topic like state the historical, economical, social, environmental, cultural factors of your topic. And then conclude your essay by summarizing the key message and the takeaways of the essay. Try to practice with this framework and in due course of time, you will be able to write an excellent essay. Also, try to read from some great essays.

7. What is the process of planet formation called?

The process by which planets are formed is called planetesimals. In the process, the clouds of gasses came together due to gravitational differences . The area of more clouds had higher gravitation and thus attracted more clouds towards them. The ball of clouds takes a round shape through the process of accretion.  

Read the article on Solar systems on the website of Vedantu.

8. What are terrestrial and jovian planets?

Terrestrial planets are planets closer to the Sun, it is also called inner planets. These planets are also called Earth-like planets as their features are similar to the Earth. It includes four planets which are Mercury, Venus, Earth, and Mars. Whereas jovian planets are the outer planets which are farther from the Sun. They are also called Jupiter-like planets as they share features similar to Jupiter. It includes Jupiter, Saturn, Uranus, and Neptune.

9. Can we draw diagrams in an essay? 

Some diagrammatic representation in an essay can be done. However, it is recommended that we should avoid drawing diagrams in an essay as it breaks the flow of the writing. Read some good essays to improve your writing style.

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17 Origin of the Universe and Our Solar System

Learning Objectives

By the end of this chapter, students should be able to:

• Explain the formation of the universe and how we observe it.
• Understand the origin of our solar system .
• Describe how the objects in our solar system are identified, explored, and characterized.
• Describe the types of small bodies in our solar system, their locations, and how they formed.
• Describe the characteristics of the giant planets , terrestrial planets , and small bodies in the solar system.
• Explain what influences the temperature of a planet’s surface.
• Explain why there is geological activity on some planets and not on others.
• Describe different methods for dating planets and the age of the solar system.
• Describe how the characteristics of extrasolar systems help us to model our own solar system.

The universe began 13.77 billion years ago when energy, matter, and space expanded from a single point. Evidence for the big bang is the cosmic “afterglow” from when the universe was still very dense, and red-shifted light from distant galaxies, which tell us the universe is still expanding.

The big bang produced hydrogen, helium, and lithium, but heavier elements come from nuclear fusion reactions in stars. Large stars make elements such as silicon, iron, and magnesium, which are important in forming terrestrial planets. Large stars explode as supernovae and scatter the elements into space.

Planetary systems begin with the collapse of a cloud of gas and dust. Material drawn to the center forms a star, and the remainder forms a disk around the star. Material within the disk clumps together to form planets. In our solar system , rocky planets are closer to the Sun, and ice and gas giants are farther away. This is because temperatures near the Sun were too high for ice to form, but silicate minerals and metals could solidify.

Early Earth was heated by radioactive decay, collisions with bodies from space, and gravitational compression. Heating melted Earth, causing molten metal to sink to Earth’s center and form a core, and silicate melt to float to the surface and form the mantle and crust. A collision with a planet the size of Mars knocked debris into orbit around Earth, and the debris coalesced into the moon. Earth’s atmosphere is the result of volcanic degassing, contributions by comets and meteorites, and photosynthesis.

The search for exoplanets has identified 12 planets that are similar in size to Earth and within the habitable zone of their stars. These are thought to be rocky worlds like Earth, but the compositions of these planets are not known for certain.

17.1 The Big Bang

According to the big bang theory , the universe blinked violently into existence 13.77 billion years ago. The big bang is often described as an explosion, but imagining it as an enormous fireball isn’t accurate. The big bang involved a sudden expansion of matter, energy, and space from a single point. The kind of Hollywood explosion that might come to mind involves expansion of matter and energy within space, but during the big bang, space itself was created.

At the start of the big bang, the universe was too hot and dense to be anything but a sizzle of particles smaller than atoms, but as it expanded, it also cooled. Eventually some of the particles collided and stuck together. Those collisions produced hydrogen and helium, the most common elements in the universe, along with a small amount of lithium.

You may wonder how a universe can be created out of nothing, or how we can know that the big bang happened at all. Creating a universe out of nothing is mostly beyond the scope of this chapter, but there is a way to think about it. The particles that make up the universe have opposites that cancel each other out, similar to the way that we can add the numbers 1 and −1 to get zero (also known as “nothing”). As far as the math goes, having zero is exactly the same as having a 1 and a −1. It is also exactly the same as having a 2 and a −2, a 3 and a −3, two −1s and a 2, and so on. In other words,  nothing is really the potential for something if you divide it into its opposite parts. As for how we can know that the big bang happened at all, there are very good reasons to accept that it is indeed how our universe came to be.

17.1.1 Looking Back to the Early Stages of the Big Bang

The notion of seeing the past is often used metaphorically when we talk about ancient events, but in this case it is meant literally. In our everyday experience, when we watch an event take place, we perceive that we are watching it as it unfolds in real time. In fact, this isn’t true. To see the event, light from that event must travel to our eyes. Light travels very rapidly, but it does not travel instantly. If we were watching a digital clock 1 m away from us change from 11:59 a.m. to 12:00 p.m., we would actually see it turn to 12:00 p.m. three billionths of a second after it happened. This isn’t enough of a delay to cause us to be late for an appointment, but the universe is a very big place, and the “digital clock” in question is often much, much farther away. In fact, the universe is so big that it is convenient to describe distances in terms of light years , or the distance light travels in one year. What this means is that light from distant objects takes so long to get to us that we see those objects as they were at some considerable time in the past. For example, the star Proxima Centauri is 4.24 light years from the sun. If you viewed Proxima Centauri from Earth on January 1, 2015, you would actually see it as it appeared in early October 2010.

We now have tools that are powerful enough to look deep into space and see the arrival of light from early in the universe’s history. Astronomers can detect light from approximately 375,000 years after the big bang is thought to have occurred. Physicists tell us that if the big bang happened, then particles within the universe would still be very close together at this time. They would be so close that light wouldn’t be able to travel far without bumping into another particle and getting scattered in another direction. The effect would be to fill the sky with glowing fog, the “afterglow” from the formation of the universe.

In fact, this is exactly what we see when we look at light from 375,000 years after the big bang. The fog is referred to as the cosmic microwave background (or CMB), and it has been carefully mapped throughout the sky. The map displays the cosmic microwave background as temperature variations, but these variations translate to differences in the density of matter in the early universe. The red patches are the highest density regions and the blue patches are the lowest density. Higher density regions represent the eventual beginnings of stars and planets. The map has been likened to a baby picture of the universe.

17.1.2 The Big Bang is Still Happening, and We Can See the Universe Expanding

The expansion that started with the big bang never stopped. It continues today, and we can see it happening by observing galaxies that large clusters of billions of stars, called galaxies, are moving away from us. (The exception is the Andromeda galaxy with which we are on a collision course.) The astronomer Edwin Hubble came to this conclusion when he observed that the light from other galaxies was red-shifted. The red shift is a consequence of the Doppler effect. This refers to how we see waves when the object that is creating the waves is moving toward us or away from us.

Before we get to the Doppler effect as it pertains to the red shift, let’s see how it works on something more tangible. The swimming duckling is generating waves as it moves through the water. It is generating waves that move forward as well as back, but notice that the ripples ahead of the duckling are closer to each other than the ripples behind the duckling. The distance from one ripple to the next is called the wavelength . The wavelength is shorter in the direction that the duckling is moving, and longer as the duckling moves away.

When waves are in air as sound waves rather than in water as ripples, the different wavelengths manifest as sounds with different pitches—the short wavelengths have a higher pitch, and the long wavelengths have a lower pitch. This is why the pitch of a car’s engine changes as the car races past you.

If you are using an offline version of this text, access the quiz for section 17.1 via the QR code.

17.2 Overview of Our Planetary System [1]

The solar system consists of the Sun and many smaller objects: the planets, their moons and rings, and such “debris” as asteroids , comets , and dust. Decades of observation and spacecraft exploration have revealed that most of these objects formed together with the Sun about 4.5 billion years ago. They represent clumps of material that condensed from an enormous cloud of gas and dust. The central part of this cloud became the Sun, and a small fraction of the material in the outer parts eventually formed the other objects.

During the past 50 years, we have learned more about the solar system than anyone imagined before the space age. In addition to gathering information with powerful new telescopes, we have sent spacecraft directly to many members of the planetary system . (Planetary astronomy is the only branch of our science in which we can, at least vicariously, travel to the objects we want to study.) With evocative names such as  Voyager ,  Pioneer ,  Curiosity , and  Pathfinder , our robot explorers have flown past, orbited, or landed on every planet, returning images and data that have dazzled both astronomers and the public. In the process, we have also investigated two dwarf planets , hundreds of fascinating moons, four ring systems, a dozen asteroids, and several comets (smaller members of our solar system that we will discuss later).

Our probes have penetrated the atmosphere of Jupiter and landed on the surfaces of Venus, Mars, our  Moon , Saturn’s moon Titan, the asteroids Eros, Itokawa, Ryugu, and Bennu, and the Comet Churyumov-Gerasimenko (usually referred to as 67P). Humans have set foot on the Moon and returned samples of its surface soil for laboratory analysis. We have flown a helicopter drone on Mars. We have even discovered other places in our solar system that might be able to support some kind of life.

17.2.1 An Inventory

The Sun, a star that is brighter than about 80% of the stars in the Galaxy, is by far the most massive member of the solar system. It is an enormous ball about 1.4 million kilometers in diameter, with surface layers of incandescent gas and an interior temperature of millions of degrees. The Sun will be discussed in later chapters as our first, and best-studied, example of a star.

Table 17.1: Mass of members of the solar system. Note that the Sun is by far the most massive member of the solar system.

Most of the material of the planets in the solar system is actually concentrated in the largest one, Jupiter , which is more massive than all the rest of the planets combined. Astronomers were able to determine the masses of the planets centuries ago using Kepler’s laws of planetary motion and Newton’s law of gravity to measure the planets’ gravitational effects on one another or on moons that orbit them. Today, we make even more precise measurements of their masses by tracking their gravitational effects on the motion of spacecraft that pass near them.

Beside Earth, five other planets were known to the ancients—Mercury, Venus, Mars, Jupiter, and Saturn—and two were discovered after the invention of the telescope: Uranus and Neptune. The eight planets all revolve in the same direction around the Sun. They orbit in approximately the same plane, like cars traveling on concentric tracks on a giant, flat racecourse. Each planet stays in its own “traffic lane,” following a nearly circular orbit about the Sun and obeying the “traffic” laws discovered by Galileo, Kepler, and Newton. Besides these planets, we have also been discovering smaller worlds beyond Neptune that are called trans-Neptunian object s or TNOs. The first to be found, in 1930, was  Pluto , but others have been discovered during the twenty-first century. One of them,  Eris , is about the same size as Pluto and has at least one moon (Pluto has five known moons). The largest TNOs are also classed as dwarf planets ,  as is the largest asteroid,  Ceres . To date, more than 2600 of these TNOs have been discovered, and one, called Arrokoth, was explored by the New Horizons spacecraft.

Each of the planets and dwarf planets also rotates (spins) about an axis running through it, and in most cases the direction of rotation is the same as the direction of revolution about the Sun. The exceptions are  Venus , which rotates backward very slowly (that is, in a retrograde direction), and Uranus and  Pluto , which also have strange rotations, each spinning about an axis tipped nearly on its side. We do not yet know the spin orientations of Eris, Haumea, and Makemake.

The four planets closest to the Sun (Mercury through Mars) are called the inner or  terrestrial planets . Often, the  Moon is also discussed as a part of this group, bringing the total of terrestrial objects to five (we generally call Earth’s satellite “the Moon,” with a capital M, and the other satellites “moons,” with lowercase m’s). The terrestrial planets are relatively small worlds, composed primarily of rock and metal. All of them have solid surfaces that bear the records of their geological history in the forms of craters , mountains , and volcanoes .

The next four planets (Jupiter through Neptune) are much larger and are composed primarily of lighter ices, liquids, and gases. We call these four the Jovian planets (after “Jove,” another name for Jupiter in mythology) or giant planets —a name they richly deserve. About 1,300 Earths could fit inside Jupiter, for example. These planets do not have solid surfaces on which future explorers might land. They are more like vast, spherical oceans with much smaller, dense cores.

Near the outer edge of the system lies  Pluto , which was the first of the distant icy worlds to be discovered beyond Neptune (Pluto was visited by a spacecraft, the NASA New Horizons mission, in 2015).

Table 17.2: The planets.

The outermost part of the solar system is known as the Kuiper belt , which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud , a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well.

The IAU narrowed the definition of a planet to three criteria: 1) it must orbit a star (in our cosmic neighborhood, the Sun), 2) it must be big enough to have enough gravity to force it into a spherical shape, and 3) it must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets.

17.2.2 Smaller Members of the Solar System

Most of the planets are accompanied by one or more moons ; only Mercury and Venus move through space alone. There are more than 210 known moons orbiting planets and dwarf planets, and undoubtedly many other small ones remain undiscovered. The largest of the moons are as big as small planets and just as interesting. In addition to our Moon, they include the four largest moons of Jupiter (called the Galilean moons, after their discoverer) and the largest moons of Saturn and Neptune (confusingly named Titan and Triton).

Each of the giant planets also has rings made up of countless small bodies ranging in size from mountains to mere grains of dust, all in orbit about the equator of the planet. The bright rings of Saturn are, by far, the easiest to see. They are among the most beautiful sights in the solar system.   But, all four ring systems are interesting to scientists because of their complicated forms, influenced by the pull of the moons that also orbit these giant planets.

The solar system has many other less-conspicuous members. Another group is the  asteroids , rocky bodies that orbit the Sun like miniature planets, mostly in the space between Mars and Jupiter (although some do cross the orbits of planets like Earth). Most asteroids are remnants of the initial population of the solar system that existed before the planets themselves formed. Some of the smallest moons of the planets, such as the moons of Mars, are very likely captured asteroids.

Another class of small bodies is composed mostly of ice, made of frozen gases such as water, carbon dioxide, and carbon monoxide; these objects are called  comets . Comets also are remnants from the formation of the solar system, but they were formed and continue (with rare exceptions) to orbit the Sun in distant, cooler regions—stored in a sort of cosmic deep freeze. This is also the realm of the larger icy worlds, called dwarf planets.

Finally, there are countless grains of broken rock, which we call cosmic dust, scattered throughout the solar system. When these particles enter Earth’s atmosphere (as millions do each day), they burn up, producing a brief flash of light in the night sky known as a meteor  (meteors are often referred to as shooting stars). Occasionally, some larger chunk of rocky or metallic material survives its passage through the atmosphere and lands on Earth. Any piece that strikes the ground is known as a  meteorite . You can see meteorites on display in many natural history museums and can sometimes even purchase pieces of them from gem and mineral dealers.

17.2.3 A Scale Model of the Solar System

Astronomy often deals with dimensions and distances that far exceed our ordinary experience. What does 1.4 billion kilometers—the distance from the Sun to Saturn—really mean to anyone? It can be helpful to visualize such large systems in terms of a scale model.

In our imaginations, let us build a scale model of the solar system, adopting a scale factor of 1 billion (10 9 )—that is, reducing the actual solar system by dividing every dimension by a factor of 10 9 . Earth, then, has a diameter of 1.3 centimeters, about the size of a grape. The Moon is a pea orbiting this at a distance of 40 centimeters, or a little more than a foot away. The Earth-Moon system fits into a standard backpack.

In this model, the Sun is nearly 1.5 meters in diameter, about the average height of an adult, and our Earth is at a distance of 150 meters—about one city block—from the Sun. Jupiter is five blocks away from the Sun, and its diameter is 15 centimeters, about the size of a very large grapefruit. Saturn is 10 blocks from the Sun; Uranus, 20 blocks; and Neptune, 30 blocks. Pluto, with a distance that varies quite a bit during its 249-year orbit, is currently just beyond 30 blocks and getting farther with time. Most of the moons of the outer solar system are the sizes of various kinds of seeds orbiting the grapefruit, oranges, and lemons that represent the outer planets.

In our scale model, a human is reduced to the dimensions of a single atom, and cars and spacecraft to the size of molecules. Sending the Voyager spacecraft to Neptune involves navigating a single molecule from the Earth–grape toward a lemon 5 kilometers away with an accuracy equivalent to the width of a thread in a spider’s web.

If that model represents the solar system, where would the nearest stars be? If we keep the same scale, the closest stars would be tens of thousands of kilometers away. If you built this scale model in the city where you live, you would have to place the representations of these stars on the other side of Earth or beyond.

By the way, model solar systems like the one we just presented have been built in cities throughout the world. In Sweden, for example, Stockholm’s huge Globe Arena has become a model for the Sun, and Pluto is represented by a 12-centimeter sculpture in the small town of Delsbo, 300 kilometers away. Another model solar system is in Washington on the Mall between the White House and Congress (perhaps proving they are worlds apart?).

If you are using an offline version of this text, access the quiz for section 17.2 via the QR code.

17.3 Composition and Structure of Planets [2]

The fact that there are two distinct kinds of planets—the rocky terrestrial planets and the gas-rich Jovian planets—leads us to believe that they formed under different conditions. Certainly their compositions are dominated by different elements . Let us look at each type in more detail.

17.3.1 The Giant Planets

The two largest planets,  Jupiter  and  Saturn , have nearly the same chemical makeup as the Sun; they are composed primarily of the two elements hydrogen and helium, with 75% of their mass being hydrogen and 25% helium. On Earth, both hydrogen and helium are gases, so Jupiter and Saturn are sometimes called gas planets. But, this name is misleading. Jupiter and Saturn are so large that the gas is compressed in their interior until the hydrogen becomes a liquid. Because the bulk of both planets consists of compressed, liquefied hydrogen, we should really call them liquid planets.

Under the force of gravity, the heavier elements sink toward the inner parts of a liquid or gaseous planet. Both Jupiter and Saturn, therefore, have cores composed of heavier rock, metal, and ice, but we cannot see these regions directly. In fact, when we look down from above, all we see is the atmosphere with its swirling clouds. We must infer the existence of the denser core inside these planets from studies of each planet’s gravity.

Uranus  and  Neptune are much smaller than Jupiter and Saturn, but each also has a core of rock, metal, and ice. Uranus and Neptune were less efficient at attracting hydrogen and helium gas, so they have much smaller atmospheres in proportion to their cores.

Chemically, each giant planet is dominated by hydrogen and its many compounds. Nearly all the oxygen present is combined chemically with hydrogen to form water (H 2 O). Chemists call such a hydrogen-dominated composition  reduced . Throughout the outer solar system, we find abundant water (mostly in the form of ice) and reducing chemistry.

17.3.2 The Terrestrial Planets

The terrestrial planets are quite different from the giants. In addition to being much smaller, they are composed primarily of rocks and metals. These, in turn, are made of elements that are less common in the universe as a whole. The most abundant rocks, called silicates , are made of silicon and oxygen, and the most common metal is iron. We can tell from their densities that  Mercury  has the greatest proportion of metals (which are denser) and the Moon has the lowest.  Earth ,  Venus , and  Mars  all have roughly similar bulk compositions: about one third of their mass consists of iron-nickel or iron-sulfur combinations; two thirds is made of silicates. Because these planets are largely composed of oxygen compounds (such as the silicate minerals of their crusts), their chemistry is said to be  oxidized .

When we look at the internal structure of each of the terrestrial planets, we find that the densest metals are in a central core, with the lighter silicates near the surface. If these planets were liquid, like the giant planets, we could understand this effect as the result the sinking of heavier elements due to the pull of gravity. This leads us to conclude that, although the terrestrial planets are solid today, at one time they must have been hot enough to melt.

Differentiation  is the process by which gravity helps separate a planet’s interior into layers of different compositions and densities. The heavier metals sink to form a core, while the lightest minerals float to the surface to form a crust. Later, when the planet cools, this layered structure is preserved. In order for a rocky planet to differentiate, it must be heated to the melting point of rocks, which is typically more than 1300 K.

17.3.3 Moons, Asteroids, and Comets

Chemically and structurally, Earth’s Moon is like the terrestrial planets, but most moons are in the outer solar system, and they have compositions similar to the cores of the giant planets around which they orbit. The three largest moons—Ganymede and Callisto in the Jovian system, and  Titan in the Saturnian system—are composed half of frozen water, and half of rocks and metals. Most of these moons differentiated during formation, and today they have cores of rock and metal, with upper layers and crusts of very cold and—thus very hard—ice.

Most of the asteroids and comets , as well as the smallest moons , were probably never heated to the melting point. However, some of the largest asteroids, such as  Vesta , appear to be differentiated; others are fragments from differentiated bodies. Many of the smaller objects seem to be fragments or rubble piles that are the result of collisions. Because most asteroids and comets retain their original composition, they represent relatively unmodified material dating back to the time of the formation of the solar system. In a sense, they act as chemical fossils, helping us to learn about a time long ago whose traces have been erased on larger worlds.

17.3.4 Temperatures: Going to Extremes

Generally speaking, the farther a planet or moon is from the Sun, the cooler its surface. The planets are heated by the radiant energy of the Sun, which gets weaker with the square of the distance. You know how rapidly the heating effect of a fireplace or an outdoor radiant heater diminishes as you walk away from it; the same effect applies to the Sun.  Mercury , the closest planet to the Sun, has a blistering surface temperature that ranges from 280–430 °C on its sunlit side, whereas the surface temperature on  Pluto is only about –220 °C, colder than liquid air.

Mathematically, the temperatures decrease approximately in proportion to the square root of the distance from the Sun. Pluto is about 30 AU at its closest to the Sun (or 100 times the distance of Mercury) and about 49 AU at its farthest from the Sun. Thus, Pluto’s temperature is less than that of Mercury by the square root of 100, or a factor of 10: from 500 K to 50 K.

In addition to its distance from the Sun, the surface temperature of a planet can be influenced strongly by its atmosphere . Without our atmospheric insulation (the greenhouse effect, which keeps the heat in), the oceans of Earth would be permanently frozen. Conversely, if Mars once had a larger atmosphere in the past, it could have supported a more temperate climate than it has today. Venus is an even more extreme example, where its thick atmosphere of carbon dioxide acts as insulation, reducing the escape of heat built up at the surface, resulting in temperatures greater than those on Mercury. Today, Earth is the only planet where surface temperatures generally lie between the freezing and boiling points of water. As far as we know, Earth is the only planet to support life.

17.3.5 Dating Planetary Surfaces [3]

How do we know the age of the surfaces we see on planets and moons? If a world has a surface (as opposed to being mostly gas and liquid), astronomers have developed some techniques for estimating how long ago that surface solidified. Note that the age of these surfaces is not necessarily the age of the planet as a whole. On geologically active objects (including Earth), vast outpourings of molten rock or the erosive effects of water and ice, which we call planet weathering , have erased evidence of earlier epochs and present us with only a relatively young surface for investigation.

One way to estimate the age of a surface is by counting the number of impact  craters . This technique works because the rate at which impacts have occurred in the solar system has been roughly constant for several billion years. Thus, in the absence of forces to eliminate craters, the number of craters is simply proportional to the length of time the surface has been exposed. This technique has been applied successfully to many solid planets and moons .

Bear in mind that crater counts can tell us only the time since the surface experienced a major change that could modify or erase preexisting craters. Estimating ages from crater counts is a little like walking along a sidewalk in a snowstorm after the snow has been falling steadily for a day or more. You may notice that in front of one house the snow is deep, while next door the sidewalk may be almost clear. Do you conclude that less snow has fallen in front of Ms. Jones’ house than Mr. Smith’s? More likely, you conclude that Jones has recently swept the walk clean and Smith has not. Similarly, the numbers of craters indicate how long it has been since a planetary surface was last “swept clean” by ongoing lava flows or by molten materials ejected when a large impact happened nearby.

Still, astronomers can use the numbers of craters on different parts of the same world to provide important clues about how regions on that world evolved. On a given planet or moon, the more heavily cratered terrain will generally be older (that is, more time will have elapsed there since something swept the region clean).

17.3.6 Radioactive Rocks

Another way to trace the history of a solid world is to measure the age of individual rocks. After samples were brought back from the Moon  by Apollo astronauts, the techniques that had been developed to date rocks on Earth were applied to rock samples from the Moon to establish a geological chronology for the Moon. Furthermore, a few samples of material from the Moon, Mars, and the large asteroid  Vesta have fallen to Earth as meteorites and can be examined directly.

Scientists measure the age of rocks using the properties of natural  radioactivity . Around the beginning of the twentieth century, physicists began to understand that some atomic nuclei are not stable but can split apart (decay) spontaneously into smaller nuclei. The process of radioactive decay involves the emission of particles such as electrons , or of radiation in the form of gamma rays .

For any one radioactive nucleus, it is not possible to predict when the decay process will happen. Such decay is random in nature, like the throw of dice: as gamblers have found all too often, it is impossible to say just when the dice will come up 7 or 11. But, for a very large number of dice tosses, we can calculate the odds that 7 or 11 will come up. Similarly, if we have a very large number of radioactive atoms of one type (say, uranium), there is a specific time period, called its  half-life , during which the chances are fifty-fifty that decay will occur for any of the nuclei.

A particular nucleus may last a shorter or longer time than its half-life, but in a large sample, almost exactly half of the nuclei will have decayed after a time equal to one half-life. Half of the remaining nuclei will have decayed after two half-lives pass, leaving only one half of a half—or one quarter—of the original sample.

If you had 1 gram of pure radioactive nuclei with a half-life of 100 years, then after 100 years you would have 1/2 gram; after 200 years, 1/4 gram; after 300 years, only 1/8 gram; and so forth. However, the material does not disappear. Instead, the radioactive atoms are replaced with their decay products. Sometimes the radioactive atoms are called  parents  and the decay products are called  daughter elements.

In this way, radioactive elements with half-lives we have determined can provide accurate nuclear clocks. By comparing how much of a radioactive parent element is left in a rock to how much of its daughter products have accumulated, we can learn how long the decay process has been going on and hence how long ago the rock formed. The following table summarizes the decay reactions used most often to date lunar and terrestrial rocks.

Table 17.3: Radioactive decay reaction used to date rocks. The number after each element is its atomic weight, equal to the number of protons plus neutrons in its nucleus. This specifies the isotope of the element, different isotopes of the same element differ in the number of neutrons.

When astronauts first flew to the Moon, one of their most important tasks was to bring back lunar rocks for radioactive age-dating. Until then, astronomers and geologists had no reliable way to measure the age of the lunar surface. Counting craters had let us calculate relative ages (for example, the heavily cratered lunar highlands were older than the dark lava plains), but scientists could not measure the actual age in years. Some thought that the ages were as young as those of Earth’s surface, which has been resurfaced by many geological events. For the Moon’s surface to be so young would imply active geology on our satellite. Only in 1969, when the first Apollo samples were dated, did we learn that the Moon is an ancient, geologically dead world. Using such dating techniques, we have been able to determine the ages of both Earth and the Moon: each was formed about 4.5 billion years ago (although, as we shall see, Earth probably formed earlier than the Moon).

We should also note that the decay of radioactive nuclei generally releases energy in the form of heat. Although the energy from a single nucleus is not very large (in human terms), the enormous numbers of radioactive nuclei in a planet or moon (especially early in its existence) can be a significant source of internal energy for that world. Geologists estimate that about half of Earth’s current internal heat budget comes from the decay of radioactive isotopes in its interior.

If you are using an offline version of this text, access the quiz for section 17.3 via the QR code.

17.4 Origin of the Solar System [4]

Much of astronomy is motivated by a desire to understand the origin of things: to find at least partial answers to age-old questions of where the universe , the Sun, Earth, and we ourselves came from. Each planet and moon is a fascinating place that may stimulate our imagination as we try to picture what it would be like to visit. Taken together, the members of the solar system preserve patterns that can tell us about the formation of the entire system. As we begin our exploration of the planets, we want to introduce our modern picture of how the solar system formed.

The recent discovery of thousands of planets in orbit around other stars has shown astronomers that many exoplanetary systems can be quite different from our own solar system. For example, it is common for these systems to include planets intermediate in size between our terrestrial and giant planets. These are often called  superearths . Some exoplanet systems even have giant planets close to the star, reversing the order we see in our system.

17.4.1 Looking for Patterns

One way to approach our question of origin is to look for regularities among the planets. We found, for example, that all the planets lie in nearly the same plane and revolve in the same direction around the Sun. The Sun also spins in the same direction about its own axis. Astronomers interpret this pattern as evidence that the Sun and planets formed together from a spinning cloud of gas and dust that we call the  solar nebula .

The composition of the planets gives another clue about origins. Spectroscopic analysis allows us to determine which elements are present in the Sun and the planets. The Sun has the same hydrogen-dominated composition as Jupiter and Saturn, and therefore appears to have been formed from the same reservoir of material. In comparison, the terrestrial planets and our Moon are relatively deficient in the light gases and the various ices that form from the common elements oxygen, carbon, and nitrogen. Instead, on Earth and its neighbors, we see mostly the rarer heavy elements such as iron and silicon. This pattern suggests that the processes that led to planet formation in the inner solar system must somehow have excluded much of the lighter materials that are common elsewhere. These lighter materials must have escaped, leaving a residue of heavy stuff.

The reason for this is not hard to guess, bearing in mind the heat of the Sun. The inner planets and most of the asteroids are made of rock and metal, which can survive heat, but they contain very little ice or gas, which evaporate when temperatures are high (to see what we mean, just compare how long a rock and an ice cube survive when they are placed in the sunlight). In the outer solar system, where it has always been cooler, the planets and their moons, as well as icy dwarf planets and comets , are composed mostly of ice and gas.

17.4.2 The Evidence from Far Away

A second approach to understanding the origins of the solar system is to look outward for evidence that other systems of planets are forming elsewhere. We cannot look back in time to the formation of our own system, but many stars in space are much younger than the Sun. In these systems, the processes of planet formation might still be accessible to direct observation. We observe that there are many other “ solar nebulas ” or  circumstellar disks —flattened, spinning clouds of gas and dust surrounding young stars. These disks resemble our own solar system’s initial stages of formation billions of years ago.

17.4.3 Building Planets

Circumstellar disks are a common occurrence around very young stars, suggesting that disks and stars form together. Astronomers can use theoretical calculations to see how solid bodies might form from the gas and dust in these disks as they cool. These models show that material begins to coalesce first by forming smaller objects, precursors of the planets, which we call  planetesimals .

Today’s fast computers can simulate the way millions of planetesimals, probably no larger than 100 kilometers in diameter, might gather together under their mutual gravity to form the planets we see today. We are beginning to understand that this process was a violent one, with planetesimals crashing into each other and sometimes even disrupting the growing planets themselves. As a consequence of those violent impacts (and the heat from radioactive elements in them), all the planets were heated until they were liquid and gas, and therefore differentiated, which helps explain their present internal structures.

The process of impacts and collisions in the early solar system was complex and, apparently, often random. The solar nebula model can explain many of the regularities we find in the solar system, but the random collisions of massive collections of planetesimals could be the reason for some exceptions to the “rules” of solar system behavior. For example, why do the planets Uranus and Pluto spin on their sides? Why does Venus spin slowly and in the opposite direction from the other planets? Why does the composition of the Moon resemble Earth in many ways and yet exhibit substantial differences? The answers to such questions probably lie in enormous collisions that took place in the solar system long before life on Earth began.

Today, some 4.5 billion years after its origin, the solar system is—thank goodness—a much less violent place. However, some planetesimals have continued to interact and collide, and their fragments move about the solar system as roving “transients” that can make trouble for the established members of the Sun’s family, such as our own Earth.

If you are using an offline version of this text, access the quiz for section 17.4 via the QR code.

Our solar system currently consists of the Sun, eight planets, five dwarf planets, nearly 200 known moons , and a host of smaller objects. The planets can be divided into two groups: the inner terrestrial planets and the outer giant planets. Smaller members of the solar system include asteroids (including the dwarf planet Ceres), which are rocky and metallic objects found mostly between Mars and Jupiter; comets , which are made mostly of frozen gases and generally orbit far from the Sun; and countless smaller grains of cosmic dust. When a meteor survives its passage through our atmosphere and falls to Earth, we call it a meteorite .

The ages of the surfaces of objects in the solar system can be estimated by counting craters: on a given world, a more heavily cratered region will generally be older than one that is less cratered. We can also use samples of rocks with radioactive elements in them to obtain the time since the layer in which the rock formed last solidified. The half-life of a radioactive element is the time it takes for half the sample to decay; we determine how many half-lives have passed by how much of a sample remains the radioactive element and how much has become the decay product. In this way, we have estimated the age of the Moon and Earth to be roughly 4.5 billion years.

Regularities among the planets have led astronomers to hypothesize that the Sun and the planets formed together in a giant, spinning cloud of gas and dust called the solar nebula . Astronomical observations show tantalizingly similar circumstellar disks around other stars. Within the solar nebula, material first coalesced into planetesimals ; many of these gathered together to make the planets and moons. The remainder can still be seen as comets and asteroids. Probably all planetary systems have formed in similar ways, but many exoplanet systems have evolved along quite different paths.

If you are using an offline version of this text, access the quiz for chapter 17 via the QR code.

Text References

Parts of this chapter are from OpenStax’s Astronomy (chapter 7) . 2016. CC BY 4.0 .

Chapter 17 Origin of Earth and the Solar System ( CC BY 4.0)  by Karla Panchuk was added from Earle, Steven (2019) Physical Geology, 2nd edition. BC Campus https://opentextbc.ca/physicalgeology2ed/chapter/22-1-starting-with-a-big-bang

Figure References

Figure 17.1: The big bang. NASA/WMAP Science Team. 2006. Public domain. https://en.wikipedia.org/wiki/File:CMB_Timeline300_no_WMAP.jpg

Figure 17.2: Cosmic microwave background (CMB) map of the sky, a baby picture of the universe. NASA / WMAP Science Team. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Ilc_9yr_moll4096.png

Figure 17.3: Doppler effect. Charly Whisky. 2007. CC BY-SA 3.0 . https://commons.wikimedia.org/wiki/File%3ADopplerfrequenz.gif

Figure 17.4: Red shift in light from the supercluster BAS11 compared to the sun’s light. Kindred Grey. 2022. CC BY 4.0 . Includes Duck by parkjisun from Noun Project ( Noun Project license ).

Figure 17.5: Astronauts on the Moon. NASA Johnson Space Center; Restored by Bammesk. 1971. Public domain. https://en.wikipedia.org/wiki/File:AS15-88-11866_-_Apollo_15_flag,_rover,_LM,_Irwin_-_restoration1.jpg

Figure 17.6: Orbits of the planets. Arabik4892. 2022. CC BY-SA 4.0 . https://commons.wikimedia.org/wiki/File:Planet_Orbits.jpg

Figure 17.7: Surface of Mercury. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Mercury_Double-Ring_Impact_Basin.png

Figure 17.8: The four giant planets. Solar System Exploration, NASA. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Gas_planet_size_comparisons.jpg

Figure 17.9: This intriguing image from the New Horizons spacecraft, taken when it flew by the dwarf planet Pluto in July 2015, shows some of its complex surface features. NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute. 2015. Public domain. https://en.wikipedia.org/wiki/File:Pluto-01_Stern_03_Pluto_Color_TXT.jpg

Figure 17.10: Saturn and its A, B, and C rings in visible and (inset) infrared light. NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic (ISS), NASA/JPL-Caltech/University of Arizona/CNRS/LPG-Nantes (VIMS). 2019. Public domain. https://commons.wikimedia.org/wiki/File:PIA23170-Saturn-Rings-IR-Map-20190613.jpg

Figure 17.11: Asteroid Eros. NASA/JPL/JHUAPL. 2000. Public domain. https://commons.wikimedia.org/wiki/File:Eros_-_PIA02923_(color).jpg

Figure 17.12: Comet Churyumov-Gerasimenko (67P). ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA. 2014. CC BY-SA 4.0 . https://commons.wikimedia.org/wiki/File:Comet_67P_True_color.jpg

Figure 17.13: Jupiter with its moon Europa on the left. NASA, ESA, STScI, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley) and the OPAL team. 2020. Public domain. https://commons.wikimedia.org/wiki/File:Jupiter_and_Europa_2020.tiff

Figure 17.14: Jupiter’s moon Ganymede. NOAA. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Moon_Ganymede_by_NOAA.jpg

Figure 17.15: Our cratered Moon. NASA/Goddard/Arizona State University. 2011. Public domain. https://www.nasa.gov/mission_pages/LRO/news/lro-farside.html

Figure 17.16: Radioactive decay. Andrew Fraknoi, David Morrison, and Sidney Wolff. 2015. CC BY 4.0 . https://en.wikipedia.org/wiki/File:OSC_Astro_07_03_Decay_(1).jpg

Figure 17.17: NASA artist’s conception of various planet formation processes, including exocomets and other planetesimals, around Beta Pictoris, a very young type A V star. NASA/FUSE/Lynette Cook. 2007. Public domain. https://commons.wikimedia.org/wiki/File:NASA-ExocometsAroundBetaPictoris-ArtistView.jpg

Figure 17.18: Atlas of Planetary Nurseries. NASA/ESA and L. Ricci (ESO). 2009. CC BY 4.0 . https://esahubble.org/copyright/

• Source: OpenStax Astronomy (CC BY). Access for free at https://openstax.org/books/astronomy-2e/pages/7-1-overview-of-our-planetary-system ↵
• Source: OpenStax Astronomy (CC BY). Access for free at https://openstax.org/books/astronomy/pages/7-2-composition-and-structure-of-planets ↵
• Source: OpenStax Astronomy (CC BY). Access for free at https://openstax.org/books/astronomy/pages/7-3-dating-planetary-surfaces ↵
• Source: OpenStax Astronomy (CC BY). Access for free at https://openstax.org/books/astronomy/pages/7-4-origin-of-the-solar-system ↵

All of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy.

The generic term for a group of planets and other bodies circling a star is planetary system. Our planetary system is the only one officially called “solar system,” because our Sun is sometimes called Sol.

A large astronomical body that is neither a star nor a stellar remnant.

The measure of the vibrational (kinetic) energy of a substance.

Originating or existing outside the solar system.

The theory that the Universe started with a expansive explosion. Shortly after, elements were created (mostly hydrogen) and galaxies started to form.

A process inside stars where smaller atoms combine and form larger atoms.

The generic term for a group of planets and other bodies circling a star.

The process of atoms breaking down randomly and spontaneously.

Any planet beyond our solar system.

A group of all atoms with a specific number of protons, having specific, universal, and unique properties.

The distance that light can travel through space in a year. One light year is 9.4607 × 10^12 km.

Radiation left over from the an early stage in the development of the universe at the time when protons and neutrons were recombining to form atoms.

A gravitationally-bound system of stars and interstellar matter.

The increase in wavelength of light resulting from the fact that the source of the light is moving away from the observer.

The distance between any two repeating portions of a wave (e.g., two successive wave crests).

to move in a circular or curving course or orbit. Not to be confused with rotate, when something spins on an axis

An object that orbits a planet or something else that is not a star. Besides planets, moons can circle dwarf planets, large asteroids, and other bodies.

A small rocky body orbiting the sun.

a celestial object consisting of a nucleus of ice and dust and, when near the Sun, a “tail” of gas and dust particles pointing away from the Sun

A small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right.

The layers of gases surrounding a planet or other celestial body.

To move in a circular or curving course or orbit. Not to be confused with rotate, when something spins on an axis.

To spin on an axis. Not to be confused with revolve, when something moves in a circular or curving course or orbit.

A bowl-shaped depression, or hollowed-out area, produced by the impact of a meteorite, volcanic activity, or an explosion.

A landform that rises above its surrounding area.

Place where lava is erupted at the surface.

A circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun.

A spherical layer of icy objects surrounding our Sun; likely occupies space at a distance between about 2,000 and 100,000 astronomical units (AU) from the Sun.

The gases that are part of the Earth, which are mainly nitrogen and oxygen.

A small body of matter from outer space that enters the Earth's atmosphere, becoming incandescent as a result of friction and appearing as a streak of light.

A stoney and/or metallic object from our solar system which was never incorporated into a planet and has fallen onto Earth. Meteorite is used for the rock on Earth, meteoroid for the object in space, and meteor as the object travels in Earth's atmosphere.

The branch of science which deals with celestial objects, space, and the physical universe as a whole.

A straight line passing from side to side through the center of a body or figure, especially a circle or sphere.

Reduction involves a half-reaction in which a chemical species decreases its oxidation number, usually by gaining electrons.

Mineral group in which the silica tetrahedra, SiO4-4, is the building block.

A solid material that is typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivity.

Oxidation is the loss of electrons or an increase in the oxidation state of a chemical or atoms within it.

In planetary science, differentiation is the process of separating out the different components within a planetary body as a consequence of their physical or chemical behavior (e.g. density and chemical affinities).

An AU (or astronomical unit) is the average distance from Earth to the Sun.

The kelvin, symbol K, is the SI base unit of temperature. Absolute zero is 0 K, the equivalent of −273.15°C.

Breaking down rocks into small pieces by chemical or mechanical means.

A stable subatomic particle with a charge of negative electricity, found in all atoms and acting as the primary carrier of electricity in solids.

A penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays.

A radioactive atom that can and will decay.

The atom that is made after a radioactive decay.

Of or pertaining to an exoplanet, a planet outside the solar system.

Rotating, flattened disk of gas and dust from which the solar system originated.

Turn from liquid into vapor.

A body that could or did come together with many others under gravitation to form a planet.

The calculated amount of time that half of the mass of an original (parent) radioactive isotope breaks down into a new (daughter) isotope.

Introduction to Earth Science Copyright © 2023 by Laura Neser is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Planet Classification: How to Group Exoplanets

With thousands of exoplanet candidates discovered, astronomers are starting to figure out how to group them in order to describe them and understand them better. Many planet classification schemes have been proposed over the years, ranging from science fiction to more scientific ones. But we still know little about exoplanets, and some scientists still debate what the definition of a planet should be.

What is a planet?

Before discussing how to classify planets, it's important to understand what a planet is. The International Astronomical Union came out with an official definition in 2006, but that definition has remained controversial. The definition states that a planet is a celestial body that

• is in orbit around the sun, 
• has sufficient mass to have a nearly round shape, 
• has "cleared the neighborhood" around its orbit. 

The definition arose after astronomers, including California Institute of Technology astronomer Mike Brown, found several small worlds at the edge of the solar system. These bodies were approximately the size of Pluto, which was then considered a planet. With a new definition, the small worlds and Pluto were grouped into a new category called "dwarf planet."

The decision did not meet with universal approval. Alan Stern is the principal investigator of the New Horizons mission to Pluto, which flew by the world in 2015. He has repeatedly argued that the phrase " cleared the neighborhood " is vague and does not account for the fact that, for example, Earth has many asteroids in its orbit. Further, the New Horizons pictures of Pluto showed a surprisingly complex world that includes mountains, frozen lakes and other features – which he again argued makes it more like a planet. 

The IAU responded to the New Horizons discoveries as follows: "These results raise fundamental questions about how a small, cold planet can remain active over the age of the Solar System. They demonstrate that dwarf planets can be every bit as scientifically interesting as planets. Equally important is that all three major Kuiper belt bodies visited by spacecraft so far – Pluto, Charon, and Triton – are more different than similar, bearing witness to the potential diversity awaiting the exploration of their realm." 

In 2017, a group of scientists, including Stern, proposed a new definition of planet , which they plan to submit to the IAU: "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters." 

Classifying planets

The urge to classify planets has increased since exoplanet discoveries became more frequent. The first confirmed exoplanet discovery was in 1992, with the discovery of PSR B1257+12 around a pulsar star; the first main-sequence star discovery (51 Pegasi b) was found in 1995. 

Since then, thousands of exoplanet candidates have been found, most of them with the Kepler Space Telescope . While Kepler's mission is focused on finding planets like Earth orbiting in the "habitable zones" (where liquid water may exist on the planet's surface) of their stars, the telescope has discovered a wide variety of planets. 

Many of the exoplanets discovered early on were so-called "hot Jupiters," large gas giants that orbit very close to their parent star. Some planets are very old, such as PSR 1620-26 b (nicknamed Methuselah as it's only 1 billion years younger than the universe itself.) Some planets are so close to their parent star that their atmosphere is evaporating , such as the case of HD 209458b. Further, planets have been found orbiting two, three or even more stars. 

With such a wide range of planets, it is perhaps understandable that there is no single classification system for all planets. For the most part, astronomers focus on the degree to which planets may be habitable, which is perhaps best demonstrated with the Habitable Exoplanets Catalog . This is a list of the most promising habitable planets as determined by experts at the University of Puerto Rico at Arecibo's Planetary Habitability Laboratory (PHL). 

The challenge is, habitability is usually defined solely by a planet's orbit and mass. Telescopes of today are not sensitive enough to look at atmospheres except for the very largest and closest planets. That said, observatories of the future may be able to examine atmospheres directly. The James Webb Space Telescope , which launches in 2018, should be capable of looking at certain planets' atmospheres, although it's unclear how much information it can obtain about smaller, rocky planets close to Earth's mass.

Solar System classification schemes

The word "planet" comes from a Greek word meaning "wanderer", meaning that the planets wander in Earth's sky compared with the (relatively fixed) stars. Planet movements were known by all ancient cultures, but they were limited to those that could be seen with the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. The discoveries of Uranus and Neptune came after the telescope was used in astronomy starting in the 1600s.

In our own solar system, astronomers typically distinguish between "rocky" planets and "gas" planets . The rocky planets are Mercury, Venus, Earth and Mars. They have small atmospheres compared with their size, and are closer to the sun. 

Long-standing theory is that when the sun was young and the solar system was just forming , radiation blew most of the gas to the outer solar system, depriving these planets of the chance to pick up a lot of atmosphere. However, other solar systems have huge, gassy exoplanets close to their parent stars. Perhaps these exoplanets migrated, or perhaps the formation theory needs tweaking.

The gas planets in our solar system are Jupiter, Saturn, Uranus and Neptune, although there are vast differences among them. Uranus and Neptune still have rocky cores (as best as theory can tell), but have very large atmospheres compared with those cores. While the cores of Jupiter and Saturn also remain enigmatic, physics predicts that because of the planets' much larger size relative to Uranus and Neptune, the cores may be liquid-metallic – or perhaps more solid. More study will be needed.

At least one classification scheme distinguishes the planets in our solar system with their position relative to Earth . Under this scheme, "inferior" planets (those inside Earth's orbit) are Mercury and Venus. "Superior" planets (those outside Earth's orbit) are Mars, Jupiter, Saturn, Uranus and Neptune. 

Sometimes, planets in our solar system are classified with their position relative to the asteroid belt , which lies approximately between Mars and Jupiter. With this scenario, "inner" planets are Mercury, Venus, Earth and Mars. "Outer" planets are Jupiter, Saturn, Uranus and Neptune. 

Exoplanet classification schemes

Perhaps the most famous attempt at exoplanet classification is that used by "Star Trek." A habitable planet like the Earth is referred to as an M-class planet; often, members of the crew would call out that they were orbiting an M-class planet, or this would be noted specifically in a captain's log. 

Star Trek fan site Memory Alpha has a class list as follows:

• Class D (planetoid or moon with little to no atmosphere)
• Class H (generally uninhabitable)
• Class J (gas giant)
• Class K (habitable, as long as pressure domes are used)
• Class L (marginally habitable, with vegetation but no animal life)
• Class M (terrestrial)
• Class N (sulfuric)
• Class R (a rogue planet, not as habitable as a terrestrial planet)
• Class T (gas giant)
• Class Y (toxic atmosphere, high temperatures)

The Planetary Habitability Laboratory lists several "obscure" examples of classification, as well as more scientific examples. Of the more scientific examples, the suggestions include using mass as a classification scheme (Stern and Levison, 2002) or the abundance of elements more important for life (Lineweaver and Robles, 2006). Stern and Levison further argue, according to PHL, that "any classification should be physically based, determinable on easily observed characteristics, quantitative, uniquely, robust to new discoveries, and be based of the fewest possible criteria."

PHL also has a proposed classification scheme that uses mass as a basis — a metric that can be obtained with today's telescopic observations. Mass can be estimated based on radial velocity measurements obtained by instruments such as the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph at the European Southern Observatory's La Silla 3.6m telescope. Simply put, this method measures the "tug" a planet exerts as it goes around its parent star, providing an estimate of the mass.

The PHL's proposed classification list is as follows:

Minor planets, moons and comets

• Less than 0.00001 Earth masses = asteroidan
• 0.00001 to 0.1 Earth masses = mercurian

Terrestrial planets (rocky composition)

• 0.1-0.5 Earth masses = subterran
• 0.5-2 Earth masses = terran (Earths)
• 2-10 Earth masses = superterran (super-Earths)

Gas giant planets

• 10-50 Earth masses = Neptunian (Neptunes)
• 50-5000 Earth masses = Jovian (Jupiters)

Additional resources

• IAU: Pluto and the Solar System
• PHL: Exoplanet Mass Classification (EMC)

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Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, " Why Am I Taller ?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace

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Essay On Solar System – 10 Lines, Short and Long Essay for Children and Students

Key Points to Remember When Writing an Essay on the Solar System

10 lines on solar system, a paragraph on solar system, short essay on solar system, long essay of the solar system in english, what will your child learn from the essay on the solar system.

Writing essays can be an incredible journey of exploration, especially when diving into fascinating topics like the solar system. A solar system essay, like the one we’re about to embark on, provides an opportunity to understand the vast universe we are a part of. By attempting this essay in English, students can improve their language skills, enhance their creativity, and develop a deeper appreciation for the wonders beyond our planet. Now, let’s travel through space and time to understand the marvellous entity we call the solar system.

When you embark on the enlightening journey of writing an essay on the solar system, it’s essential to remember some fundamental aspects to make your essay stand out. These points ensure that your content is rich and informative and captivates your readers.

• Research Thoroughly:  Before starting, gather information from credible sources. The solar system is vast, and discoveries are made regularly.
• Keep It Organised:  Structure your essay with a proper introduction, body, and conclusion. This will help readers follow your thoughts.
• Use Simple Language:  If it’s meant for children and students, keep your language simple and avoid jargon.
• Include Visuals:  Include images or diagrams of planets, orbits, or other celestial bodies to make your essay more engaging and to help explain complex concepts.
• Discuss Recent Discoveries:  Astronomy is a constantly evolving field. To keep your essay current, mention any new findings or missions.
• Maintain Accuracy:  When mentioning facts or figures, ensure they are accurate. Mistakes in such essays can misinform readers.
• Personal Touch:  Share anecdotes or experiences related to stargazing or space exploration. This adds a warm, personal touch to the essay.
• Include Interesting Facts:  Sprinkle your essay with fascinating tidbits about the  solar system , like the storms on Jupiter or the possibility of water on Mars .
• Stay Updated:  The realm of space exploration and astronomy is constantly advancing. Ensure you are updated with the latest information.
• Proofread:  After finishing your essay, review it for any grammatical or factual errors. A well-polished essay makes a better impression.

For primary class students just beginning their exploration into the vast wonders of space, breaking down the vastness of the solar system into digestible bites is essential. The solar system can be awe-inspiring with its planets, moons, and other celestial wonders. Here’s a simple solar system 10-line essay perfect for budding astronomers and an essay for primary-class students.

1. The solar system comprises the sun and all the celestial objects around it.

2. There are eight planets:  Mercury , Venus, Earth, Mars, Jupiter,  Saturn , Uranus, and Neptune.

3. The sun is a giant star that gives us light and warmth.

4.  Earth , our home, is the third planet from the sun and the only one known to have life.

5. The  moon  is Earth’s natural satellite and orbits around us.

6.  Jupiter , the largest planet, has a giant red storm raging for centuries.

7. Between Mars and Jupiter, there’s an asteroid belt filled with rocky objects  (4) .

8. The solar system also includes comets with tails that glow when close to the sun.

9. Neptune, the farthest planet, has strong winds and dark storms  (5) .

10. Exploring our solar system helps us learn more about our place in the universe.

Young students can use these lines as a foundation and further expand their knowledge by exploring each point in depth as they grow.

Writing an essay for classes 1 & 2 can be a great way for young students to understand the solar system. To help them grasp the essentials, here’s a short essay in 100 words tailored to their comprehension level.

The solar system is like a big family in space. At the centre is the  sun , shining bright and giving us light. Around the sun, eight planets move in circles called orbits.  Earth  is one of them, and it’s where we live. Some planets have rings, like Saturn, and some have many moons. There are also tiny rocks called asteroids and icy bodies known as comets. Every member of this space family has its own unique story. By reading and learning about the solar system, kids can begin to understand the vast world beyond our blue sky.

The allure of the night sky, dotted with twinkling stars and distant planets, has always been a source of wonder for humans. Exploring the solar system’s mysteries offers profound insights into the cosmos and our place within it. The following essay, in 200 words, captures the essence of this mesmerising expanse.

Our solar system is a cosmic marvel, a vast expanse dominated by the sun’s brilliant glow. The centre of the solar system is occupied by the sun, a colossal sphere of fiery gas that makes up over 99% of the solar system’s total mass  (3) . Orbits around this central star are eight diverse planets with unique features and mysteries. The rocky planets Mercury, Venus , Earth, and  Mars  are nearest to the sun. These are followed by the gas giants, Jupiter and Saturn, and the ice giants,  Uranus  and Neptune.

In contrast, each planet provides a distinct study, from Mercury and Venus’s scorching surfaces to Neptune’s frozen realms. Beyond the planets, the solar system also shelters asteroids, comets, and dwarf planets like Pluto. As we send probes and satellites farther into space, our understanding of this vast system deepens, revealing secrets that challenge our understanding of existence. The solar system, with its intricate dance of celestial bodies, is a testament to the grandeur of the universe, beckoning us to explore and discover.

For every student and reader, understanding our solar system is the first step towards unravelling the deeper mysteries of the cosmos.

The cosmos has always fascinated mankind. Its vastness and mysteries have piqued our curiosity for centuries. To comprehend the universe’s grandeur, we must begin with our neighbourhood in space: the solar system. This solar system essay for class 3 and above offers more profound insights into our cosmic home.

What Is the Solar System?

The solar system comprises various celestial bodies held together by the sun’s gravitational pull, which sits at its centre. This dynamic system is located in the  Milky Way galaxy  and spans a distance of billions of miles. The major constituents of the solar system are the sun, eight planets, their moons, and a range of smaller objects like asteroids, comets, and dwarf planets. It is an intricate dance of objects revolving around the sun, each following its unique path and exhibiting individual characteristics.

How Does the Solar System Work?

The sun is the heart of the solar system, a colossal ball of gas undergoing nuclear fusion. It emits immense heat and light, making life possible on Earth. The sun’s gravitational force is so strong that it keeps all the planets and celestial bodies in their orbits.

The planets orbit the sun in elliptical paths. Like Mercury and Venus, those closest to the sun complete their orbits quicker than those farther away, such as  Neptune . The force of gravity also ensures that moons orbit planets. For example, our Earth has one moon, while Jupiter boasts 79 known moons!

The balance of gravity and the momentum of celestial objects keep everything in place. Without the sun’s gravitational pull, planets would drift away into the vastness of space.

Celestial Bodies Exist in the Solar System

Our solar system’s central star provides energy and light that drive life on Earth.

There are eight in total. The inner planets (Mercury, Venus, Earth, and Mars) are rocky, while the outer planets (Jupiter, Saturn, Uranus, and Neptune) are gas giants or ice giants.

Natural satellites that orbit planets. Their number varies from planet to planet.

4. Asteroids

Rocky fragments remain from the formation of the solar system. Most are found in the asteroid belt between Mars and Jupiter.

Comets are icy bodies that come from the solar system’s outer regions. When they approach the sun, they develop glowing tails.

6. Dwarf Planets

These celestial bodies orbit the sun and have enough mass for their self-gravity to overcome rigid body forces  (2) . However, they still need to clear their neighbouring region of other objects. Pluto  is the most famous dwarf planet.

7. Kuiper Belt & Oort Cloud

These are regions beyond Neptune filled with millions of icy objects. The Kuiper Belt is closer than the Oort Cloud and is the birthplace of short-term comets  (1) .

Our solar system is breathtakingly vast and dynamic, filled with various celestial bodies. Its complex mechanisms and operations provide invaluable insights into the universe’s workings. Understanding the solar system is not just a part of the curriculum for students in class 3 and above; it is a journey into the wondrous realm of space. This essay aims to be a guide, igniting young minds’ curiosity and exploration.

Through the essay on the solar system, your child will gain a foundational understanding of our cosmic neighbourhood, grasping the vastness and intricacies of space. Beyond mere facts, the essay fosters curiosity, inspiring them to dig deeper into the mysteries of the universe and comprehend the grandeur and significance of the celestial dance above us.

1. Where is the solar system situated?

The solar system is in the Milky Way galaxy, in one of its spiral arms called the Orion Arm.

2. How many total solar systems exist?

Numerous solar systems exist, with billions believed to reside in our Milky Way galaxy alone. This showcases the vast expanse and diversity of solar systems.

The solar system’s myriad celestial bodies and dynamic interplays provide a window into the cosmos’s infinite wonders. Understanding and appreciating its grandeur satiates our innate curiosity and helps us find our humble place within the vast tapestry of the universe.

References/Resources:

1. Relationship of the Kuiper Belt to the Oort Cloud; The European Space Agency; https://esahubble.org/images/opo0204i/

2. What is a Dwarf Planet?; Jet Propulsion Laboratory; https://www.jpl.nasa.gov/infographics/what-is-a-dwarf-planet ; April 2015

3. Our Sun: Facts; NASA; https://science.nasa.gov/sun/facts/

4. Asteroids: Facts; NASA; https://science.nasa.gov/solar-system/asteroids/facts/

5. Neptune Facts; NASA; https://science.nasa.gov/neptune/facts/

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Neptune is about four times wider than Earth.

Number Eight

Neptune is 30 AU from the Sun. Earth = 1 AU.

A Neptunian Year

Neptune takes 165 Earth years to go around the Sun.

The most dense of the giant planets.

Named for sea gods and nymphs in Greek mythology.

Rings and Arcs

Neptune has five rings and four more ring arcs,

Solo Voyager

Voyager 2 is the only spacecraft to visit Neptune.

Bring a Spacesuit

Atmosphere: molecular hydrogen and atomic helium with a bit of methane.

No Life Signs

Neptune cannot support life as we know it.

Orbit Crossing

Pluto sometimes comes closer to the Sun than Neptune.

Planet Neptune Overview

Dark, cold and whipped by supersonic winds, giant Neptune is the eighth and most distant major planet orbiting our Sun. More than 30 times as far from the Sun as Earth, Neptune is not visible to the naked eye. In 2011, Neptune completed its first 165-year orbit since its discovery.

The planet’s rich blue color comes from methane in its atmosphere, which absorbs red wavelengths of light, but allows blue ones to be reflected back into space.

Neptune was the first planet located through mathematical calculations. Using predictions sent to him by French astronomer Urbain Le Verrier, based on disturbances in the orbit of Uranus, German astronomer Johann Galle was the first to observe the planet in 1846. The planet is named after the Roman god of the sea, as suggested by Le Verrier.

Pop Culture

Even though Neptune is the farthest planet from our Sun, it's a frequent stop in pop culture and fiction. The planet served as the backdrop for the 1997 science fiction horror film "Event Horizon," while in the cartoon series "Futurama," the character Robot Santa Claus has his home base on Neptune's north pole. "Dr. Who" fans will remember that an episode entitled "Sleep No More" is set on a space station orbiting Neptune. And in the "Star Trek: Enterprise" pilot episode, "Broken Bow," viewers learn that at warp 4.5 speed, it is possible to fly to Neptune and back to Earth in six minutes.

Latest News

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For Kids: All About Neptune

Facts about Neptune for Kids.

NASA Photojournal: Neptune

Images of Neptune from NASA spacecraft.

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