A montage of the planets (plus Pluto, a dwarf planet) in our solar system presented in correct relative sizes. The orbits in the background are also drawn to scale.

In this chapter, you will discover…In this chapter, you will discover… how the solar system formedhow the solar system formed why the environment of the early solar system was much why the environment of the early solar system was much

more violent than it is todaymore violent than it is today how astronomers define the various types of objects in the how astronomers define the various types of objects in the

solar systemsolar system the relationships between planets, dwarf planets, small solar the relationships between planets, dwarf planets, small solar

system bodies, and other classifications of objects in the system bodies, and other classifications of objects in the solar systemsolar system

how the planets are grouped as they arehow the planets are grouped as they are how the moons formed throughout the solar systemhow the moons formed throughout the solar system the composition of the debris scattered throughout the solar the composition of the debris scattered throughout the solar

systemsystem that disks of gas and dust, as well as planets, have been that disks of gas and dust, as well as planets, have been

observed around a growing number of starsobserved around a growing number of stars that newly forming stars and planetary systems are being that newly forming stars and planetary systems are being

discovered every yeardiscovered every year

How Stars Lose Mass

(a) The brightest star in Scorpius, Antares, is nearing the end of its existence. Strongwinds from its surface are expelling large quantities of gas and dust, creating this nebula reminiscent of an Impressionist painting. The scattering of starlight off this material makes it appear especially bright, even at a distance of 604 light-years. (b) The planetary nebula Abell 39 is 7000 light-years from Earth. With a relatively gentle emission of matter, the central star shed its outer layers of gas and dust in an expanding spherical shell now about 6 light-years across. (c) A supernova is the most powerful known mechanism for a star to shed mass. The Crab Nebula, even though it is about 6000 light-years from Earth, was visible during the day for three weeks During 1054.

Stars Transform MatterHydrogen, helium, and traces of lithium, Hydrogen, helium, and traces of lithium,

the three lightest elements, were formed the three lightest elements, were formed shortly after the formation of the universe. shortly after the formation of the universe.

The heavier elements were produced much The heavier elements were produced much later by stars and are cast into space when later by stars and are cast into space when stars die. stars die.

By mass, 98% of the observed matter in By mass, 98% of the observed matter in the universe is hydrogen and helium.the universe is hydrogen and helium.

Dusty Regions of Star Formation

(a) The three bright young stars shown in the inset of this image of the Cone Nebula in the constellation Monoceros are still surrounded by much of the gas and dust from which they formed. Astronomers hypothesize that the solar system formed from a similarly small fragment of a giant interstellar gas and dust cloud. (b) Newly formed stars in the Orion Nebula. Although visible light from many of the stars is blocked by the nebula, their infrared emission travels through the gas and dust to us.

The Formation of the

Solar System

Formation of the Solar System The solar system formed about 4.6 billion years ago from The solar system formed about 4.6 billion years ago from

a swirling, disk-shaped cloud of gas, ice, and dust called a swirling, disk-shaped cloud of gas, ice, and dust called the solar nebula.the solar nebula.

As the solar nebula began to collapse rotation rate, As the solar nebula began to collapse rotation rate, density, and temperature increased as size decreased. density, and temperature increased as size decreased.

The planets and other debris found in the solar system The planets and other debris found in the solar system today formed from gas, ice, and dust in the solar nebula today formed from gas, ice, and dust in the solar nebula orbiting the protosun, forming a disk.orbiting the protosun, forming a disk.

The outer solar system, beyond the snow line (ice line), The outer solar system, beyond the snow line (ice line), had both dust and ice (including hydrogen and helium), had both dust and ice (including hydrogen and helium), while inside the snow line, such ices were vaporized by while inside the snow line, such ices were vaporized by the protosun.the protosun.

Collisions between dust particles created large debris, Collisions between dust particles created large debris, including moon-sized bodies.including moon-sized bodies.

Young Circumstellar Disks of Matter

This is the heart of the Orion Nebula as seen through the Hubble Space Telescope. The four insets are false-color images of protoplanetary disks within the nebula. A recently formed star is at the center of each disk. The disk in the upper right is seen nearly edge on. Our solar system is drawn to scale in the lower left image.

According to the Nice model, Jupiter formed first, followed According to the Nice model, Jupiter formed first, followed by Saturn, and then by Neptune and Uranus, which were by Saturn, and then by Neptune and Uranus, which were flung out to their present orbits by gravitational forces from flung out to their present orbits by gravitational forces from Jupiter and Saturn.Jupiter and Saturn.

Jupiter, cleared the gas and debris from its orbit. The rest of Jupiter, cleared the gas and debris from its orbit. The rest of the gas giants also cleared gas and debris from their orbits.the gas giants also cleared gas and debris from their orbits.

Jupiter and Saturn were initially worlds of rock and metal Jupiter and Saturn were initially worlds of rock and metal that pulled onto themselves large amounts of hydrogen and that pulled onto themselves large amounts of hydrogen and helium, along with some water.helium, along with some water.

Uranus and Neptune were also initially worlds of rock and Uranus and Neptune were also initially worlds of rock and metal, but they attracted more water and less hydrogen and metal, but they attracted more water and less hydrogen and helium than the other giant planets.helium than the other giant planets.

Saturn, Neptune, and Uranus probably formed closer to the Saturn, Neptune, and Uranus probably formed closer to the protosun than they are to the Sun today.protosun than they are to the Sun today.

Formation of the Solar System

Formation of the Solar System Debris from outside the snowline spiraled inward Debris from outside the snowline spiraled inward

forming the Terrestrial planets and orbiting Moon-forming the Terrestrial planets and orbiting Moon-sized bodies as they collided.sized bodies as they collided.

The Sun formed at the center of the solar nebula. The Sun formed at the center of the solar nebula. After about 100 million years, the temperature at After about 100 million years, the temperature at the protosun’s center was high enough to ignite the protosun’s center was high enough to ignite thermonuclear fusion reactions.thermonuclear fusion reactions.

Saturn, Neptune, and Uranus spiraled outward, Saturn, Neptune, and Uranus spiraled outward, with Neptune and Uranus changing places. Much with Neptune and Uranus changing places. Much debris was sent inward and far outward, impacting debris was sent inward and far outward, impacting the newly formed planets and creating the Kuiper the newly formed planets and creating the Kuiper belt and Oort cloud.belt and Oort cloud.

Accretion of the Inner Planets

This computer simulation shows the formation of the inner planets as a result of myriad collisions.

The Nice Model and the Outer Solar System

The classical Kuiper belt of comets spreads from Neptune out 50 AU from the Sun. Most of the estimated 200 million Kuiper belt comets are believed to orbit in or near the plane of the ecliptic. The spherical Oort cloud, containing billions of comets, extends out beyond the Kuiper belt.

The Kuiper Belt and Oort Cloud

(b) Positions of known bodies in the Kuiper belt and Oort cloud.

This picture of the asteroid Gaspra was taken in 1991 by theGalileo spacecraft on its way to Jupiter. The asteroid measures 12 × 20 × 11 km. Millions of similar chunks of rock orbit the Sun between the orbits of Mars and Jupiter. Even smaller rocky bodies called meteoroids are scattered throughout the solar system.

An Asteroid

Thousands of lunar craters were produced by impacts of leftover rocky debris from the formation of the solar system. Age-dating of lunar rocks brought back by the astronauts indicates that the Moon is about 4.5 billion years old. Most of the lunar craters were formed during the Moon’s first 700 million years of existence, when the rate of bombardment was much greater than it is now. The large dark regions are the maria.

Our Moon

Here are classifications of some solar system objects. A planet is an object that1) orbits the sun; 2) has enough mass so that its own gravitational attraction causes it to be essentially spherical; 3) has enough gravitational attraction to clear its neighborhood of other orbiting debris.A dwarf planets fulfills conditions (1) and (2), but not (3).A small solar system object only fulfills (1). Note that some of the objects overlap on this diagram

Different Classifications of Solar System Objects

Astronomical objects smaller than the eight Astronomical objects smaller than the eight planets are classified as dwarf planets or small planets are classified as dwarf planets or small solar system bodies (SSSBs).solar system bodies (SSSBs).

A variety of other names, including asteroids, A variety of other names, including asteroids, comets, meteoroids, trans-Neptunian objects, comets, meteoroids, trans-Neptunian objects, plutinos, plutoids, Kuiper belt objects (KBOs), plutinos, plutoids, Kuiper belt objects (KBOs), and Oort cloud objects, overlap with the and Oort cloud objects, overlap with the designations designations “dwarf planet” and “SSSB.”“dwarf planet” and “SSSB.”

KBOs and Oort cloud objects are trans-KBOs and Oort cloud objects are trans-Neptunian objectsNeptunian objects——they orbit farther from the they orbit farther from the Sun than the outermost planet.Sun than the outermost planet.

The largest asteroids are also classified as dwarf The largest asteroids are also classified as dwarf planets or comets. planets or comets.

Different Classifications of Solar System Objects

Different Classifications of Solar System Objects

This scale drawing shows the distribution of planetary orbits around the Sun. All orbits are counterclockwisebecause the view is from above Earth’s North Pole. The four terrestrial planets are located close to the Sun; the four giant planets orbit at much greater distances. Seen from above the disk of the solar system, most of the orbits appear nearly circular. Mercury has the most elliptical orbit of any planet.

The Sun and the planets are drawn to size scale in order of their distance from the Sun (distances not to scale). The four planets that orbit nearest the Sun (Mercury, Venus, Earth, and Mars) are small and made of rock and metal. The next two planets (Jupiter and Saturn) are large and composed primarily of hydrogen and helium. Uranus and Neptune are intermediate in size and contain roughly equal amounts of ices, hydrogen and helium, and terrestrial material.

The Sun and the Planets

Comparative Planetology The four inner planets of the solar system share many

characteristics and are distinctly different from the four giant outer planets.

The four inner, terrestrial planets are relatively small, have high average densities, and are composed primarily of rock and metal.

Jupiter and Saturn have large diameters and low densities and are composed primarily of hydrogen and helium. Uranus and Neptune have large quantities of water as well as much hydrogen and helium.

All four giants have terrestrial cores, rings, and numerous satellites.

Asteroids are rocky and metallic debris in the solar system, are larger than about 10 m in diameter, and are found primarily between the orbits of Mars and Jupiter. Meteoroids are smaller pieces of such debris.

All of the objects in this image have the same mass (total number of particles). However, the chemicals from which they form have different densities (number of particles per volume), so they each take up different amounts of space (volume).

The Volumes of Objects with Different Densities

A Circumstellar Disk of Matter

(a) This is a Hubble view of Beta Pictoris, an edge-on disk of material 225 billion km (140 billion mi) across that orbits the star Beta Pictoris (blocked out in this image) 50 light-years from Earth. Twenty million years old, this disk is believed to be composed primarily of iceberglike bodies that orbit the star. The smaller disk is believed to have been formed by the gravitational pull of a roughly Jupiter-mass planet in that orbit. Because the secondary disk is so dim, the labeling for this image is added in (b).

Off-Center Disk

Visible Image of an Exoplanet

The star Fomalhaut, blocked out so that its light does not obscure the disk, is surrounded by gas and dust in a ring whose center is separated from the star by 15 AU, nearly as far as Uranus is from the Sun. This offset is due to the gravitational effects of giant planet Fomalhaut b orbiting the star. This system is 25 light-years from Earth. The dimmer debris in that system and between it and Earth scatters light that is called “noise” in this image.

This infrared image of an almost-extrasolar object was taken at the European Southern Observatory. It shows the two bodies 2M1207 and 2M1207b. Neither is quite large nor massive enough to be a star, and evidence suggests that 2M1207b did not form from a disk of gas and dust surrounding the larger body; hence, it is not a planet. This system is about 170 light-years from the solar system in the constellation Hydra.

Image of an Almost-Extrasolar Planet

Direct Image of an Extrasolar Planet

A planet with 8 times the mass of Jupiter orbiting the Sunlike-star 1RXS 1609.

Three Traditional Methods of Detecting Exoplanets

(a) A planet and its star both orbit around their common center of mass, always staying on opposite sides of that point. The star’s motion around the center of mass provides astronomers with the information that a planet is present. (b) As a planet moves toward or away from us, its star moves in the opposite direction. Using spectroscopy, we can measure the Doppler shift of the star’s spectrum, which reveals the effects of the unseen planet or planets. (c) If a star and its planet are moving across the sky, the motion of the planet causes the star to orbit its center of mass. This motion appears as a wobbling of the star across the celestial sphere. (d) If a planet happens to move in a plane that takes it across its star (that is, the planet transits the star), as seen from Earth, then the planet will hide some of the starlight, causing the star to dim. This change in brightness will occur periodically and can reveal the presence of a planet.

This figure shows the separations between some exoplanets and their stars. The corresponding star names are given on the left of each line. Note that many systems have giant planets that orbit much closer than 1 AU from their stars. (MJ is shorthand for the mass of Jupiter.) For comparison, the solar system is shown at top.

Planets and Their Stars

Microlensing Reveals an Extrasolar Planet

(a) Gravitational fields cause light to change direction. As a star with a planet passes between Earth and a more distant star (b), the light from the distant star is focused toward us, making the distant star appear brighter. The focusing of the distant star’s light occurs twice, once by the closer star and once by its planet (c), making the distant star change brightness. For these simulations, the closer star and planet are 17,000 light-years away, while the distant star is 24,000 light-years away.

A Star with Three Planets

(a) The star Upsilon Andromedae has at least three planets, discovered by measuring the complex Doppler shift of the star. This star system is located 44 light-years from Earth, and the planets all have masses similar to Jupiter’s. (b) The orbital paths of the planets, labeled B, C, and D, along with the orbits of Venus, Earth, and Mars, are drawn for comparison.

A Star with Four Planets

Direct image of four planets orbiting the star HR 8799.

Summary of Key IdeasSummary of Key Ideas

Formation of the Solar SystemFormation of the Solar System Hydrogen, helium, and traces of lithium, the three lightest Hydrogen, helium, and traces of lithium, the three lightest

elements, were formed shortly after the formation of the elements, were formed shortly after the formation of the universe. The heavier elements were produced much later universe. The heavier elements were produced much later by stars and are cast into space when stars die. By mass, by stars and are cast into space when stars die. By mass, 98% of the observed matter in the universe is hydrogen and 98% of the observed matter in the universe is hydrogen and helium.helium.

The solar system formed 4.6 billion years ago from a The solar system formed 4.6 billion years ago from a swirling, disk-shaped cloud of gas, ice, and dust called the swirling, disk-shaped cloud of gas, ice, and dust called the solar nebula.solar nebula.

The planets and other debris in the solar system today The planets and other debris in the solar system today formed from gas, ice, and dust in the solar nebula orbiting formed from gas, ice, and dust in the solar nebula orbiting the protosun.the protosun.

The outer solar system, beyond the snow line, had both dust The outer solar system, beyond the snow line, had both dust and ice (including hydrogen and helium), while inside the and ice (including hydrogen and helium), while inside the snow line, such ices were vaporized by the protosun.snow line, such ices were vaporized by the protosun.

Formation of the Solar SystemFormation of the Solar System Jupiter and Saturn were initially worlds of rock and metal Jupiter and Saturn were initially worlds of rock and metal

that pulled onto themselves large amounts of hydrogen that pulled onto themselves large amounts of hydrogen and helium, along with some water.and helium, along with some water.

Uranus and Neptune were also initially worlds of rock and Uranus and Neptune were also initially worlds of rock and metal, but they attracted more water and less hydrogen metal, but they attracted more water and less hydrogen and helium than the other giant planets.and helium than the other giant planets.

The Nice model of solar system formation proposes that The Nice model of solar system formation proposes that in the outer solar system, Jupiter formed first, followed by in the outer solar system, Jupiter formed first, followed by Saturn, and then by Neptune and Uranus, which were Saturn, and then by Neptune and Uranus, which were flung out to their present orbits by gravitational forces flung out to their present orbits by gravitational forces from Jupiter and Saturn.from Jupiter and Saturn.

The four inner planets formed through the collisions of The four inner planets formed through the collisions of Moon-sized bodies, probably after the outer four planets Moon-sized bodies, probably after the outer four planets were formed.were formed.

Formation of the Solar SystemFormation of the Solar System The Sun formed at the center of the solar nebula. After The Sun formed at the center of the solar nebula. After

about 100 million years, the temperature at the about 100 million years, the temperature at the protosun’s center was high enough to ignite protosun’s center was high enough to ignite thermonuclear fusion reactions.thermonuclear fusion reactions.

For 800 million years after the Sun formed, impacts of For 800 million years after the Sun formed, impacts of asteroidike objects on the young planets dominated the asteroidike objects on the young planets dominated the history of the solar system.history of the solar system.

Categories of Solar System ObjectsCategories of Solar System Objects Astronomical objects smaller than the eight planets are Astronomical objects smaller than the eight planets are

classified as dwarf planets or small solar system bodies classified as dwarf planets or small solar system bodies (SSSBs).(SSSBs).

A variety of other names, including asteroids, comets, A variety of other names, including asteroids, comets, meteoroids, trans-Neptunian objects, plutinos, plutoids, meteoroids, trans-Neptunian objects, plutinos, plutoids, Kuiper belt objects (KBOs), and Oort cloud objects, overlap Kuiper belt objects (KBOs), and Oort cloud objects, overlap with the designations with the designations “dwarf planet” and “SSSB.”“dwarf planet” and “SSSB.”

KBOs and Oort cloud objects are trans-Neptunian objectsKBOs and Oort cloud objects are trans-Neptunian objects——they orbit farther from the Sun than the outermost planet.they orbit farther from the Sun than the outermost planet.

To date, five objectsTo date, five objects——Pluto, Ceres, Eris, Haumea, and Pluto, Ceres, Eris, Haumea, and MakemakeMakemake——have been classified as dwarf planets.have been classified as dwarf planets.

Other objects orbit the Sun beyond Neptune. At least 1500 Other objects orbit the Sun beyond Neptune. At least 1500 KBOs have been observed. A few potential Oort cloud KBOs have been observed. A few potential Oort cloud objects have also been identified.objects have also been identified.

Comparative PlanetologyComparative Planetology The four inner planets of the solar system share many The four inner planets of the solar system share many

characteristics and are distinctly different from the four characteristics and are distinctly different from the four giant outer planets.giant outer planets.

The four inner, terrestrial planets are relatively small, The four inner, terrestrial planets are relatively small, have high average densities, and are composed have high average densities, and are composed primarily of rock and metal.primarily of rock and metal.

Jupiter and Saturn have large diameters and low Jupiter and Saturn have large diameters and low densities and are composed primarily of hydrogen and densities and are composed primarily of hydrogen and helium. Uranus and Neptune have large quantities of helium. Uranus and Neptune have large quantities of water as well as much hydrogen and helium.water as well as much hydrogen and helium.

All four giants have terrestrial cores. All four giants have terrestrial cores.

Comparative PlanetologyComparative Planetology

Pluto, once considered the smallest planet, has a size, Pluto, once considered the smallest planet, has a size, density, and composition consistent with other large density, and composition consistent with other large Kuiper belt objects (KBOs).Kuiper belt objects (KBOs).

Asteroids are rocky and metallic debris in the solar Asteroids are rocky and metallic debris in the solar system, are larger than about 10 m in diameter, and are system, are larger than about 10 m in diameter, and are found primarily between the orbits of Mars and Jupiter. found primarily between the orbits of Mars and Jupiter. Meteoroids are smaller pieces of such debris. Comets Meteoroids are smaller pieces of such debris. Comets are debris that contain both ice and rock.are debris that contain both ice and rock.

Planets Outside Our Solar SystemPlanets Outside Our Solar System

Astronomers have observed disks of gas and dust Astronomers have observed disks of gas and dust orbiting young stars.orbiting young stars.

At least 1000 exoplanets have been discovered orbiting At least 1000 exoplanets have been discovered orbiting other stars.other stars.

Exoplanets ranging in mass from less the mass of the Exoplanets ranging in mass from less the mass of the Earth to many times the mass of Jupiter have been Earth to many times the mass of Jupiter have been detected.detected.

Most of the exoplanets that have been discovered have Most of the exoplanets that have been discovered have masses roughly equal to the mass of Jupiter.masses roughly equal to the mass of Jupiter.

Some exoplanets are observed directly, while most are Some exoplanets are observed directly, while most are detected indirectly as a result of their effects on the stars detected indirectly as a result of their effects on the stars they orbit.they orbit.

Planets Outside Our Solar SystemPlanets Outside Our Solar System

Exoplanets orbiting virtually all types of Exoplanets orbiting virtually all types of stars have been observed.stars have been observed.

Some planets that are not orbiting stars Some planets that are not orbiting stars have been observed.have been observed.

Key TermsKey Terms

accretionalbedoasteroidasteroid beltaverage densitycometcraterdense coredwarf planetJeans instabilityKuiper beltKuiper belt object (KBO) metalsmeteoroid

microlensingmoon (natural satellite)Nice modelOort cloudorbital inclinationplanetplanetesimalprotoplanetary disks (proplyds)protosunsmall solar system body (SSSB)snow line solar nebulasolar systemterrestrial planettrans-Neptunian objects (TNOs)