ch.5 Astro compton

2. 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.

3. 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 muchwhy 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 thehow astronomers define the various types of objects in the
solar systemsolar system
 the relationships between planets, dwarf planets, small solarthe relationships between planets, dwarf planets, small solar
system bodies, and other classifications of objects in thesystem 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 solarthe composition of the debris scattered throughout the solar
systemsystem
 that disks of gas and dust, as well as planets, have beenthat 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 beingthat newly forming stars and planetary systems are being
discovered every yeardiscovered every year

4. How Stars Lose Mass
(a) The brightest star in Scorpius, Antares, is nearing the end of its existence. Strong
winds 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

5. Stars Transform Matter
 Hydrogen, helium, and traces of lithium,Hydrogen, helium, and traces of lithium,
the three lightest elements, were formedthe three lightest elements, were formed
shortly after the formation of the universe.shortly after the formation of the universe.
 The heavier elements were produced muchThe heavier elements were produced much
later by stars and are cast into space whenlater by stars and are cast into space when
stars die.stars die.
 By mass, 98% of the observed matter inBy mass, 98% of the observed matter in
the universe is hydrogen and helium.the universe is hydrogen and helium.

6. 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.

7. The Formation
of the
Solar System

8. Formation of the Solar System
 The solar system formed about 4.6 billion years ago fromThe solar system formed about 4.6 billion years ago from
a swirling, disk-shaped cloud of gas, ice, and dust calleda 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 systemThe planets and other debris found in the solar system
today formed from gas, ice, and dust in the solar nebulatoday 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 bywhile 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.

9. 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.

10.  According to the Nice model, Jupiter formed first, followedAccording to the Nice model, Jupiter formed first, followed
by Saturn, and then by Neptune and Uranus, which wereby Saturn, and then by Neptune and Uranus, which were
flung out to their present orbits by gravitational forces fromflung 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 ofJupiter, 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 metalJupiter and Saturn were initially worlds of rock and metal
that pulled onto themselves large amounts of hydrogen andthat 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 andUranus and Neptune were also initially worlds of rock and
metal, but they attracted more water and less hydrogen andmetal, 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 theSaturn, 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

11. Formation of the Solar System
 Debris from outside the snowline spiraled inwardDebris 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 atAfter about 100 million years, the temperature at
the protosun’s center was high enough to ignitethe 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. Muchwith Neptune and Uranus changing places. Much
debris was sent inward and far outward, impactingdebris was sent inward and far outward, impacting
the newly formed planets and creating the Kuiperthe newly formed planets and creating the Kuiper
belt and Oort cloud.belt and Oort cloud.

12. Accretion of the Inner Planets
This computer simulation shows the formation of
the inner planets as a result of myriad collisions.

13. 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.

14. The Kuiper Belt and Oort Cloud
(b) Positions of known bodies in the Kuiper belt and Oort cloud.

15. This picture of the asteroid Gaspra was taken in 1991 by the
Galileo 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

16. 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

17. Here are classifications of some
solar system objects.
A planet is an object that
1) 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

18.  Astronomical objects smaller than the eightAstronomical objects smaller than the eight
planets are classified as dwarf planets or smallplanets 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 theand Oort cloud objects, overlap with the
designationsdesignations “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 thethey orbit farther from the
Sun than the outermost planet.Sun than the outermost planet.
 The largest asteroids are also classified as dwarfThe largest asteroids are also classified as dwarf
planets or comets.planets or comets.
Different Classifications of Solar System Objects

19. Different Classifications of Solar System Objects

20. This scale drawing
shows the distribution of
planetary orbits around
the Sun. All orbits are
counterclockwise
because 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.

21. 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

22. 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.

23. 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

24. 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).

25. 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.

26. 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

27. Direct Image of an Extrasolar Planet
A planet with 8 times the mass of Jupiter
orbiting the Sunlike-star 1RXS 1609.

28. 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.

29. 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

30. 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.

31. 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.

32. A Star with Four Planets
Direct image of four planets orbiting the star HR
8799.

33. Summary of Key IdeasSummary of Key Ideas

34. Formation of the Solar SystemFormation of the Solar System
 Hydrogen, helium, and traces of lithium, the three lightestHydrogen, helium, and traces of lithium, the three lightest
elements, were formed shortly after the formation of theelements, were formed shortly after the formation of the
universe. The heavier elements were produced much lateruniverse. 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 and98% of the observed matter in the universe is hydrogen and
helium.helium.
 The solar system formed 4.6 billion years ago from aThe solar system formed 4.6 billion years ago from a
swirling, disk-shaped cloud of gas, ice, and dust called theswirling, disk-shaped cloud of gas, ice, and dust called the
solar nebula.solar nebula.
 The planets and other debris in the solar system todayThe planets and other debris in the solar system today
formed from gas, ice, and dust in the solar nebula orbitingformed from gas, ice, and dust in the solar nebula orbiting
the protosun.the protosun.
 The outer solar system, beyond the snow line, had both dustThe outer solar system, beyond the snow line, had both dust
and ice (including hydrogen and helium), while inside theand 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.

35. Formation of the Solar SystemFormation of the Solar System
 Jupiter and Saturn were initially worlds of rock and metalJupiter and Saturn were initially worlds of rock and metal
that pulled onto themselves large amounts of hydrogenthat 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 andUranus and Neptune were also initially worlds of rock and
metal, but they attracted more water and less hydrogenmetal, 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 thatThe Nice model of solar system formation proposes that
in the outer solar system, Jupiter formed first, followed byin the outer solar system, Jupiter formed first, followed by
Saturn, and then by Neptune and Uranus, which wereSaturn, and then by Neptune and Uranus, which were
flung out to their present orbits by gravitational forcesflung out to their present orbits by gravitational forces
from Jupiter and Saturn.from Jupiter and Saturn.
 The four inner planets formed through the collisions ofThe four inner planets formed through the collisions of
Moon-sized bodies, probably after the outer four planetsMoon-sized bodies, probably after the outer four planets
were formed.were formed.

36. Formation of the Solar SystemFormation of the Solar System
 The Sun formed at the center of the solar nebula. AfterThe Sun formed at the center of the solar nebula. After
about 100 million years, the temperature at theabout 100 million years, the temperature at the
protosun’s center was high enough to igniteprotosun’s center was high enough to ignite
thermonuclear fusion reactions.thermonuclear fusion reactions.
 For 800 million years after the Sun formed, impacts ofFor 800 million years after the Sun formed, impacts of
asteroidike objects on the young planets dominated theasteroidike objects on the young planets dominated the
history of the solar system.history of the solar system.

37. Categories of Solar System ObjectsCategories of Solar System Objects
 Astronomical objects smaller than the eight planets areAstronomical objects smaller than the eight planets are
classified as dwarf planets or small solar system bodiesclassified 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, overlapKuiper belt objects (KBOs), and Oort cloud objects, overlap
with the designationswith 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, andPluto, 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 1500Other objects orbit the Sun beyond Neptune. At least 1500
KBOs have been observed. A few potential Oort cloudKBOs have been observed. A few potential Oort cloud
objects have also been identified.objects have also been identified.

38. Comparative PlanetologyComparative Planetology
 The four inner planets of the solar system share manyThe four inner planets of the solar system share many
characteristics and are distinctly different from the fourcharacteristics 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 composedhave high average densities, and are composed
primarily of rock and metal.primarily of rock and metal.
 Jupiter and Saturn have large diameters and lowJupiter and Saturn have large diameters and low
densities and are composed primarily of hydrogen anddensities and are composed primarily of hydrogen and
helium. Uranus and Neptune have large quantities ofhelium. 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.

39. 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 largedensity, and composition consistent with other large
Kuiper belt objects (KBOs).Kuiper belt objects (KBOs).
 Asteroids are rocky and metallic debris in the solarAsteroids are rocky and metallic debris in the solar
system, are larger than about 10 m in diameter, and aresystem, 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. CometsMeteoroids are smaller pieces of such debris. Comets
are debris that contain both ice and rock.are debris that contain both ice and rock.

40. Planets Outside Our Solar SystemPlanets Outside Our Solar System
 Astronomers have observed disks of gas and dustAstronomers have observed disks of gas and dust
orbiting young stars.orbiting young stars.
 At least 1000 exoplanets have been discovered orbitingAt least 1000 exoplanets have been discovered orbiting
other stars.other stars.
 Exoplanets ranging in mass from less the mass of theExoplanets ranging in mass from less the mass of the
Earth to many times the mass of Jupiter have beenEarth to many times the mass of Jupiter have been
detected.detected.
 Most of the exoplanets that have been discovered haveMost 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 areSome exoplanets are observed directly, while most are
detected indirectly as a result of their effects on the starsdetected indirectly as a result of their effects on the stars
they orbit.they orbit.

41. Planets Outside Our Solar SystemPlanets Outside Our Solar System
 Exoplanets orbiting virtually all types ofExoplanets orbiting virtually all types of
stars have been observed.stars have been observed.
 Some planets that are not orbiting starsSome planets that are not orbiting stars
have been observed.have been observed.

42. Key TermsKey Terms
accretion
albedo
asteroid
asteroid belt
average density
comet
crater
dense core
dwarf planet
Jeans instability
Kuiper belt
Kuiper belt object (KBO)
metals
meteoroid
microlensing
moon (natural satellite)
Nice model
Oort cloud
orbital inclination
planet
planetesimal
protoplanetary disks (proplyds)
protosun
small solar system body (SSSB)
snow line
solar nebula
solar system
terrestrial planet
trans-Neptunian objects (TNOs)