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DTU 8e Chap 5 Formation of the Solar System


Discovering the Universe Ninth Edition CHAPTER 5 Formation of the Solar System and Other Planetary Systems * FIGURE 5-17 Planets and Their Stars This figure shows the ... – PowerPoint PPT presentation

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Title: DTU 8e Chap 5 Formation of the Solar System

Neil F. Comins William J. Kaufmann III
Discovering the Universe Ninth Edition
CHAPTER 5 Formation of the Solar System and Other
Planetary Systems
A montage of the planets in our solar system
presented in correct relative sizes. The orbits
in the background are also drawn to scale.
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  1. How many stars are there in the solar system?
  2. Were the Sun and planets among the first
    generation of objects created in the universe?
  3. How long has Earth existed, and how do we know
  4. What typical shape(s) do moons have, and why?
  5. Have any Earthlike planets been discovered
    orbiting Sunlike stars?

In this chapter you will discover
  • how the solar system formed
  • why the environment of the early solar system was
    much more violent than it is today
  • how astronomers define the various types of
    objects in the solar system
  • the relationships between planets, dwarf planets,
    small solar system bodies, and other
    classifications of objects in the solar system
  • how the planets are grouped as they are
  • how the moons formed throughout the solar system
  • what the debris of the solar system is made of
  • that disks of gas and dust, as well as planets,
    have been observed around a growing number of
  • that newly forming stars and planetary systems
    are being discovered every year

How Stars Lose Mass
(a) Antares is nearing the end of its existence.
Strong winds from its surface are expelling large
quantities of gas and dust, creating this nebula.
The scattering of starlight off this material
makes it appear especially bright. (b) The
planetary nebula Abell 39 exhibits 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 ly across. (c) The
Crab Nebula A supernova is the most powerful
known mechanism for a star to shed mass.
Dusty Regions of Star Formation
(a) These three bright young stars in the
constellation Monoceros are still surrounded by
much of the gas and dust from which they formed.
This is a tiny part of a much larger cloud, known
as the Cone Nebula. Astronomers hypothesize that
the solar system formed from a similar fragment
of an interstellar gas and dust cloud. (b) These
are 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
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.
This computer simulation shows the formation of
the inner planets over time.
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The Nice Model and the Outer Solar System
The Nice model suggests that the outer planets
formed much closer to the sun. Flinging material
into the inner solar system caused them to spiral
outward in their orbits. Later, they sent rocky
and icy material out to the outer reaches of the
solar system to stop their outward motion. This
resulted in the formation of the Kuiper belt and
Oort cloud.
This picture of the asteroid Gaspra was taken in
1991 by the Galileo spacecraft on its way to
Jupiter. The asteroid measures 12 x 20 x 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.
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 Moons first 700 million years of
existence, when the rate of bombardment was much
greater than it is now.
Different Classifications of Solar System Objects
  • Here are classifications of solar system objects.
  • A planet is an object that
  • orbits the sun
  • has enough mass so that its own gravitational
    attraction causes it to be essentially spherical
  • 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).

This scale drawing shows the distribution of
planetary orbits around the Sun. All orbits are
counterclockwise because the view is from above
Earths 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.
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
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 ly 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
The star Fomalhaut, blocked out so that its light
does not obscure the off-center 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
believed to be due to the gravitational effects
of a giant planet orbiting Fomalhaut. This system
is 25 ly from Earth. The dimmer debris in that
system and between us and it scatters light that
is considered noise in such images.
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 ly from the
solar system in the constellation Hydra.
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 stars 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 stars 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 separation between
extrasolar planets 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.
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 stars 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 ly away, while the distant star is 24,000
ly 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 ly from Earth, and the
planets all have masses similar to Jupiters. (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.
Summary of Key Ideas
Formation of the Solar System
  • Hydrogen, helium, and traces of lithium, the
    three lightest elements, were formed shortly
    after the formation of the universe. The heavier
    elements were produced much later by stars and
    are cast into space when stars die. By mass, 98
    of the observed matter in the universe is
    hydrogen and helium.
  • The solar system formed 4.6 billion years ago
    from a swirling, disk-shaped cloud of gas, ice,
    and dust called the solar nebula.
  • The planets and other debris in the solar system
    today formed from gas, ice, and dust in the solar
    nebula orbiting the protosun.
  • The outer solar system, beyond the snow line, had
    both dust and ice (including hydrogen and
    helium), while inside the snow line, such ices
    were vaporized by the protosun.

Formation of the Solar System
  • Jupiter and Saturn were initially worlds of rock
    and metal that pulled onto themselves large
    amounts of hydrogen and helium, along with some
  • Uranus and Neptune were also initially worlds of
    rock and metal, but they attracted more water and
    less hydrogen and helium than the other giant
  • The Nice model of solar system formation proposes
    that in the outer solar system, Jupiter formed
    first, followed by Saturn, and then by Neptune
    and Uranus, which were flung out to their present
    orbits by gravitational forces from Jupiter and
  • The four inner planets formed through the
    collisions of Moon-sized bodies, probably after
    the outer four planets were formed.

Formation of the Solar System
  • The Sun formed at the center of the solar nebula.
    After about 100 million years, the temperature at
    the protosuns center was high enough to ignite
    thermonuclear fusion reactions.
  • For 800 million years after the Sun formed,
    impacts of asteroidike objects on the young
    planets dominated the history of the solar system.

Categories of Solar System Objects
  • Astronomical objects smaller than the eight
    planets are classified as dwarf planets or small
    solar system bodies (SSSBs).
  • A variety of other names, including asteroids,
    comets, meteoroids, trans-Neptunian objects,
    plutinos, plutoids, Kuiper belt objects (KBOs),
    and Oort cloud objects, overlap with the
    designations dwarf planet and SSSB.
  • KBOs and Oort cloud objects are trans-Neptunian
    objectsthey orbit farther from the Sun than the
    outermost planet.
  • To date, five objectsPluto, Ceres, Eris, Haumea,
    and Makemakehave been classified as dwarf
  • Other objects orbit the Sun beyond Neptune. At
    least 1500 KBOs have been observed. A few
    potential Oort cloud objects have also been

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

Comparative Planetology
  • Pluto, once considered the smallest planet, has a
    size, density, and composition consistent with
    other large Kuiper belt objects (KBOs).
  • 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. Comets are debris
    that contain both ice and rock.

Planets Outside Our Solar System
  • Astronomers have observed disks of gas and dust
    orbiting young stars.
  • At least 506 exoplanets have been discovered
    orbiting other stars.
  • Most of the exoplanets that have been discovered
    have masses roughly equal to the mass of Jupiter.
  • Exoplanets are discovered indirectly as a result
    of their effects on the stars they orbit.

Key Terms
accretion albedo asteroid asteroid belt average
density comet crater dense core dwarf
planet Jeans instability Kuiper belt Kuiper belt
object (KBO) meteoroid metals
microlensing moon (natural satellite) Nice
model Oort cloud orbital inclination planet planet
esimal protoplanetary disks (proplyds) protosun sm
all solar system body (SSSB) solar nebula solar
system terrestrial planet trans-Neptunian object
  • How many stars are there in the solar system?
  • One, the Sun.

  • Were the Sun and planets among the first
    generation of objects created in the universe?
  • No. All matter and energy were created by the Big
    Bang. However, much of the material that exists
    in our solar system was processed inside stars
    that evolved before the solar system existed. The
    solar system formed billions of years after the
    Big Bang occurred.

  • How long has Earth existed, and how do we know
  • Earth formed along with the rest of the solar
    system, about 4.6 billion years ago. The age is
    determined from the amount of radioactive decay
    that has occurred in it.

  • What typical shape(s) do moons have, and why?
  • Although some moons are spherical, most look
    roughly like potatoes. Those that are spherical
    are held together by the force of gravity, which
    pulls down high regions. Those that are
    potato-shaped are held together by the
    electromagnetic interaction between atoms, just
    like rocks. These latter moons are too small to
    be reshaped by gravity.

  • Have any Earthlike planets been discovered
    orbiting Sunlike stars?
  • Not really. Most exoplanets are Jupiterlike gas
    giants. The planets similar in mass and size to
    Earth are either orbiting remnants of stars that
    exploded or, in the case of Gliese 581, a star
    much less massive and much cooler than the Sun.
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