Download 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.
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
How many stars are there in the solar
system?
Were the Sun and planets among the
first generation of objects created in the
universe?
How long has Earth existed, and how do
we know this?
What typical shape(s) do moons have,
and why?
Have any Earthlike planets been
discovered orbiting Sunlike stars?
In this chapter you will discover…
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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 stars
that newly forming stars and planetary systems are
being discovered every year
How Stars Lose Mass
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 “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.
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
Orion Nebula with Hubble Space Telescope. The four
insets: false-color images of protoplanetary disks within
the nebula. A newborn star is at the center of each disk.
This computer simulation shows the formation of
the inner planets over time.
The Nice Model and the Outer Solar System
The outer planets formed much closer to the sun. Flinging
material into the inner solar system caused them to spiral
outward 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.
Asteroid Gaspra, 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 Moon’s first 700 million years of existence.
Different Classifications of Solar System Objects
Here are classifications of 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).
Planetary orbits
around the Sun, to
scale. All orbits are
counterclockwise
as viewed from above
Earth’s North Pole.
The orbits appear
nearly circular, with
Mercury having the
most elliptical orbit.
Edge view shows that
all orbits are nearly in
the ecliptic plane.
The Sun and the planets with size drawn 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 (volume).
A Circumstellar Disk of Matter
Hubble view of Beta Pictoris, with an edge-on disk of material 225
billion km (1500 AU) across that orbits the star 50 ly from Earth.
Twenty million years old, this disk is believed to be composed
primarily of icy 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.
Off-Center Disk
The star Fomalhaut, blocked out, 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 planet was taken at
the European Southern Observatory. Neither object 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 .
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
separation between extrasolar
planets and their stars. 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
Gravitational fields
cause light to change
direction. The
deflection itself is too
small to be detectable,
but it does increase
detectably the apparent
brightness of the distant
star. A planet makes it
non-syymetric.
A Star with Three Planets
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 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.
Summary of Key Ideas
Formation of the Solar System
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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
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Jupiter and Saturn were initially worlds of rock and metal
that pulled onto themselves large amounts of hydrogen
and helium, along with some water.
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 planets.
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 Saturn.
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
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The Sun formed at the center of the solar nebula. After
about 100 million years, the temperature at the
protosun’s 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
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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
objects—they orbit farther from the Sun than the
outermost planet.
To date, five objects—Pluto, Ceres, Eris, Haumea, and
Makemake—have been classified as dwarf planets.
Other objects orbit the Sun beyond Neptune. At least
1500 KBOs have been observed. A few potential Oort
cloud objects have also been identified.
Comparative Planetology
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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.
Comparative Planetology
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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
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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
planetesimal
protoplanetary disks (proplyds)
protosun
small solar system body (SSSB)
solar nebula
solar system
terrestrial planet
trans-Neptunian object (TNO)
WHAT DID YOU THINK?
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How many stars are there in the solar system?
One, the Sun.
WHAT DID YOU THINK?
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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.
WHAT DID YOU THINK?
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How long has Earth existed, and how do we know this?
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 DID YOU THINK?
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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.
WHAT DID YOU THINK?
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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.