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Chapter 8
Survey of Solar Systems
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Upon completing this chapter you should be able to:
• Identify the primary components of the Solar
System, and describe their distinctive properties.
• Discuss the differences between terrestrial,
Jovian, and dwarf planets, and their satellites, and
recount why astronomers reclassified Pluto.
• Explain how astronomers measure masses and
radii for bodies in the Solar System, and carry out
a calculation to find a body's density from these
measurements.
• Describe the densities of different classes of
objects in the Solar System and relate this to their
composition.
Upon completing this chapter you should be able to:
• Recall the age of the Solar System and explain
how it is determined.
• Describe the steps in the formation of the Solar
System according to the nebular theory and relate
these to the properties of the planets and other
bodies.
• Explain why disks are expected to form around
stars, and describe the observations that indicate
disks are present around yound stars.
• Describe the role of planetesimals in planet
formation and modification, and where some can
still be found.
Upon completing this chapter you should be able to:
• Discuss the roles of rocky, icy, and gaseous
materials in the formation of planets and their
atmospheres.
• Explain the various methods currently being used
to detect exoplanets, the information each
provides about an exoplanet, and the limitations
of these methods.
• Discuss the ways in which exoplanetary systems
appear to differ from the Solar System.
The Solar System consists of the Sun and the bodies in its
gravitational domain: the eight planets, dozens of dwarf
planets, and swarms of moons, asteroids, and comets.
Although we have only walked on the Earth and the Moon, we have
detailed pictures sent to us from spacecraft of most of the planets
and their satellites. Some are baren balls of rock; others are
mostly ice. Some have thin, frigid atmospheres so cold that
ordinary gases crystallize as snow on their cratered surfaces;
others have thick atmospheres that constistency of molten lava
and no solid surface at all.
All the planets circle the Sun in the same direction, and most of them
spin in the same direction. The moons also form flattened
systems, generally orbiting in the same direction.
The Solar System
• The Solar System is occupied by a diversity of
objects, but shows an underlying order in the
dynamics of their movements
• The planets form two main families:
– solid rocky inner planets (terrestrial)
– gaseous/liquid outer planets (Jovian)
• Astronomers deduce that the Solar System
formed some 4.5 billion years ago out of the
collapse of a huge cloud of gas and dust
The Sun: the only star in our solar system
Image of the Sun made with ultraviolet
telescope that reveals high temperature
gases in the Sun's atmosphere.
• The Sun is a star, a ball • Composed mainly of
of incandescent gas
hydrogen (71%) and helium
whose output is
(27%), it also contains
generated by nuclear
traces of nearly all the other
reactions in its core
chemical elements
The Sun
• It is the most
massive object
in the Solar
System – 700
times the mass
of the rest of the
Solar System
combined
• Emits its own
• Its large mass provides the
light
gravitational force to hold all the Solar
System bodies in their orbital patterns
around the Sun
The Planets
• Orbits are almost circular lying in nearly the same plane
• They emit no visible light of their own but shine by reflected
sunlight
• Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and
Neptune
The Planets
The orbits are shown in the correct relative scale in the 2 drawings.
Because of the great difference in scale, the inner and outer Solar
System are displayed separately.
The Planets
• All of the planets travel
counterclockwise
around the Sun (as seen
from high above the
Earth’s north pole)
• Six planets rotate
counterclockwise;
Venus rotates clockwise
(retrograde rotation),
and Uranus appears to
rotate on its side
The Planets
The planets' orbits from the side. The drawf planets Ceres, Pluto,
and Eris are also shown, illustrating their highly inclined orbits.
This view also shows that while Pluto sometimes gets closer to the
Sun than Neptune, its orbit actually remains well separated from
Neptune's.
According to the IAU, a planet is a celestrial body
that is in orbit around the sun, is massive enought
for its self-gravity to create a nearly round shape,
and has cleared the neighborhood around its orbit.
By this definition, our solar system has eight
planets: Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, and Neptune.
Some astronmers call these the major planets.
Instead of "inner" and "outer" planets, astronomers
sometimes use "terrestrial" and "Jovian" to
describe the two types of planets.
The terrestrial planets (Mercury to Mars) are so
named because of their resemblance to the Earth.
The Jovian planets (Jupiter to Neptune) are named
for their resemblance to Jupiter.
Inner Planets
• Mercury, Venus, Earth, Mars
• Small rocky (mainly silicon and
oxygen) bodies with relatively thin or
no atmospheres
• Also known as terrestrial planets
• It is believed that only the rocky
material could condense at the
higher temperatures of the inner
part of the solar nebula.
Outer Planets
• Jupiter, Saturn, Uranus,
and Neptune
• Gaseous, liquid, or icy
(H2O, CO2, CH4, NH3)
• Also referred to as
Jovian planets
• Jovian planets are much
larger than terrestrial
planets and do not have a
well-defined surface
• Compared to the
Terrestrial planets, the
Jovian planets have many
satellites.
A drawf planet shares all the characteristics of a
planet except that it has not cleared the
neighborhood around its orbit.
Ceres, Pluto, Haumea, Makemake, and Eris fall into
this category...
Dwarf Planets
• Pluto (it has not cleared its neighborhood) and
similar objects fail to fit into either family
• Recently, scientists have discovered more than
200 similar objects orbiting the Sun at the same
distance as Pluto
• In 2008, dwarf planets that orbit beyond Neptune
are called Plutoids.
• In 2006, a new family was introduced – the dwarf
planets
– Massive enough to pull themselves spherical
– Orbits have not been swept clear of debris
Spins and Axial Tilts
• Most planets spin (rotate) in the same direction,
clounterclockwise as viewed from above the
north pole.
• Venus and Uranus are exceptions
• Axial tilts range from 0º to 98º.
Asteroids and Comets
• Composition and size
– Asteroids are rocky or metallic bodies ranging in size
from a few meters to 1000 km across (about 1/10 the
Earth’s diameter)
– Comets are icy bodies about 10 km or less across that
can grow very long tails of gas and dust as they near
the Sun and are vaporized by its heat
– Although the majority of comets probably originate in
the Oort cloud, some come from the Kuiper belt.
Together they probably contain more than 1 trillion
comet nuclei, only a few of which get close enough to
the Sun to be detected.
Asteroids and Comets
• Their location within Solar System
– Most asteroids are in the asteroid belt between
Mars and Jupiter indicating that these asteroids are
the failed building-blocks of a planet
– Some comets may also come from a disk-like
swarm of icy objects that lies beyond Neptune and
extends to perhaps 1000 AU, a region called the
Kuiper ( KY -per) Belt
Asteroids and Comets
Most comets orbit the Sun far beyond Pluto in the
Oort cloud, a spherical shell extending from
40,000 to 100,000 AU from the Sun
Bode's Rule: The search for order
In the 1700's astronomers noticed that the spacing
between the orbits of the planets seems to follow
a fairly regular progression.
This mathematical progression may indicate
something about the natural spacing between
orbits of large bodies, or it may be a chance
pattern ...
Bode’s Rule
• Very roughly, each planet is about twice as far from
the Sun as its inner neighbor. The progression of
distance from the Sun can be expressed by a simple math
relation...Bode's Rule.
• First noted in 1766, formalized mathematically by J. E. Bode in
1778
– 0, 3, 6, 12 ,24, 48, 96, 192, 384
(nine numbers...)
– 4, 7, 10, 16, 28, 52, 100, 196, 388
"
– 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, 19.6, 38.8
"
• Does a pretty good job, up to a point...
Measuring Composition: Density
• A planet’s average density is determined by
dividing a planet’s mass by its volume
– Mass determined from Kepler’s modified third law
– Volume derived from a planet’s measured radius
Measuring Composition
• Since the inner and outer planets differ
dramatically in composition, it is important
to understand how composition is
determined
• A planet’s reflection spectrum can reveal
a planet’s atmospheric contents and the
nature of surface rocks
• Seismic activity has only been measured on
Earth for the purposes of determining
interior composition
Measuring Composition: Density
• Once average density known, the following
factors are taken into account to determine a
planet’s interior composition and structure:
– Densities of abundant, candidate materials
– Variation of these densities as a result of
compression due to gravity
– Surface composition determined from reflection
spectra
– Material separation by density differentiation
– Mathematical analysis of equatorial bulges
Analysis Concludes:
• The terrestrial planets, with average densities ranging
from 3.9 to 5.5 g/cm3, are largely rock and iron, have
iron cores, and have relative element ratios similar to
the Sun except for deficiencies in lightweight gasses
Analysis Concludes:
• The Jovian planets, with average densities ranging from
0.71 to 1.67 g/cm3, have relative element ratios
similar to the Sun and have Earth-sized rocky cores
Satellites
• The number of planetary satellites
changes frequently as more are
discovered!
–
–
–
–
–
–
–
–
Jupiter 63
Saturn 60
Uranus 27
Neptune 13
Mars 2
Earth 1
Mercury and Venus are moonless
Even Pluto and Eris have moons!
Age of the Solar System
• All objects in the Solar System seem to have
formed at nearly the same time, out of the same
original cloud of gas and dust
• Radioactive dating of rocks from the Earth,
Moon, and some asteroids suggests an age of
about 4.5 billion yrs
• A similar age is found for the Sun based on
current observations and nuclear reaction rates
Formation of Planetary Systems
• A theory of the Solar System’s formation must
account for the following:
– Planets orbit in the same direction and in the same
plane
– Rocky inner planets and gaseous/liquid/icy outer
planets
– Compositional trends in the solar system
– All Solar System bodies appear to be less than 4.5
billion years old
– Other details – structure of asteroids, cratering of
planetary surfaces, detailed chemical composition of
surface rocks and atmospheres, etc.
The Solar Nebula Hypothesis
• Derived from 18th
century ideas of Laplace
and Kant
• Proposes that Solar
System evolved from a
rotating, flattened disk
of gas and dust (an
interstellar cloud), the
outer part of the disk
becoming the planets
and the inner part
becoming the Sun
The Solar Nebula Hypothesis
• This hypothesis
naturally explains the
Solar System’s flatness
and the common
direction of motion of
the planets around the
Sun
• Interstellar clouds are
common between the
stars in our galaxy and
this suggests that most
stars may have planets
around them
Interstellar Clouds
• Come in many shapes
and sizes – one that
formed Solar System
was probably a few
light years in
diameter and 2 solar
masses
• Typical clouds are
71% hydrogen, 27%
helium, and traces of
the other elements
Interstellar Clouds
• Clouds also contain
tiny dust particles
called interstellar
grains
– Grain size from large
molecules to a few
micrometers
– They are a mixture of
silicates, iron and
carbon compounds,
and water ice
Formation of the Solar Nebula
• Triggered by a
collision with
another cloud or a
nearby exploding
star, rotation forces
clouds to
gravitationally
collapse into a
rotating disk
The Solar Nebula
• A few million years pass
for a cloud to collapse
into a rotating disk with a
bulge in the center
• This disk, about 200 AU
across and 10 AU thick,
is called the solar nebula
with the bulge becoming
the Sun and the disk
condensing into planets
• The jovian planets has a
composition that most
resembles the original
solar nebula.
Disk Observations
A) The small blobs in this are stars in the process of formation and
their surrounding disk of dust and gas. These are in the Orion
Nebula, a huge gas cloudabout 1500 ly from Earth.
B) Dust ring that surounds the star Fomalhaut. A small mask in the
telescope blots out the star's direct light, which would otherwise
overexpose the image.
Temperatures in the Solar Nebula
• Before the planets formed, the inner part of
the disk was hot, heated by gas falling onto
the disk and a young Sun – the outer disk
was colder than the freezing point of water
Condensation
• Condensation occurs when
gas cools below a critical
temperature at a given gas
pressure and its molecules
bind together to form
liquid/solid particles
• The process in which a gas
cools and its molecules stick
together to form liquid
particles ....
Condensation in the Solar Nebula
– Iron vapor will condense at 1300 K, silicates will
condense at 1200 K, and water vapor will condense
at room temperature in air
– In a mixture of gases, materials with the highest
vaporization temperature condense first
– Condensation ceases when the temperature never
drops low enough
– Sun kept inner solar nebula (out to almost Jupiter’s
orbit) too hot for anything but iron and silicate
materials to condense
– Outer solar nebula cold enough for ice to condense
Formation of Planets
An artist's dipiction of how the planets may have formed in the
nebula.
nebular hypothesis: Interstellar cloud, solar nebula, accretion,
collisions between planetesimals, planets.
Accretion
– Next step is for the tiny
particles to stick together,
perhaps by electrical forces,
into bigger pieces in a process
called accretion
– As long as collisions are not
too violent, accretion leads to
objects, called planetesimals,
ranging in size from
millimeters to kilometers
– Astronomers believe that the
satellites of the Jovian planets
were planetesimals orbiting
the growing planet.
Planetesimals
• Planetesimals in the inner
solar nebula were rockyiron composites, while
planetesimals in the outer
solar nebula were icyrocky-iron composites
• Planets formed from
“gentle” collisions of the
planetesimals, which
dominated over more
violent shattering collisions
Formation of the Planets
• Simulations show that
planetesimal collisions
gradually lead to
approximately circular
planetary orbits
• As planetesimals grew in
size and mass their
increased gravitational
attraction helped them
grow faster into clumps
and rings surrounding the
Sun
Formation of the Planets
• Planet growth was
especially fast in the
outer solar nebula due
to:
– Larger volume of
material to draw upon
– Larger objects (bigger
than Earth) could start
gravitationally capturing
gases like H and He
Final Stages
• Rain of
planetesimals
cratered surfaces
• Remaining
planetesimals
became small
moons, comets, and
asteroids
Continuous Bombardment
• Continued
planetesimal
bombardment and
internal radioactivity
melted the planets
and led to the
density
differentiation of
planetary interiors
Formation of Moons
• Moons of the outer
planets were probably
formed from
planetesimals orbiting
the growing planets
• Not large enough to
capture H or He, the
outer moons are mainly
rock and ice giving
them solid surfaces
Formation of Atmospheres
• Atmospheres were the last planet-forming
process
• Outer planets gravitationally captured their
atmospheres from the solar nebula
• Inner planets created their atmospheres by
volcanic activity and perhaps from comets and
asteroids that vaporized on impact
• Objects like Mercury and the Moon are too small
– not enough gravity – to retain any gases on their
surfaces
Exosolar Planets
• Evidence exists for planets around other nearby stars
• The new planets are not observed directly, but rather by their
gravitational effects on their parent star
• These new planets are a surprise - they have huge planets very
close to their parent stars
• The recent discovery that planets more massive than Jupiter orbit
nearby stars in small orbits surprising because according to the
nebular hypothesis, massive planets should only form away from
their star.
Exosolar Planets
• Idea: The huge planets
formed far from their stars
as current theory would
project, but their orbits
subsequently shrank
• This migration of planets
may be caused by
interactions between
forming planets and
leftover gas and dust in
the disk
Detecting Exoplanets
• We can detect
planets around other
stars by using the
doppler shift method.
– A planet and its stars
revolve around a
common center of
mass.
– As the star approaches
us in its motion around
the center of mass, its
spectrum will be blueshifted.
– As it recedes, the
spectrum will be redshifted.
Gravitational Lensing Method
• Another method utilizes
a prediction of General
Relativity called
gravitational lensing.
• Gravity bends the path
of light when it passes
near a massive object.
• If that object is a star
with companion planets,
we can use the lensing
effect to detect them.
• We have not found many
planetary systems using
this method, but it has
the potential to find lowmass planets!
Transit Method
• We can also use the
transit method for
detection.
– Look for dimming of
light from the central
star.
A Sample of Exoplanets
Comparison of the orbital radii and relative sizes of expoplanetary
system...