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Chapter 7
The Solar System
Components of the Solar
System
The Sun
• The Sun is a star, a
ball of incandescent
gas whose light and
heat are generated by
nuclear reactions in
the core. It’s mass is
more than 700 times
the mass of
everything else in the
solar system put
together.
The Sun-2
The Sun’s gravitational
force holds the planets
other bodies in the solar
system in their orbital
patterns.
The Sun is mostly
hydrogen, 71%, and
helium, 27%.
It also contains very small
proportions of all of the
other elements.
The Planets
• The planets are much smaller than the
Sun and orbit around it.
• They emit no visible light of their own but
shine by reflected sunlight, a property
known as albedo.
• The planets move around the Sun in
approximately circular orbits, all lying on
nearly the same plane.
The Planets-2
• The Solar System is like a spinning disc
with the planets traveling around the sun
in the same counterclockwise direction.
• Because of this, the planets appear to lie
in a line in the sky.
• As the planets orbit the Sun, each spins
on its rotation axis.
• The spin is generally in the same direction.
The Planets-3
• The tilt of the rotational axes relative to the
planetary orbit is not far from perpendicular.
• There are 2 exceptions, Venus and Uranus.
Their tilts are extremely large.
• The flattened structure and the orderly orbital
and spin properties of the Solar System are 2 of
its most fundamental features and any theory of
the Solar System must explain them.
• A third, but equally important feature is that the
planets fall into 2 families called inner and outer
planets.
The Planets-4
• The inner and outer planets are classified
based on their size, composition and
location in the Solar System.
Two Types of Planets
• Inner Planets
• Mercury
• Venus
• Earth
• Mars
• Outer Planets
• Jupiter
• Saturn
• Uranus
• Neptune
Two Types of Planets-2
• The inner planets are small rocky bodies
with relatively thin or no atmospheres.
• The outer planets are gaseous, liquid or
icy. They have deep, hydrogen-rich
atmospheres.
• The term “rocky” for the inner planets
means material composed of silicon and
oxygen (SiO2-sand) with a mixture of other
elements such Al, Mg, S and Fe.
Two Types of Planets-3
• By ice, we mean frozen liquids and gases,
not just frozen water.
• This would include CO2, NH3, CH4 etc.
• Rock is rare in the Solar System by
percentage because of the abundance of
Hydrogen compared to Silicon.
• The inner part of the system is mostly rock
because of the heat of the sun.
Two Types of Planets-4
• The gases and liquids cannot condense to
mix with the Silicon in the heat.
• The outer planets generally have no true
surface.
• The atmospheres of the outer planets
thicken with depth and eventually liquefy.
• There is no distinct boundary between
atmosphere and crust.
Two Types of Planets-5
• Deep in the interior, the liquid may
compress enough to form a solid.
• The transition from liquid to solid is no
sharply defined.
• We probably will never land on Jupiter, we
would simply sink deeper and deeper into
its interior.
Two Types of Planets-6
• The inner planets are sometimes referred
to as the “Terrestrial” or Earth-like planets.
• The outer planets are sometimes referred
to as the “Jovian” or Jupiter-like planets.
Satellites
• Many of the planets have satellites themselves.
• Only Mercury and Venus do not have moons.
• The moons usually move in a roughly circular
path around the equator of the planet.
• Only the moons of Uranus and Pluto are not
near the equatorial plane, an important clue to
the origin of the moons.
• Jupiter, Saturn and Neptune have large families
of moons, 62,31 and 27 respectively.
Asteroids and Comets
• Asteroids and comets are much smaller
than planetary bodies.
• Asteroids are rocky or metallic bodies with
diameters that range from a few meters up
to about 1000 km.
• Comets are icy bodies about 10 km or less
in diameter.
• Comets grow huge tails of gas and dust as
they approach the sun.
Asteroids and Comets-2
• The comets are partially vaporized by the
flow of energy from the sun.
• Their composition puts them into 2
families, much the same as the planets.
• Asteroids and comets also differ in their
location within the Solar System.
• Most asteroids are found in a large gap
between Mars and Jupiter called the
“asteroid belt”.
Asteroids and Comets-3
• It may be material that failed to aggregate
into a planet as a result of disturbance by
the gravity of Jupiter.
• Most comets are found in an orbit far
beyond Pluto in an area called the Oort
cloud.
• It is named after the Dutch Astronomer
who proposed its existence.
Asteroids and Comets-4
• It completely surrounds the Solar System in a
spherical region 40,000 to 100,000 astronomical
units from the sun.
• Even though most comets originate in the Oort
cloud, some may come from a disk-like swarm of
icy objects that lies just beyond the orbit of
Neptune and extends to about 60 au from the
Sun.
• This area is called the “Kuiper Belt”.
• Together the cloud and belt may hold 1012 comet
nuclei.
Composition Differences
• Astronomers can deduce a planet’s
composition in several ways.
• From the planet’s spectrum we can
measure its atmospheric composition and
get some information about the nature of
its surface rocks.
• The spectrum does not give a clue about
internal composition.
Composition Differences-2
• Earthquake waves could give us
information, but to date we do not have
any working detectors on the inner
planets.
• The outer planets provide a different
problem because of the lack of surface.
• Density is another way to give us an idea
of composition.
Density as a Measure of a Planet’s
Composition
• By using Newton’s modification of Kepler’s
3rd law, we can determine the mass of the
planet by observing the effect of the mass
of the planet has on an orbiting body such
as a moon.
• Volume can be determined by measuring
the planets radius.
• (V=4πR3/3; R is the planets radius)
Density as a Measure of a Planet’s
Composition-2
• Radius can be measured by angular size and
distance.
• With both mass and volume, we can determine
density of the planet. (D=M/V)
• Once the planet’s average density is known, we
can compare it with the density of abundant
candidate materials.
• Earth has a density of about 5.5, about
intermediate between silicate rock (3 g/cm3) and
iron (7.8 g/cm3).
Density as a Measure of a Planet’s
Composition-3
• Because of this data, we can deduce that
the Earth has a silicate rock surface with
and iron core.
• This has also been confirmed using
earthquake information.
• Density comparison is a powerful tool to
use to study planetary composition, but it
also has drawbacks.
Density as a Measure of a Planet’s
Composition-4
• There may be similar substances that
might match the given density.
• The density of a material can be affected
by a planet’s gravitational force.
• A massive planet may crush rock whose
normal density is 3 to a density of 7 or 8.
• All of the terrestrial planets have a density
similar to that of the Earth (3.9-5.5).
Density as a Measure of a Planet’s
Composition-5
• The jovian planets have a density that is
much smaller (0.71-1.67), about that of
ice.
• After correcting for gravitational
compression, we can conclude that all of
the inner planets contain large amounts of
rock and iron and that the iron has sunk to
the core.
Density as a Measure of a Planet’s
Composition-6
• The outer planets contain mainly light
materials such as hydrogen, helium,
methane, ammonia and water.
• The outer planets probably have core of
iron about the size of the Earth, beneath
their deep atmosphere.
• Astronomers deduce the existence of the
cores in 2 ways.
Density as a Measure of a Planet’s
Composition-7
• If the outer planets have the same relative
amounts of heavy elements as the Sun, they
should contain several Earth masses of iron and
silicates.
• Because they are much more dense than the
gases that make up the majority of the planet’s
mass, they sink to form the core.
• Analysis of rotational data shows that the
equatorial bulges can best be explained if they
have small dense core.
Density as a Measure of a Planet’s
Composition-8
• Composition studies show the differences
between families but also furnishes astronomers
with another clue to the origin of the planets.
• The Sun and the planets were made from the
same material.
• Because Jupiter and Saturn have a composition
nearly identical to the Sun, and the inner planets
have a similar composition if you remove the
hydrogen and helium component, we can
conclude that the process must keep the inner
planets from capturing the light gases.
Bode’s Law
(0+4)/10
(3+4)/10
(6+4)/10
(12+4)/10
0.4
0.7
1.0
1.6
(24+4)/10
2.8
(48+4)/10
5.2
(96+4)/10 10.0
(192+4)/10 19.6
Mercury
Venus
Earth
Mars
0.39
0.72
1.00
1.52
?
Jupiter
Saturn
Uranus
?
5.2
9.5
19.2
Bode’s Law
• Bode’s Law is a curious relationship, that
is stilled unexplained.
• The gap in the prediction corresponds
nicely with the location of the asteroid
Ceres, first discovered by Giuseppi Piazzi.
• Verification of Bodes Law will come when
we can establish the same relationship in
other solar systems.
Age of the Solar System
• In spite of the differences in size, structure and
composition, the planets, asteroids and comets
all seem to have formed at nearly the same time.
• We can directly measure the Earth, Moon and
some asteroids from the radioactivity of their
rocks (about 4.6 billion years old).
• The Sun’s age is similar based on it’s current
brightness and it’s presumed rate nuclear fuel
consumption.
Origin of the Solar System
Origin of the Solar System
•
Observations that have to be accounted
for when formulating a theory about the
origin of the solar system.
1. The Solar System is flat, with all of the
planets orbiting in the same direction.
2. There are 2 types of planets, inner and
outer, with the rocky ones near the Sun
and the gaseous or liquid ones farther
out.
Origin of the Solar System-2
3. The composition of the outer planets is
similar to the Sun’s while that of the inner
planets is like the Sun’s minus the gases
that condense only at low temperatures.
4. All of the bodies in the Solar System
whose ages have so far been
determined to be 4.6 billion years old.
Origin of the Solar System-3
• Other details that could also be explained are
structure of asteroids, number of craters on
planetary and satellite surfaces as well as
detailed chemical composition of surface rocks
and atmosphere.
• The currently favored theory for the origin of the
Solar System derives from the theories
proposed in the eighteenth century by Immanuel
Kant and Pierre Simon LaPlace.
Origin of the Solar System-4
• They independently proposed the “Solar Nebula
Hypothesis”.
• The Solar System originated from a rotating flat
disk of gas and dust, with the outer part of the
disk becoming the planets and the center
becoming the Sun.
• This theory offers a natural explanation for the
flattened shape of the system and the common
direction of motion of planets around the Sun.
Interstellar Clouds
• Interstellar clouds are the raw material of
the Solar System.
• The clouds are found in many sizes and
shapes .
• The one that formed the Solar System was
probably a few light years in diameter and
contained about twice the present mass of
the Sun.
Interstellar Clouds-2
• If the cloud was typical of today’s clouds, it
contained about 71% hydrogen and 27%
helium with tiny traces of other elements
such as carbon, oxygen and silicon.
• In addition to the gases, interstellar clouds
also contain tiny dust particles called
interstellar grains.
Interstellar Clouds-3
• Interstellar grains range in size from large
molecules to micrometers or larger.
• They are believed to be made of a mixture
of silicates, iron compounds, carbon
compounds and water frozen into ice.
• This is determined by analyzing the
spectrum of light that passes through the
cloud.
Interstellar Clouds-4
• The cloud began its transformation into the
Sun and planets when the gravitational
attraction between the particles in the
densest part of the cloud caused it to
collapse inward.
• The collapse may have been triggered by
a star exploding nearby or by a collision
with another cloud.
Interstellar Clouds-5
• The infall was not directly to the center.
• Because the cloud was rotating, it
flattened.
• Flattening occurred because rotation
retarded the collapse perpendicular to the
rotation.
• A similar effect occurs when pizza dough
is flattened by tossing it into the air with a
spin.
Formation of the Solar Nebula
• It probably took a few million years for the
cloud to collapse and to become the
rotating disk with a bulge in the center.
• The disk is called a solar nebula.
• The disk eventually condensed into the
planets and the bulge into the Sun.
• This helps to explain the property of the
solar system with the planets on the same
plane.
Formation of the Solar Nebula-2
• The solar nebula was probably about 200
AU in diameter and 10 AU thick.
• These measurements seem consistent
with stars and disks around them.
• The stars in the center are not yet hot
enough to emit visible light
Condensation of the Solar
Nebula
• Condensation occurs when a gas cools
and its molecules begin to stick together to
form liquid or solid particles.
• The gas must cool below a critical
temperature.
• If we cool a cloud of vaporized iron (2000
K) to 1300 K, tiny flakes of iron will
condense from it.
Condensation of the Solar
Nebula-2
• If we condense a cloud of silicates to
about 1200 K, it will begin to condense.
• At lower temperatures, other substances
will condense.
• As the vaporized material cools, its
molecules move more slowly, so that when
they collide, they bond.
• They first bond into pairs, then clumps and
eventually into droplets.
Condensation of the Solar
Nebula-3
• As the droplets cool at different
temperatures, they begin to solidify.
• If the temperature does not drop low
enough, the material fails to condense.
• If the temperature does not drop below
500 K, water will not condense and only
the silicates and iron form.
• This kind of condensation occurred in the
solar nebula.
Condensation of the Solar
Nebula-4
• The Sun was so hot in the inner disk that
water could not condense nearly out to the
orbit of Jupiter.
• Iron and silicate on the other hand were
able to condense throughout the system.
• The nebula became divided into 2 regions;
the inner region with iron-silicate particles
and the outer region with similar particles
plus ice.
Condensation of the Solar
Nebula-5
• Water, hydrogen and other easily
vaporized materials were present as
gases in the inner solar system.
• These gases chemically combined with
the silicates so that the rocky materials
contained the gases in small amounts.
Accretion and Planetesimals
• In the next stage of planet formation, the tiny
particles condensed from the nebula must have
begun to stick together into bigger pieces. This
process is called accretion.
• The process is similar to building a snowman.
• In the solar nebula, the tiny grains stuck together
and formed bigger grains that grew into clumps.
• Subsequent collisions, if not too violent, allowed
the objects to grow in size from millimeters to
kilometers.
Accretion and Planetesimals-2
• The larger objects are called
planetesimals.
• Because some of the planetesimals
formed near the Sun and some far enough
away to also include ice in their structure.
• There are 2 forms; the rocky iron-silicates
and the icy-iron-silicates.
Formation of the Planets
• As planetsimals moved within the disk and
collided with one another, planets formed.
• Some collisions led to the shattering of the 2.
• Some collisions led to a merging of the 2 bodies.
• There orbits gradually became nearly circular.
• These orbits are similar to the orbits given by
Bode’s Law.
• Merging of the planetesimals increased their
mass and thus their gravitational attraction.
Formation of the Planets-2
• This increased their attraction for other particles.
• The planet forming process probably took place
over about a 100,000 year span of time.
• The planet growth occurred more rapidly in the
outer part of the solar system.
• There was more material to work with because
ice was about 10 times more abundant than the
other materials.
Formation of the Planets-3
• The planetsimals in the outer solar system could
be 10 times as large as the inner bodies.
• At some point, the planet was massive enough
to attract and retain gas by its own gravity.
• Since hydrogen and helium were very abundant
in the solar nebula, planets that were large
enough could tap the reservoir and increase
their size even more.
Formation of the Planets-4
• Jupiter, Saturn, Uranus and Neptune may
have begun as Earth-sized and then grew
due to the attraction of the additional
gases.
• As planetesimals struck the growing
planets, their impact released gravitational
energy that heated both of the bodies.
• Planetesimals hitting the crust give energy
to the particles that appears as heating.
Formation of the Planets-5
• This heat in combination with radioactive
heating served to melt the planet and
allowed the material with a higher density
to sink to their cores.
• Lower denser material, such as silicate
rock floated.
Direct Formation of Giant
Planets
Formation of Moons
• The moons of the outer planets probably formed
from planetesimals orbiting the growing planets.
• Once a body became massive enough that it’s
gravitational force could draw in additional
material, it became ringed with debris.
• Moon formation in the outer planets is probably
a scaled-down version of planet formation.
• The outer moons have the same regularities as
the planets around the Sun.
Formation of Moons-2
• All 4 outer planets have a flattened
satellite system, in which, with few
exceptions, the satellites orbit in the same
direction.
• Many of these satellites are about as large
as Mercury and would be considered
planets if they were orbiting the Sun.
• Some of them even have atmospheres.
Formation of Moons-3
• They do not have enough mass to
maintain the gases of the solar nebula
(i.e. Hydrogen and Helium), so they are
mostly composed of rock and ice.
• Because of the solid surface, these moons
can become cratered and show signs of
volcanic activity.
Final Stages of Planet Formation
• The last stage of planet formation was a
bombardment of planetesimals that
created the huge craters we see
throughout the solar system where we find
solid surfaces.
• Sometimes the incoming planetesimals
was so large that it did more than leave a
crater.
Final Stages of Planet Formation-2
• Our Moon may have been created as a
result of a collision with a Mars-size body.
• Mercury may have lost its crust due to a
massive collision.
• Uranus may be atilt because of a collision
as well.
• Most of the planetesimals were consumed
in the process, but some survived to form
small moons, comets and asteroids.
Final Stages of Planet Formation-3
• Many of the remaining bodies and their
fragments remained between Mars and
Jupiter, unable to accrete because of the
huge impact of Jupiter’s gravity.
• The icy planetesimals have their orbits
disrupted by the giant planets are flung in
towards the Sun and some outwards
forming the Oort cloud.
Formation of Atmospheres
• Atmospheres are the last part of the
planet-forming process.
• The 2 types of planets form atmospheres
differently.
• The outer planets probably captured most
of their atmosphere from the solar nebula.
• The inner planets were not massive
enough or too hot to capture gas from the
solar nebula.
Formation of Atmospheres-2
• Venus, Earth and Mars probably created
their original atmospheres from volcanic
eruptions and retaining gases from
infalling comets and icy planetesimals that
vaporized on impact.
• Bodies too small to small to have captured
atmospheres directly, but show clear signs
of volcanic activity have atmospheres.
Formation of Atmospheres-3
• More quiescent ones do not.
• Small bodies such as Mercury and our
Moon essentially show no atmosphere
because of weak gravitational force being
unable to hold an atmosphere.
Cleaning Up the Solar System
• The solar system assembled the planets
over a relatively short period of time
(hundreds of thousands to a few million
years).
• The rain of planetesimals continued for a
much longer period.
• The last thing that happened in the
process was the removal of the residual
gas and dust.
Cleaning Up the Solar System-2
• The intense heat of the Sun swept the gas and
debris with the solar “gusts” to the fringes of the
solar system.
• The gusts were super-heated blasts of gas
escaping from the young, less-structured Sun.
• Gas flows like this are seen mostly in young
stars.
• Similar occurrences still happen today, but less
often than the early days.
Cleaning Up the Solar System-3
• There are still questions to be answered
(and asked) about the origin of the Solar
System.
• How can we confirm the theory?
• The best way would be confirmation of the
theory by observing other stars just
beginning the process.
Other Planetary Systems
Other Planetary Systems
• We are very interested in searching for
extra-solar planets.
• The hope is that in finding these far-flung
planets we will be better able to
understand how our own planet formed.
• Directly seeing these planets is currently
impossible.
• We do have other methods that will allow
us to “see” their presence.
Other Planetary Systems-2
• The search is on to find systems that are just
beginning to form, according to the solar nebula
hypothesis.
• They should be surrounded by a disk of dust
and gas many AU across.
• Recently, disks like these have been found.
• Some of them even have “lumps” which may
represent the first clustering of planetesimals.
Other Planetary Systems-3
• In some cases the disks show a strong
ring, perhaps caused by planets that have
already formed.
• The disks exist, but do the planets?
• Most present evidence is from the effect of
the planets on the star they orbit.
• Newton’s 3rd Law explains the “wobble”
that we see when we observe some stars.
Other Planetary Systems-4
• The wobble of the star creates a Doppler shift in
the star’s light that astronomers can measure.
• From the shift and its change in time,
astronomers can deduce the planet’s orbital
period, mass and distance from the star.
• Based on current data collected, the systems
that have been discovered are very different
from ours.
• Most of these planets are very large and orbit
close to their star.
Other Planetary Systems-5
• Does this disprove our model of solar
system formation?
• Maybe. But more likely it is simply a byproduct of how we are searching.
Other Planetary Systems
Other Planetary Systems
Other Planetary Systems
Other Planetary Systems
Migrating Planets