Astronomy 100:
Formation and Structure of the Solar System
What are the properties of the solar system?
How are these properties explained by theories
of the formation of the solar system?
Why are the planets different from one another?
Characteristics of the Solar System
(formation theory must explain all)
1. All of the planets orbit in the same direction and in the same
plane. (Within a few degrees) This plane corresponds with
the equator of the sun.
– Exceptions: Mercury (7 degrees), Pluto (17 degrees)
2. Most of the planets rotate in the same direction and have
their equators roughly aligned with the plane of the solar
system.
– Exception: Venus rotates in the opposite direction (retrograde).
– Exception: Uranus and Pluto highly tilted (~90 degrees) with respect
to the plane of solar system.
Characteristics of the Solar System
3. Orbits of moons around planets are in the planet’s equatorial
plane.
– Exception: Earth’s moon rotates in plane of the solar system
– One of Neptune’s moons orbits retrograde.
4. Two (three?) types of planets…
– Small rocky planets (Mercury, Venus, Earth, Mars)
– Large gaseous planets (Jupiter, Saturn, Uranus, Neptune)
– Tiny icy planets? (Pluto?)
Terrestrial Planets
Small and Rocky. Crust mainly composed of Silicon
and Oxygen (Silicates).
Atmosphere ranges from none to thick. Atmosphere
Terrestrial Planets
Near to the sun. (< 5 astronomical units)
– Reminder of what an astronomical unit is….
Small and Rocky. (From 0.1 to 1 Me)
– High density (volume/mass) (3-5x the density of water).
• How do we measure the density?
– Crust mainly composed of Silicon and Oxygen (Silicates)
with small fraction of Sulfates.
• Exception: Earth has higher percentage of carbonates in its rocks.
– Cores of Nickel and Iron
• Some solid, some liquid
Terrestrial Planets
Cratering is common on the
surface of terrestrial planets.
From the ages of the craters, we
can tell that impacts were
much more common early in
the history of the solar system
Terrestrial Planets
Atmosphere ranges from none (Mercury) to thick
(Venus).
– Typical atmospheric components are Carbon Dioxide
(CO2) and Nitrogen (N2)
• Exception: Earth has Oxygen (O2) and Water (H2O) in its
atmosphere.
Typically have no moons.
– Exception: Earth has a very large moon.
– Exception: Mars has two tiny moons.
Terrestrial Planet Interiors
Jovian (Gas Giant) Planets
Jovian (Gas Giant) Planets
Primarily composed of hydrogen (H2) and Helium (He)
Massive (15 to 300 Me)
Thick Atmosphere
– H2, He, Methane (CH4), Ammonia (NH3)
Low density (0.7 to 1.8x the density of water)
– Saturn would float!
Small rocky core surrounded by huge ocean of liquid hydrogen.
– Is the core a terrestrial planet?
All are found more distant than about 5 astronomical units from
the sun.
Jovian Planets
Rings! All Jovian planets have them.
Jovian Planets
Moons! Jovian planets tend to have very many
Jovian Planets
Interior
Atmosphere
Liquid
Liquid
Metallic
Hydrogen
Hydrogen
Core
Jovian Planets
• Some (Jupiter and Saturn at least) radiate
more energy than they receive from the sun.
– They generate energy from gravitational
contraction.
Tiny Icy Planets
• Pluto? Charon? Are they really planets
• The moons of the Jovian planets?
• These are like the terrestrial planets, but
instead of SiO2 they have H2O
• Tiny rocky core underneath the ice.
Characteristics of the Solar System
5. Three types of space debris:
– Asteroids:
• Chunks of rock between 10
meters and a thousand km
in size.
• ~20,000 of them
• Concentrated in the plane
of the solar system between
Mars and Jupiter
Characteristics of the Solar System
• Comets:
– Chunks of ice and rock between
10 meters and a thousand km in
size.
– Billions? of them
– Most reside outside the orbit of
Pluto in the Oort cloud. Not
concentrated into the plane of
the solar system
– Occasionally one will fall into
the inner solar system (on a
very elliptical orbit).
Characteristics of the Solar System
• Meteoroids:
– Tiny bits of rock and metal
– Most <1 gram
– Heated by atmospheric
friction until they glow.
– Most follow along the orbits
of comets
• Debris left behind when a
comet goes by.
Characteristic of the Solar System
6. Age:
• The objects in the solar system are all about 4.6
billion years old.
• How do we know this?
– Radioactive dating
• A radioactive element decays into a daughter
element.
– We don’t know what time a specific atom will decay but
we know how long it will take for half the atoms to decay.
– (Demo)
Radioactive Dating
• U238>Pb206
– Halflife:
• 4.5 billion years
– Oldest earth rocks
• 3.96 billion years
– Meteors and Moon
rocks
• 4.6 billion years
• This is the time they
solidified… The
solar system is older
than this.
Theory of the formation of the Solar
System
• A story the fits the facts….
– Needs to explain, or at least be consistent with all the
characteristics that we listed.
– Needs to also be consistent with what we know about the rest of
the galaxy. Other stars and solar systems should form in the same
way.
The Solar Nebula Theory
• The sun and solar system formed from the collapse of a
cloud of gas and dust.
– The cloud was slowly rotating, so centrifugal force made it into a
disk (accretion disk) transferring matter to the center.
– Conservation of angular momentum made it rotate more quickly
The Solar Nebula Theory
– Instabilities in the disk may have formed smaller sub-disks where
giant planets formed
The Solar Nebula Theory
– Dust, rock and ice condense and stick together to make small
bodies called planetesimals.
– Heat from the forming sun only allowed certain elements to
condense nearby. Ices could only condense far away.
The Solar Nebula Theory
• Two ways of building planets
– Larger planetesimals attract smaller ones. They collide and merge
to make a bigger planetesimals. These attract more and eventually
form the planets
– Near the sun, the nebular hydrogen gas is too hot (moving to fast)
to form an atmosphere around the planets. Distant planets begin to
form hydrogen atmospheres once they get big enough.
• The Jovian planets captured their atmospheres.
• As time goes on the nebula cools, making the “frost line”
move inward.
The Solar Nebula Theory
• The young planets start out fairly warm (in a liquid or nearly liquid state). Heavy
elements start to sink… This concentrates the radioactive elements in the center
(and explains why the earth’s core is hot).
Differentiation
•
This process is still occurring in the giant planets. It releases gravitational energy.
The Solar Nebula Theory
• On the terrestrial planets, gasses are released from the hot
interior to form atmospheres.
– Volcanic processes release H2S, SO2, CO2, H2O, NH3, N2
– Solar UV radiation breaks apart NH3, hydrogen escapes, leaving
N2
– Solar UV radiation also breaks apart H2O, hydrogen escapes
leaving O, which reacts with rock to form solid oxides.
– H2O combines with H2S, and SO2 to make sulfuric and sulfurous
acid. This eats away rock to form solid sulfates.
• What’s left? CO2 and N2 in the atmosphere. Oxygen and Sulfur in
the rocks. (Why is Earth’s atmosphere different?)
Where did the nebula go?
• Solar wind, heat, and light pressure drove the gas away.
• What about the left over planetesimals?
– Most of the rocky ones in the inner solar system eventually collided with
planets. (That’s why the rate of impacts was high 4 Gya, but is low now.)
• There’s about 20,000 left over mostly between Mars and Jupiter (Asteroids!)
• Jupiter’s gravity prevented a planet from forming there.
– Encounters with the giant Jovian planets kicked most of the remaining icy ones
into the outer solar system or interstellar space
• These are comets!
• The encounters would kick them in any direction. (This explains why comets aren’t
concentrated in the plane of the solar system.)
How does this theory fit the
characteristics of the Solar System?
1. & 2. Collapse to a disk explains the concentration in the plane of the
solar system, and why almost everything moves in the same direction.
3. The giant planets had disks of their own so their moons orbit in their
equatorial plane
4a. Because the inner solar system was hot, only rock and metal could
condense which resulted in terrestrial planets
4b. The outer solar system was cold enough for ices to condense and for
hydrogen gas to be captured by a massive enough body. This resulted
in Jovian planets.
4c. If an object in the outer solar system wasn’t massive enough to
capture hydrogen gas, it remained as a small icy body. (Pluto, the
outer planet moons, comets)
How does this theory fit the
characteristics of the Solar System?
4d. The terrestrial planets released their atmospheres from their interiors.
The Jovian planets captured theirs. The icy planets weren’t massive
enough to capture one, or hot enough to release one.
4e. The inner structure of the planets is explained by differentiation.
Heavier elements sink to the core. Lighter ones float to the surface.
5. Asteroids and comets are left over planetesimals. Meteors are bits of
dust that have fallen off of comets
6. Everything is the same age because it all formed at about the same
time.
What about the exceptions?
For every exception there is a rule...
• Tilted orbits of Mercury and Pluto.
– Mercury probably suffered a large impact late in its formation
– Pluto might be a left-over planetesimal.
• Retrograde rotation of Venus:
– Probably due to a large impact late in formation.
– Probability favors, but does not require, rotation in the same direction as the
orbit.
• High axial tilt of Uranus and Pluto:
– Also likely to be due to a large impact
– Also, in the outer solar system, computer models suggest the nebula was less
concentrated in the plane, which could result in large tilt of sub-disks.
For every exception there is a rule...
• Retrograde moon of Neptune.
– Probably a captured planetesimal.
• Oxygen in the atmosphere of earth.
– Earth’s atmosphere is highly modified by life.
• Earth’s moon orbits in the plane of the solar system.
– This is likely because the moon was formed from an impact with another body
traveling in the plane of the solar system.