Origin of the Solar System
Stars spew out 1/2 their mass as gas & dust as they die
In the interstellar medium, dust and gas coalesces into clouds
New generations of stars (and their planets, if any) form in
these clouds
Nebular theory
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Interstellar cloud of gas &
dust collapsed under its own
gravity
Prediction: protoplanetary
nebulae should be observed
Explains all of the major
features of solar system, and
also the exceptions
Observations continue to
support this theory
Protoplanetary disks
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Protoplanetary disks last for only about 1-10 million years
The next billion years: Debris disks
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Gas and fine dust blows away after
~ 10 million years
Jupiter must have formed by then
Older stars have ‘debris disks’
around them
Need a supply of larger objects to
regenerate the dust that gets
blown away
evidence of planets forming around
other stars
Debris disks are analogous to the
Oort cloud and Kuiper belt of
comets, and the asteroid belt
Debris disks around stars > 100 million years old are very
common!
(artist’s drawing of a debris disk)
Zodiacal light
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Any GOOD hypothesis about the origin of the solar system
must explain most - if not all - of its characteristics:
1. All of the planets orbit the sun in the same direction,
and in the same plane
2. The planets closest to the sun are small and rocky,
have few moons
3. The planets further from the sun are large and
contain more gas and icy materials
4. Most of the Moons orbit their planets in the same
direction as the planets orbit the sun
5. Oldest meteorites are about 4.566 billion years old
6. Planetary surfaces are all younger than the oldest
meteorites
Relative sizes of the planets
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Sizes of the planets relative to Sun
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Sun-planet distance (relative to Earth: AU)
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
0.4 AU
0.7
1.0
1.5
5.2
9.5
19
30
1 AU = 150 million km
Other residents of the solar system:
1. Dwarf planets
diameter = 1000-3000 km, smaller than Moon, orbit the sun
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Other residents of the solar system
2. Asteroids - rocky, d < 1000 km, orbit the sun
Asteroid belt
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Asteroids are
really quite rare…
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Other residents of the solar system
3. Comets - rock & ice, wide
range of sizes (~10 m to
100 km)
Other residents of the solar system
4. Moons - orbit planets, some are larger than Mercury
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Asteroids and comets
are leftover
planetesimals
Some moons are
captured
planetesimals
Other residents of the solar system
5. Meteoroids - small fragments of asteroids that enter
earth’s atmosphere (dust to boulder sized)
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Meteor!
Zodiacal light
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Any GOOD hypothesis about the origin of the solar system
must explain most - if not all - of its characteristics:
1. All of the planets orbit the sun in the same direction,
and in the same plane
2. The planets closest to the sun are small and rocky,
have few moons
3. The planets further from the sun are large and
contain more gas and icy materials
4. Most of the Moons orbit their planets in the same
direction as the planets orbit the sun
5. Oldest meteorites are about 4.566 billion years old
6. Planetary surfaces are all younger than the oldest
meteorites
Protoplanetary disks last for only about 1-10 million years
H, He gas is present throughout the disk
Icy compounds and rock/metal
Rock & metal
ice line
Condensation: gas becomes solid
What are the planets made of?
Element
how many atoms gas or solid at
(total)
Earth
Jupiter
________________________________________________
Hydrogen
705,700
gas
gas
Helium
275,200
gas
gas
Carbon
3,032
gas
soot (solid)
Nitrogen
1,105
gas
ice
Oxygen
5,920
H2O gas
H2O ice
Silicon
653
rock
rock
Iron
1,169
metal
metal
Planet formation: Terrerstrial vs. giant planets
Giant (“jovian”)
1. Lots of solids in the
disk (cold > 5 AU)
2. Cores form from
ice, rock and metal
3. Grow large, quickly
(~1 million years)
4. Big enough to trap
H and He gas from
disk
Terrestrial (“earth like”)
1. Very little solid material in
disk at 1 AU
2. Form from rock and metal
only
3. Grow slowly (~100 million
years)
4. Too small to trap any gas
from disk
Connecting the dots: From planet formation to early Earth
Computational astrophysics meets field geology!
Hot+Dry (H2O gas)
1 million years
H2O ice
habitable zone
10 million years
Jupiter
>100 million years,
3.8 billion years ago
Terrestrial planets form by accretion of solids
Dust >rocks >planetesimals >embryos >planets
The Moon-Forming Event
t=0 : IMPACT!
6 minutes
20 minutes
32 minutes
•A protoplanet the size of Mars (1/10 Earth’s mass) struck Earth, forming t
Moon 4.5 billion years ago
•Oceans boiled away, silicate-vapor atmosphere for at least 1 Myr
•Earth had already differentiated into core & mantle structure by this time
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But what if you don’t know:
• the initial number of parent & daughter atoms?
• how much of the P & D’s have entered or left the rock?
Solution: Isochron dating, requires a 4th measurement
(the amount of a stable isotope of one of the
elements)
87
87
48.8 Gyr
Ru Sr

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Slope = D(now)/P(now)
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solid
melt
Make measurements for different minerals in rock. If
data are linear, there is a strong correlation between:
•The amount of P in each sample
•The extent to which the sample has been enriched in D
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87Sr/86Sr
Stable isotope geochronology
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87Rb/86Sr
Formation of Jovian Planets: Fast! (< 10 Myr)
Core accretion: icy planetesimals clump together first
Gravitational instability: dense clump of nebular gas
forms first
The Nebular theory predicts
most other sun-like stars
should have planets
Do they?
358 planets have been found around other stars!!!
http://www.exoplanets.org
Detecting planets around other
stars: Doppler method
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Transit method (Kepler Mission)
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