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

• 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
Nebular theory

Protoplanetary disks

Protoplanetary disks last for only about 1-10 million years

The next billion years: Debris disks
• 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

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

Sizes of the planets relative to Sun

Sun-planet distance (relative to Earth: AU)
Mercury 0.4 AU
Venus 0.7
Earth 1.0
Mars 1.5
Jupiter 5.2
Saturn 9.5
Uranus 19
Neptune 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

Other residents of the solar system
2. Asteroids - rocky, d < 1000 km, orbit the sun

Asteroid belt

Asteroids are
really quite rare…

3. Comets - rock & ice, wide
range of sizes (~10 m to
100 km)
Other residents of the solar system

Other residents of the solar system
4. Moons - orbit planets, some are larger than Mercury

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)

Meteor!

Zodiacal light

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!

1 million years
10 million years
>100 million years,
3.8 billion years ago
Hot+Dry (H2O gas) H2O ice
Jupiter
habitable zone

Terrestrial planets form by accretion of solids
Dust >rocks >planetesimals >embryos >planets

The Moon-Forming Event
•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
t=0 : IMPACT! 6 minutes 20 minutes 32 minutes

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)
48.8 Gyr
Slope = D(now)/P(now)

melt
solid

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

Stable isotope geochronology
87
Sr/
86
Sr
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

Transit method (Kepler Mission)