Download 21. Solar System Formation

Astronomy 241: Foundations of Astrophysics I
21. Solar System Formation
Star-Forming Clouds
Stars form in cold, dark clouds of dusty gas floating in
space. These clouds are called molecular clouds.
Hubble’s sharpest image of the Orion Nebula
Gravity Versus Pressure
Gravity can’t form stars unless
it can overcome the force of
thermal (gas) pressure.
For this to happen, the ‘freefall’ time, tff, must be less than
the ‘sound-crossing’ time, tsc.
There is a minimum size (and mass)
for clouds which can collapse.
1
tff ∝
√ρ
tsc ∝ D√T
Matthew Bate: The Formation of Stars and Brown Dwarfs
Collapse: Disks Around Other Stars
We can see disks around other stars, as expected if
these stars formed from collapsing gas clouds.
Disks and Jets
Rotation presents a barrier to star
formation; unless most of the
original angular momentum is lost,
gas can’t reach the center!
It seems that jets — possibly driven
by magnetic fields — may slow disk
rotation and allow gas to flow in.
Wikipedia: Protoplanetary disk
Three-Trillion-Mile-Long Jet From a Wobbly Star
Planet Formation: Terrestrial Planets
1. Within the frost line, bits of rock and metal clumped
together to make planetesimals.
2. As the planetesimals grew, they became large enough
to attract each other.
3. Finally, only a few planets were left.
Planet Formation: The Frost Line
The disk was hot at the center, and cool further out.
Inside the frost line, only
rocks & metals can condense.
Outside, hydrogen compounds
can also condense.
The frost line was between the present orbits of Mars
and Jupiter — roughly 4 AU from the Sun.
ALMA Image (Nov. 6) of HL Tau (< 1 Myr) Inner ring at ~20-­‐30 AU, second ring at ~70 AU Impacts & Encounters
1. Giant impacts in early solar system:
— explain rotation of Uranus,Venus
— form Moon from collision debris
2. Satellite capture after near-miss:
— moons of Mars captured from asteroid belt?
— Triton captured from Kuiper belt
Planet Formation: Asteroids and Comets
• “Leftovers” from early stages of planet formation
• Asteroids form inside frost line, comets outside
• Scattered by jovian planets into present orbits
Planet Migration
A planet embedded in a disk around a star can excite
spiral waves — this process robs the planet of angular
momentum, causing it to spiral inward.
Planet Migration: The Nice Model
Migration is expected whenever planets interact with
disks; did this happen in our Solar System?
Wikipedia: Nice Model
1. Giant planets
born closer to Sun;
icy planetesimals
orbit in outer disk.
2. Jupiter & Saturn
migrate into 2:1
resonance; Uranus
& Neptune switch.
3. Planetesimals are
scattered outward,
populating Kuiper
belt & Oort cloud.
Planet Migration
1. Can explain hot jupiters and eccentric orbits
— migration can move planets very close to star
— encounters between planets disturb orbits
2. Did this happen in our solar system?
— disk cleared by Sun’s wind or external effects
— some migration needed to form Oort cloud