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 ‘free-fall’ time, tff, must be less than the ‘sound-crossing’ time, tsc. tff ∝

√ρ1

tsc ∝ D√TThere is a minimum size (and mass) for clouds which can collapse.

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!

Wikipedia: Protoplanetary disk

It seems that jets — possibly driven by magnetic fields — may slow disk rotation and allow gas to flow in.

Three-Trillion-Mile-Long Jet From a Wobbly Star

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.

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: Jovian Planets

1. Outside the frost line, icy planetesimals were very common, forming planets about 10 times the mass of Earth.

2. These planets attracted nearby gas, building up giant planets composed mostly of H and He.

3. The disks around these planets produced moons.

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.

Wikipedia: Nice Model

Planet Migration: The Nice Model

Migration is expected whenever planets interact with disks; did this happen in our Solar System?

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