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 1 tff ∝ the ‘sound-crossing’ time, tsc. √ρ There is a minimum size (and mass) tsc ∝ D√T 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! 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 Outside, hydrogen compounds rocks & metals can condense. 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 2. Jupiter & Saturn 3. Planetesimals are born closer to Sun; migrate into 2:1 scattered outward, icy planetesimals resonance; Uranus populating Kuiper orbit in outer disk. & Neptune switch. 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.