Lecture 32: The Origin of the Solar System

1. Lecture 32:The Origin of the Solar System Astronomy 161 – Winter 2004

2. Key Ideas: • The present-day properties of our Solar System hold important clues to its origin. • Primordial Solar Nebula: • Process of the Sun’s formation • Condensation of grains & ices • From Planetesimals to Planets: • Aggregation of small grains into planetesimals • Aggregation of planetesimals into planets • Terrestrial vs. Jovian planet formation.

3. The Birth of the Solar System • The present-day properties of the Solar System preserves its formation history. • Relevant Observations: • Orbits of the planets and asteroids. • Rotation of the planets and the Sun. • Compositions of the planets

4. Clues from motions • Orbital Motions: • Planets all orbit in nearly the same plane. • Most planet's orbits are nearly circular. • Planets & Asteroids orbit in the same direction • Rotation: • Axes of the planets tends to align with the sense of their orbits, with notable exceptions. • Sun rotates in the same direction as planets orbit. • Jovian moon systems mimic the Solar System.

5. Pluto

7. Clues from planet composition • Inner Planets & Asteroids: • Small & rocky (silicates & iron) • Few ices or volatiles, no H or He • Jovian Planets: • Large ice & rock cores • Hydrogen atmospheres rich in volatiles. • Outer solar system moons & icy bodies: • Small ice & rock mixtures with frozen volatiles.

8. Rocky Planets Giant Gas Planets Mostly H, He, & Ices Icy Pluto

9. Formation of the Sun • Stars form out of interstellar gas clouds: • Large cold cloud of H2 molecules and dust gravitationally collapses and fragments. • Rotating fragments collapse further: • Rapid collapse along the poles, but centrifugal forces slow the collapse along the equator. • Result is collapse into a spinning disk • Central core collapses into a rotating proto-Sun surrounded by a “Solar Nebula”.

10. Cold Interstellar H2 Cloud

11. Interstellar Cloudof H2 and Dust

12. Stellar-mass fragment

13. Gas & dust disks observed around young stars

14. Primordial Solar Nebula • The rotating solar nebula is composed of • ~75% Hydrogen & 25% Helium • Traces of metals and dust grains • Starts out at ~2000 K, then cools: • As it cools, various elements condense out of the gas into solid form as grains or ices. • Which elements condense out when depends on their “condensation temperature”.

15. Condensation Temperatures

16. The “Frost Line” • Rock & Metals form anywhere the gas cooler than 1300 K. • Carbon grains & ices only form when the gas is cooler than 300 K. • Inner Solar System: • Too hot for ices & carbon grains. • Outer Solar System: • Carbon grains & ices form beyond the “frost line”.

18. From Grains to Planetesimals • Grains that have low-velocity collisions can stick together, forming bigger grains. • Beyond the “frost line”, get additional growth by condensing ices onto the grains. • Grow until their mutual gravitation assists in aggregation, accelerating the growth rate: • Form km-sized planetesimals after few 1000 years of initial growth.

19. Terrestrial Planets • Only rocky planetesimals inside the frost line: • Collide to form small rocky bodies. • Hotter closer to the Sun: • Inner proto-planets cannot capture or retain H & He gas. • Solar wind also disperses the solar nebula from the inside out, removing H & He. • Result: • Form rocky terrestrial planets with few ices.

20. Formation of a Terrestrial planet

21. Jovian Planets • Ices augment the masses of the planetesimals. • These collide to form large rock and ice cores: • Jupiter & Saturn: 10-15 MEarth rock/ice cores. • Uranus & Neptune: 1-2 MEarth rock/ice cores. • Larger masses & colder temperatures: • Accrete H & He gas from the Solar Nebula. • Planets with the biggest cores grow rapidly.

22. Formation of Jupiter Solar Nebula ProtoSun

23. Moons & Asteroids • Gas gets attracted to the proto-Jovians & forms rotating disks of material: • Get mini solar nebulae around the Jovians • Rocky/icy moons form in these disks. • Later moons added by asteroid/comet capture. • Asteroids: • Gravity of the proto-Jupiter keeps the planetesimals in the main belt stirred up. • Never get to aggregate into a larger bodies.

24. Icy Bodies & Comets • Outer reaches are the coldest and thinnest parts of the Solar Nebula: • Ices condense very quickly onto rocky cores. • Stay small because of a lack of material. • Gravity of the proto-Neptune: • Assisted the formation of Pluto-sized bodies in 3:2 resonance orbits (Pluto & Plutinos) • Disperses the others into the Kuiper Belt.

25. Mopping up... • The whole planetary assembly process took about 100 Million Years. • Followed by ~1 Billion years of heavy bombardment of the planets by the remaining rocky & icy pieces. • Sunlight dispersed the remaining gas in the Solar Nebula gas into the interstellar medium.

26. Planetary motions reflect the history of their formation. • Planets formed from a thin rotating gas disk: • The disk’s rotation was imprinted on the orbits of the planets. • Planets share the same sense of rotation, but were perturbed from perfect alignment by strong collisions during formation. • The Sun “remembers” this original rotation: • Rotates in the same direction with its axis aligned with the plane of the Solar System.

27. Planetary compositions reflect the different environments of formation. • Terrestrial planets are rock & metal: • Formed in the hot inner Solar Nebula. • Too hot to capture and retain Hydrogen & Helium. • Jovian planets contain ices, H, & He: • Formed in the cool outer Solar Nebula • Grew large enough to accrete lots of H & He.