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Lecture 32: The Origin of the Solar System Astronomy 161 – Winter 2004 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. 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 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. Pluto 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. Icy Pluto Rocky Planets Giant Gas Planets Mostly H, He, & Ices 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”. Cold Interstellar H2 Cloud Interstellar Cloud of H2 and Dust Stellar-mass fragment Gas & dust disks observed around young stars 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”. Condensation Temperatures Temp (K) >2000 K 1600 K 1400 K 1300 K Elements all are gaseous Al, Ti, Ca Iron & Nickel Silicon 300 K Carbon 300-100 K H, N Condensate Mineral oxides Metal Grains Silicate grains Carbonaceous Grains Ices (H2O, CO2, NH3, CH4) 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”. 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. 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. Formation of a Terrestrial planet 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. Formation of Jupiter Solar Nebula ProtoSun 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. 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. 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. 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. 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.