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Formation of Solar System • Nebular Hypothesis (Laplace, 18th century) – Rotating sphere of gas flattens into spinning disk – Contraction produces a protostar at the center – Material in surrounding disk becomes the planets • Problem: angular momentum… – Sun is 99% of mass of solar system – Planets have 99% of angular momentum – Shouldn’t be that way… Sun should be spinning faster than once every 25 days (because it’s at the center) Formation of Solar System • Where did all the stuff come from? • How old is all the stuff? Formation of Solar System • Where did all the stuff come from? – Stars come from clouds of molecular gas and dust. – Masses from 0.08 Ms to 90 Ms – Solar systems like our own probably limited to 1-3 Ms • Too big – too violent and short-lived • Too small – unlikely to ever form planets Formation of Solar System • How old is all the stuff? Age of meteorites = Age of solar system = Age of earth Formation of Solar System • Star forming nebula and circumstellar disks Formation of Solar System • Dust to protostars – Dense clouds litter the galaxy – Low temperature (10 K) – High density (1000 to 1 million times greater than interstellar medium) – Rich collection of molecules (but mostly H) – 1015 km in diameter – 2000 or so in Milky Way – Favorable places for star formation (and, therefore, perhaps planetary formation) Formation of Solar System • Dense clouds associated with young stars (“stellar nurseries”) Formation of Solar System • Problem… – Gravity. What does it do? ATTRACTS – So all clouds should eventually contract under their own gravity. – But they all don’t! Formation of Solar System • Solution… – Jean’s Mass • Gravity is counteracted by internal pressure of the cloud. • Internal pressure comes about by temperature and density of a spherical cloud • There will be a balance between pressure and gravity… • Until you exceed a certain mass (Jean’s mass) Formation of Solar System • Contracting dense cloud… – Becomes a disk because of angular momentum • In plane of rotation, centripetal and gravitational forces are in equilibrium • Normal to plane of rotation, gravity and pressure compete. Gravity wins, pulling material into disk – Heats up • Gravitational energy converted to kinetic energy (and temperature) via collisions • After few kyr, edge of cloud ~2000-3000 K • Cloud becomes opaque at center (due to density), radiation is trapped, and gets hotter • Star ignition temperature ca. 106 K achieved in 108 yr Formation of Solar System • Violent early phase… – Young protostars go through the “T-Tauri” phase where strong, bi-polar outflows of 50 km/s jets of material form. – protostar can loose up to 0.5 Ms within 106 yr Formation of Solar System 1. Dense cloud collapse (0.1 – 0.5 Ma) 2. Disc dissipation: some material transported toward protostar (0.05 Ma) 3. T-Tauri phase (1-2 Ma) 4. Gas dissipation: planetary accretion & residual nebula removed (3 – 30 Ma) • What about angular momentum problem…? Viscous drag: particles in disc interact, stealing/transferring some angular momentum from star (slowing its spin) towards edges Formation of Solar System • Star with a disk around it… now what? Formation of Solar System • • • • • • Condensation Coagulation Planetesimals Planetary growth Planetary migration Satellites, rings, asteroids, comets Formation of Solar System • • • • • • Condensation Coagulation Planetesimals Planetary growth Planetary migration Satellites, rings, asteroids, comets Formation of Solar System • Condensation – Away from star, disk will be relatively cool (400 K at 4.5 AU, 3000 K at 1 AU) and material will form compounds/minerals (not necessarily solid, though) H2O CH4 FeS Al2O3 CaMgSi2O6 Formation of Solar System • Temperature is key… – Refractory: condense at high T – Volatile: condense at low T Formation of Solar System Formation of Solar System • • • • • • Condensation Coagulation Planetesimals Planetary growth Planetary migration Satellites, rings, asteroids, comets Formation of Solar System • Now we have particles of condensed stuff colliding with one another. What happens? Formation of Solar System • They break. – If collisions violent enough • They bounce. – Elastic interactions • They stick. – This is coagulation – By magnetism? Static electricity? Friction? Fluffy particles with lots of cavities (e.g. velcro effect)? Formation of Solar System • Coagulation will occur rapidly… – – – • 10 mm particles at 1 AU in 2 kyr 15 mm particles at 5 AU in 5 kyr 0.3 mm particles at 30 AU in 50 kyr What limits rate (so that takes longer to happen further out)? 1. Density (e.g. column mass) – decreases outward from Sun 2. Number of collisions/time (density dependent) 3. Thickness of disk (thicker away from Sun) determines timescale for particles to get to midplane Formation of Solar System • • • • • • Condensation Coagulation Planetesimals Planetary growth Planetary migration Satellites, rings, asteroids, comets Formation of Solar System • Remember T-Tauri stage… it could blow particles up to 10 m out of the solar system. • So we have to get up to this size and larger before ca. 1 Ma! • Coagulation can’t do it, though. Formation of Solar System • How do you get 0.1 to 10 km planetesimals? – Gather coagulants into turbulent knots in thinning disk – Gravity draws material together (focusing) in collisions that have a net effect of accretion (rather than dispersal) – At given distance from Sun, runaway growth will promote development of one or two dominant planetary embryos that sweep up all material Formation of Solar System Formation of Solar System • • • • • • Condensation Coagulation Planetesimals Planetary growth Planetary migration Satellites, rings, asteroids, comets Formation of Solar System • Planetary growth in inner solar system – Embryo growth is self limiting as material used up – Embryos every 0.02 AU formed by time of T-Tauri stage – Embryos about 0.1 mass of eventual planets – Subsequent interactions (collisions) take 107-108 yr to complete planet formation Formation of Solar System Formation of Solar System • Planetary growth in outer solar system – Larger rock-ice embryos up to 5 Me – Takes 10x as long to form as in inner solar system – Embryos get large enough to gravitationally trap volatiles e.g. H, He, etc. – T-Tauri stage eventually begins, removing remaining, unbound H and He – Sizes of outer planets determined by amounts of H and He present at those distances (and amount of time available for accumulation before removed) Formation of Solar System • • • • • • Condensation Coagulation Planetesimals Planetary growth Planetary migration Satellites, rings, asteroids, comets Formation of Solar System • New, controversial idea… radii of planetary orbits can change over time! – By 2-30 % (in or out) over 0.1 Ga • Why might this be true? 1. 2. 3. 4. Discovery of icy, minor planets out to 60+ AU Discovery of extrasolar planets size of Jupiter close to their star Volatiles in Jupiter atmosphere consistent with formation at lower T (e.g. further from Sun) than present position implies Orbits of satellites demonstrably change over time (e.g. Moon), so why not that of planets around Sun? Formation of Solar System • • • • • • Condensation Coagulation Planetesimals Planetary growth Planetary migration Satellites, rings, asteroids, comets Formation of Solar System • Satellites, rings, asteroids, comets – Presence of Jupiter and orbital resonance greatly affected distribution of asteroids – Jupiter (and other giant planets) affected orbits of comets – Gravitational focusing affected impact histories of all planets – Protosatellite disks, capture, and giant impacts lead to formation of moons and ring systems