Origin of the Solar SystemLook for General PropertiesDynamical RegularitiesOrbits in plane, nearly circularOrbit sun in same direction (CCW from North pole)Rotation Axes perpendicular to orbit plane(Sun & most planets; Uranus exception)Planets contain 98% of angular momentumSpacing and CompositionSpacing increases with distance (roughly logarithmic)Composition varies with distance inner 4: rocky, small, thin atmospheres outer 4: gaseous, large, mostly atmosphereSun contains 99.9% of massd1d212*d2d1~1.5 - 2Current Properties of the Solar SystemPlutoMercuryVenus - NeptuneSide ViewDistancefrom Sun.1 1 10 100 AU 1 1 1 1 (Log scale) M V E M J S U N P M (M⊕ ) 0.06 1 < .001 95 17 .82 .11 318 15 0.002 TerrestrialGas GiantsAsteroidsRocky - iron, silicates, …“Icy” - at time of formation (H2O, NH3, CH4, …)Gaseous - H, HeThe Solar SystemWhat is a Planet? I. Small end…• Pluto much smaller than others (0.002 Mearth)• Other, similar objects found in Kuiper Belt– Including one larger than Pluto (Xena)– Known as Kuiper Belt objects or ice dwarfs• IAU committee split on definition– 1. Demote Pluto to one of KBOs– 2. Include all larger than 1000 km• At least 17 totalAll start with rotating diskMinimum mass: 0.01 MSum of planets ~ 0.001 M but most of H2, He lostNote: Similar to typical masses of disks around forming starsSome models assume more massive disksTemperature, Density decrease with distance from forming star(Observations suggest slower decrease than models usually assume)DUST PLAYS A KEY ROLETheory of Solar System FormationNo DustDust CoresIron, Silicon, Oxygen, CarbonTerrestrial PlanetsDust cores and Icy Mantles(H2O, NH3, CH4)Outer PlanetsArtist’s conception of dust in diskAccretion of Dust GrainsFig. From talk by Jurgen BlumDust sinks to midplaneGravitational instability planetesimals (~ 1 km in size)and/orAccretion of dust grains~104yrCollisions between planetesimals builds rocky planet cores106 - 108 yrproblemGas Processes (Outer Planets)Accretion of gas/gravitational collapse onto rockycoresLeads to H, He in atmosphereRings, moons (minature solar system)Core Accretion ModelInterstellar dust - core + mantleSilicate (Si + 0 + …)or GraphiteMantle: H2O, NH3, CH4, …?Planet typesInner: Only rocky cores, little or no ice survives rocky planetsOuter: Ice survives comets, icy moons of outer planetsOutgassingPlanet heats internally, so ice turns to gas (atmosphere)Uranus and Neptune (thick atmospheres, formerly icy materials)If pressure, T suitable, may form liquid and get ocean (Earth)Dust and IceFormation of Gas Giants (Jupiter, Saturn)1. Planet formation in a rotating disk with icy dust can explain most ofthe general facts about our solar system2. Planetary systems are likely to be common since disks with M > Mminare common around forming stars.If we are typical,3. Expect other planetary systems will have ~10 planets, logarithmicspacing, different planet types~General Expectations about PlanetarySystemsTheory Predicts Forming Planets Clear aGapCan we observe such gaps?Possible Evidence for Planet FormationSMM image of VegaJACH, Holland et al.SMM image of Vega shows dustpeaks off center from star (*). Fits amodel with a Neptune like planetclearing a gap. Can test by looking formotion of clumps in debris disk.Model by Wyatt (2003), ApJ, 598, 1321Issues for Planet Formation• The time to build up the giant planets fromdust particles is long in theories– Gas has to last that long to make gasgiants• How long do dust disks last?– How long does the gas last?• Are there faster ways to make planets?• What about planet building for binary stars?Time Available to form planets• The disks around young stars can formplanets• How long do the disks last?– Sets limit on time to form planets– Most gone by 3 to 5 Myr– Little evidence that gas stays longer– Some “debris” around older stars– May be evidence of planet buildingDisks versus Age of StarEvidence for CollisionsFormation of Gas Giants (Jupiter, Saturn)Binary Stars• About 2/3 of all stars are in binaries– Most common separation is 10-100 AU• Can binary stars have disks?– Yes, but binary tends to clear a gap– Disks well inside binary orbit– Or well outside binary orbitBrown Dwarfs• Stars range from 0.07 to ~100 Msun• Jupiter is about 0.001 Msun• Brown dwarfs between stars and planets– Dividing line is somewhat arbitrary– Usual choice is 13 Mjupiter– Brown dwarfs rarely seen as companions to stars– But “free-floaters” as common as stars– Many young BDs have disks• Planets around BDs?What is a Planet? II. High end…• Brown dwarfs now found to very low masses– Some clearly less than 13 Mjupiter• Can’t even fuse deuterium• Some people call these planets• Some less massive than known planets– Usual definition: planets orbit stars• Some brown dwarfs may have “planets”• Nature does not respect our human desirefor neat categories!Other Active Issues• Other planetary systems are quite different– Big planets in close– But this is probably due to selection effect• Locations may differ with mass of star– Ices survive closer to lower mass star– May get ice giants in close– Also planets may migrate inwards– May prevent formation of terrestrial planetsFormation of EarthSolid particles ⇒ silicate + ironNo gas collected ⇒ atmosphere outgassedRadioactive heating ⇒ molten core ice gas H2O gas liquid (oceans)CO2 dissolve in oceans carbonate rocksN2 gasEarly Earth AtmosphereN2, CO2 , H2O (CH4, NH3, H2 ?)Reducing (No free O2) Neutral ?Energy SourcesDifferentiation of the EarthImpact heating by planetesimals (release of gravitational potential energy)Radioactive nuclei decay (release of nuclear potential energy)unstablenucleusNuclearpotentialenergySmaller nuclei(Fission)Also emit α particles (He)electrons, gamma-rayse.g. 40K Potassium 238U UraniumKinetic energy heatResult: molten EarthIron-Nickel center (core)Silicates float upper levels (mantle)Differentiation released Grav. Potential energy hot
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