Forma on of the Solar System Overview
Observa ons of trends in our solar system, plus observa ons of young stars, yield a coherent picture of the forma on of planetary systems • The laws of physics (Week 3) come into play here. • The major dis nc on between terrestrial planets and Jovian planets comes from where in the solar nebula they formed • The “excep ons” arise from collisions and other interac ons • We can find the age of our solar system by studying radioac ve isotopes in meteorites and rocks. Making a Model A hypothesis for solar system forma on must explain: • Pa erns of mo on of the orbits • The 2 classes of planets • Asteroids and comets • Excep ons • It should be predic ve
Does it apply to other solar systems? Two models 1. Close Encounter – dal stream (Buffon 1745) Physics
• Hot gas will expand due to high pressure, rather than collapsing • Gas pressure ∼ nT – n is gas density – T is the gas temperature • If the pressure exceeds that of the interplanetary medium, it will expand Two models 2. Nebular Hypothesis (Kant 1755; LaPlace 1790) Physics
• Large, cold cloud of gas (D ~ few ly) • Collapse begins • Gravity pulls cloud together • Cloud heats (why?) • Cloud rotates (why?) • Disk forms (why?) • Sun forms at hot center How do we know this happened?
• We see disks around young stars Proplyd : Protoplanetary Disk Edge-on Disks HL Tau:
Environs and ALMA image β Pictoris Debris Disks Planet Forma on
Planet forma on in fla ened disks, dictated by conserva on of angular momentum, explains the shape of our Solar System Frac on of Stars with Disks
Hernandez et al. 2007, ApJ 662, 1067 Towards a Hypothesis:
• Disks are ubiquitous • Disks are a by-product of star forma on • Disks are planar • Disks rotate differen ally • Disk composi ons = stellar composi ons Elemental Abundances
Sun Mass Frac on Earth Mass Frac on H 0.74 Fe 0.32 He 0.24 O 0.30 O 0.010 Si 0.15 C 0.0046 Mg 0.14 Ne 0.0013 S 0.029 Fe 0.0011 Ni 0.018 N 0.00096 Ca 0.015 Si 0.00065 Al 0.014 Mg 0.00058 Cr 0.005 Planet Forma on in a Disk: Condensa on Sequence •Solar nebula had uniform composi on
•Temperature decreases outwards
•Different materials condense at different T
•H and He never condense Condensa on Sequence Temperature (K) Condensate
1500 Fe2O3, FeO, Al2O3 1300 Fe, Ni 1200 Silicates
1000 MgSiO3 680 FeS
175 H2O
150 NH3
120 CH4 65 Noble gases
Solar Composi on at Low Temperature Compe ng Models • Accre on • Gas collapse
Domina ng the Orbit
1/3 Hill radius: rH=a (mp/3M⊙) • a: distance from planet to Sun
• mp: mass of planet
• M⊙: mass of Sun
Planet will sweep up all ma er within rH of its orbit Terrestrial Planets
• Inside frost line: rock/metal condenses • Small size reflects limited material • Seed grow via accre on to make planetesimals • Planetesimals grow via gravity to 102 to 103 km • Only the largest planetesimals survive fragmenta on • This idea is supported by meteorites—metal grains embedded in rock Birth of the Earth • Small dust grains collide and s ck • Once grain becomes large enough, gravity takes over • Runaway accre on ensues.
Chondrite Jovian Planets
• Beyond frost line—H compounds can condense
(ices: CH4, NH3, H2O) • Lots of ice—planetesimals grow large • Can gravita onally capture H and He • Grow very large • Moons form in accre on disks of Jovian planets • Sub-nebula also has temperature gradient Jupiter as a Miniature Solar System
Core Accre on or Gravita onal Instability? • Core accre on difficul es: – Core growth takes longer than typical disk life mes (few Myr) – Larger disks or lower opacity can help – Turbulence in the disk may help • Gravita onal instability difficul es: – Requires disk to cool – Only operates in massive disks and at large orbital distances (> 100 AU) – May not be able to explain the higher metallici es in giant planets – Gravita onal instability works quickly—young (1 Myr old) stars should already have gas planets
End of Planet Forma on
• Solar wind / radia on pressure blows disk away • Gaseous phase ~ 10 million yr • Strong magne c field transfers angular momentum outward • Supported by observa ons of young stars Frac on of Stars with Disks
Hernandez et al. 2007, ApJ 662, 1067 Resul ng Solar System Inside Frost Line: small rocky planets
Outside Frost Line: large gaseous planets The Debris • Solar wind removed gas – Small planetesimals remained • Asteroids: remaining rocky planetesimals – Planet forma on inhibited between Mars and Jupiter – Ini ally lots of planetesimals – Most crashed into inner planets or were ejected • Comets: remaining icy planetesimals – Ini ally all throughout outer solar system • KBOs – accreted too slowly Craters
Comparison of ~ 30 km craters on different bodies.
Names and loca ons : • Golubkhina (Venus), 60.30N, 286.40E; • Kepler (Moon), 8.10N, 38.10W; • (Mars), 20.80S, 53.60E; • (Ganymede), 29.80S, 136.00W.
Image Credit: Image of Ganymede Crater contributed by Paul Schenk (Lunar and Planetary Ins tute). Image of Mars crater obtained from the Mars Mul -Scale Map, Calvin Hamilton (Los Alamos Na onal Laboratory). Images of lunar and venusian craters from Robert Herrick (Lunar and Planetary Ins tute). Barringer Crater, Arizona
Diameter: 1.2 km. Depth: 170m. Rim: 45m. Age: 50,000 years. Impactor: 50m Fe-Ni meteor
Origin of the Moon (Luna) Luna is too large to have been captured by Earth • Composi on different than Earth – Moon has lower density (less iron/nickel) – Could not have formed in same place/ me as Earth • Giant Impact – Many large planetesimals le over during SS forma on – Collision between proto-Earth and Mars-sized object • Possible outcomes – Change in axial lt – Change in rota on rate – Complete destruc on – outer layers of Earth blown off
Support for Collisional Lunar Origin
• Moon's composi on matches outer layers of Earth • Moon has deficit of vola les: all were vaporized • Other large impacts: – Pluto/Charon ? – Mercury – Uranus – Venus? (perhaps more complicated) The Nice Model Series of papers in Nature in 2005 • Solar system: Sun, planets, debris disk of planetesimals – Planets accrete or sca er planetesimals – Angular momentum exchange causes planetary migra on • Jupiter moves inward, Saturn, Uranus, and Neptune move outward • 1:2 orbital resonance between Jupiter and Saturn reached – Kick in eccentrici es, destabiliza on of orbits – Uranus and Neptune sca ered outward, switch posi ons • Small bodies move inward • Interac ons explain current orbital radii and eccentrici es • 1:2 resonance explains late heavy bombardment period – Ini al geometry can give resonance (and hence sca ering) at right me. – Asteroids will also be perturbed at this me. What is a Resonance? The Asteroid Belt • Why not another terrestrial planet? • Total mass ~ 1 lunar mass • Perturba ons by Jupiter
Kirkwood Gaps Age of the Solar System
• Radiometric da ng: measure solidifica on age – Look at propor ons of isotopes and atoms • Radioac ve decay: – Breaking apart or change (p+ into no) of nucleus – E.g. 40K becomes 40Ar – Parent isotope: 40K – Daughter isotope: 40Ar • Half-life: me it takes for ½ of parent nuclei to decay
Radiometric Da ng
Useful radioisotopes
14 14 C → N: t1/2 = 5730 years
26 26 Al → Mg: t1/2 = 717,000 years
40 40 K → Ar: t1/2 =1.25 billion years
238 206 U → Pb: t1/2 = 4.47 billion years
87 87 Rb → Sr: t1/2 = 49.4 billion years
Radiometric Da ng
• Rock forms with 40K but no 40Ar • Any 40Ar you find in the rock is due to radioac ve decay • Remains trapped in the rock unless heated • Ra o è age • Moon rock da ng uses U and Pb (note: different chemical proper es + understanding minerals) Result: lunar rocks ~ 4.4 billion years old The Age of the Solar System
• Radiometric da ng ≈ solidifica on age • Earth rock age < SS age (surface reshaping) • Moon rock age < SS age (impact) • Meteorite ages: – Have not melted or vaporized since SS forma on – Age ~ 4.55 billion years • Age consistent with solar evolu on theory