Formation of Terrestrial Planets

1. Formation of Terrestrial Planets Roman Rafikov (Institute for Advanced Study)

2. More than 150 extrasolar planets are known at present. • Most of them have masses typical of gas giants, i.e. • Only planets around pulsar PSR B1257+12 (neutron star) resemble terrestrial planets gas giants ice giants terrestrial planets The most likely scenario of the giant planet formation is a rapid gas accretion onto a preexisting massive solid core. Solar System Extrasolar planets 8 major planets: exoplanets.org nineplanets.org Formation of the Earth-like planets have likely been an important stage of the giant planet genesis.

3. Planetary diversity Lissauer 2004 Currently known to us are • terrestrial planets – consist mainly of solid refractory materials – silicates and iron • gas giants – mostly gas , with solid cores • ice giants – large ice + rock cores , covered with thick atmospheres composed of mainly H and He

4. Pictoris Mouillet et al’97 Initial conditions for planet formation Planets form in protoplanetary disks • Gaseous disks in differential rotation around parent stars • Dust (about 1% by mass) is a material for planet building • Sizes range from 100 to 1000 AU (AU – distance between Sun and Earth ) • Disks live for 1 to 10 million years • Cold (hundreds of K) and geometrically thin

5. Stagesof Terrestrial Planet Formation • From dust to km-size planetesimals • Myriads of microscopic dust particles merging together. Motion of solid objects is coupled to gas. • From planetesimals to Moon-sized objects (embryos) • Large number of gravitationally interacting objects. • Gravity is the major player. Planetesimal collisions and mergers lead to formation of bigger objects. • From embryos to terrestrial planets and cores of giants • Small number of massive, spatially isolated bodies. • Weak gravitational perturbations between them cause their orbits to cross leading to giant impacts. • Possible accretion of gas and transition to gas giants

6. From dust to planetesimals Very poorly understood!Potential planetesimal formation mechanisms: • Gravitational instability(Goldreich & Ward 1973; Youdin & Shu 2002) • Dust sediments towards midplane, forms dense layer, becomes gravitationally unstable. 1-10 km size bodies form on dynamical (about 100 yr) timescale. • ???Can dust really sediment? What is the role of turbulence in the disk? • Coagulation of dust particles(Weidenschilling & Cuzzi 1993) • Dust particles collide with each other and stick ensuring growth. 1 m bodies grow in less than 10,000 yr if 100% sticking probability. • ???Sticking mechanism is very unclear. Collisions may occur at high velocities leading to dust fission rather than fusion. • “Exotic” mechanisms: vortices, turbulent concentration, etc. • ???Do these work at all? Interdisciplinary connections: dust sticking (chemistry, surface science), dust destruction (solid state physics), physics of turbulence, etc. Need many realistic, controlled lab experiments!

7. From planetesimals to embryos • Features of this evolutionary stage: • Many planetesimals ( within 1 AU); orbits overlap. • Mutual gravitational perturbations excite their eccentricities and inclinations -energy gets pumped from circular orbital motion into random motion. • Low-velocity collisions lead to mergers and planetesimal grows, high velocity collisions cause erosion and fragmentation • System evolves under simultaneous action of all these processes planetesimals embryo Because of the huge number of bodies involved, kinetic theory should be employed to study planetesimal agglomeration, including both mass and velocity evolution. Direct N-body simulations can also probe spatial evolution but they are very limited. Particle-in-a-box simulations (modeling disk as a “gas” of gravitating particles) demonstrate growth up to g in yr at 1 AU – Moon-size embryos in the terrestrial region. (Kenyon & Luu 1998) From planetesimals to Moon-size “embryos”

8. From planetesimals to embryos ???Planet formation timescale exceeds age of the Universe in the outer Solar System. How do Uranus and Neptune form? We have interesting clues on this one! - After some coagulation has proceeded, growing embryos cause rapid dynamical evolution of surrounding planetesimals, increase their speeds dramatically - As a result, when planetesimals collide with each other they fragment into smaller pieces - Fragments are subject to strong gas drag and mutual inelastic collisions, this decreases their random velocities - Dynamically “cold” fragments are accreted by embryos much more efficiently than the original planetesimal material Embryos grind their food for better digestion! ???Details of planetesimal fragmentation? Internal strengths of planetesimals? Interdisciplinary connections: cratering, physics of impacts, crack propagation, granular flows (“rubble piles”)

9. Spatially widely separated embryos gravitationally excite each other into crossing orbits • Bigger bodies form in catastrophic collisions in about years in the inner Solar System 150 Moon-size bodies Moon-forming impact (Canup 2004) Chambers 2001 From Moon-size embryos to fully-grown planets • Evidence: • Earth-Moon system: giant impact about 30 million yrs after Earth formed. • Planetary obliquities ???Final dynamical state? Interdisciplinary connections: geology, equation of state at high T and P (shock experiments), numerical hydrodynamics

10. Conclusions • Formation of terrestrial planets provides clues to the genesis of giant planets • Growth of terrestrial planet consists of three important stages: • - Formation of planetesimals (very poorly understood) - strong coupling of dust to gas. • - Coagulation of planetesimals (general picture is within our grasp but details are often not clear) – “gas” of gravitating and merging particles, with important contribution of dissipative processes. • - Stage of giant impacts (numerical studies give reasonable picture although not without questions) – catastrophic collisions of massive protoplanetary cores. • Plenty of room for interdisciplinary studies in a wide variety of fields – chemistry, geology, physics, atmospheric sciences, etc. – both on theoretical and experimental sides. Without them progress will be stalled.

11. Planetary diversity Lissauer 2004 Currently known to us are • terrestrial planets – consist mainly of solid refractory materials – silicates and iron • gas giants – mostly gas , with solid cores • ice giants – large ice + rock cores , covered with thick atmospheres composed of mainly H and He