Formation of Terrestrial Planets. Roman Rafikov (Institute for Advanced Study). 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.
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(Institute for Advanced Study)
The most likely scenario of the giant planet formation is a rapid gas accretion onto a preexisting massive solid core.
8 major planets:
Formation of the Earth-like planets have likely been an important stage of the giant planet genesis.
Currently known to us are
Mouillet et al’97Initial conditions for planet formation
Planets form in protoplanetary disks
Very poorly understood!Potential planetesimal formation mechanisms:
Interdisciplinary connections: dust sticking (chemistry, surface science), dust destruction (solid state physics), physics of turbulence, etc. Need many realistic, controlled lab experiments!
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”
???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”)
150 Moon-size bodies
Moon-forming impact (Canup 2004)
From Moon-size embryos to fully-grown planets
???Final dynamical state?
Interdisciplinary connections: geology, equation of state at high T and P (shock experiments), numerical hydrodynamics
Planetary diversity each other into
Currently known to us are