Download Planet Formation and Dynamics of Planetesimal Disks

Formation of Terrestrial Planets
Roman Rafikov
(Institute for Advanced Study)
Solar System
8 major planets:
Extrasolar planets
• More than 150 extrasolar planets
are known at present.
• Most of them have masses typical of
gas giants, i.e. ~ M J
gas giants
ice giants
• Only planets around pulsar PSR
B1257+12 (neutron star) resemble
terrestrial planets
exoplanets.org
terrestrial planets
nineplanets.org
The most likely scenario of the giant planet formation is a rapid gas accretion
onto a preexisting massive (~ 10 M  ) solid core.
Formation of the Earth-like planets have likely been an important stage of the
giant planet genesis.
Planetary diversity
Currently known to us are
• terrestrial planets ( M  M  ) –
consist mainly of solid refractory
materials – silicates and iron
2
• gas giants – mostly gas (~ 10 M  ) ,
with solid cores (5  15 M  )
• ice giants – large ice + rock cores
(12  15 M  ) , covered with thick
atmospheres (2  3 M  ) composed
of mainly H and He
Lissauer 2004
Initial conditions for planet formation
 Pictoris 1.2 m
Planets form in protoplanetary disks
• Gaseous disks in differential rotation around
parent stars
Mouillet et al’97
• 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
Stages of Terrestrial Planet Formation
1 m
1 km
• 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.
103 km
• 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.
10 4 km
• Possible accretion of gas and transition to gas giants
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!
From planetesimals to embryos
From planetesimals to Moon-size “embryos”
Features of this evolutionary stage:
• Many planetesimals (  10
12
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
Because of the huge number of bodies
involved, kinetic theory should be employed to
study planetesimal agglomeration, including
both mass and velocity evolution.
planetesimals
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
26
growth up to 10 g in 10 5 yr at 1 AU – Moonsize embryos in the terrestrial region.
embryo
(Kenyon & Luu 1998)
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”)
From Moon-size embryos to fully-grown planets
• Spatially widely separated embryos
gravitationally excite each other into
crossing orbits
150 Moon-size
bodies
• Bigger bodies form in catastrophic
8
collisions in about 10 years in the
inner Solar System
Moon-forming impact (Canup 2004)
Evidence:
Chambers 2001
• 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
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.
Planetary diversity
Currently known to us are
• terrestrial planets ( M  M  ) –
consist mainly of solid refractory
materials – silicates and iron
2
• gas giants – mostly gas (~ 10 M  ) ,
with solid cores (5  15 M  )
• ice giants – large ice + rock cores
(12  15 M  ) , covered with thick
atmospheres (2  3 M  ) composed
of mainly H and He
Lissauer 2004