Astronomy and Astrophysics at SAO

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From dust bunnies grow planets?
Lee Hartmann, Smithonian Astrophysical Observatory
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Protostars images of (rotating) infall forming
rotating disks

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T Tauri disks

Shadow disks in Orion Nebula Stauffer et al.
(1999)
Ages ? 3 Myr opaque dusty disks
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How do planets form?
- look for them in dusty rotating protoplanetary
rotating disks
Light scatters off top and bottom of disk
Dusty protoplanetary disk seen edge-on, hiding
the central star
Are planets forming?
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HR 4796A 8 Myr old ring system

18 mm thermal emission
1.1 mm scattered light

• Binary system age 8 ? 2 Myr (Stauffer et al.
  1995)
• 70 AU inner disk hole (Jayawardhana et al. 1998,
  Koerner et al.
• 1998, Telesco et al. 1998, Schneider et al.
  1998)

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disk
star
CoKu Tau 4 DAlessio, Calvet et al
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The beginning dust particles stick together
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Dust settles to midplane Coagulation occurs to
larger bodies Radial drift also occurs, run into
larger objects (problem at meter-sized why
dont they just break apart?) At sizes 10 km,
gravitational focussing make planetary embryos
? terrestrial planets or at masses few to 10
Earth masses, can attract lots of gas ? form
giant planets.
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Chambers Wetherill 1998
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Chambers Wetherill 1998
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Terrestrial planet formation is MESSY
Multiple body (Ngt2) systems are UNSTABLE (if
spaced far enough apart, collisions are
rare) But at early stages, planetary-sized
bodies collide (e.g., craters) 65 Myr ago - a
last gasp of a general process which formed the
Earth over 10-30 Myr Also a Mars-sized body hit
the Earth, forming the Moon
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Canup and Asphaug
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The recipe for planets (we think)

• angular momentum ? make rotating disk
• dust settles, coagulates ? make larger bodies
• gravity makes many tiny planets
• planetesimals collide with each other because
  multi-body system is unstable, chaotic
• slower growth ? make small (terrestrial) planets
• faster growth plus gas ? make giant planets
• outcomes are highly variable
• Moon, extinctions ? late collisions