Chapter 6: Planetological foundations for origins of life

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Chapter 6 Planetological foundations for
origins of life
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2 Planet formation magic in the residue of
stellar formation!
Emmanual Kant and Pierre-Simon Laplace 18th
century giants

• Kant-Laplace hypothesis planets form in disks
• verification 200 years later!
• Two major kinds terrestrial (rocky) planets
  like Earth
• giants (gaseous) planets like Jupiter.
• Formation terrestrial planets form by
  collisions of smaller bodies like asteroids?
• gas giants gas accreting onto a massive
  rocky core or by gravitational instability of
  disk?

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HH 30 (from HST)
Star formation sets the stage for planet formation
Gas Accretion Gap-formation
Protoplanet
http//www.astro.psu.edu/users/niel/astro1/slidesh
ows/class43/slides-43.html
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Planet formation theories

• Giant planet formation two mechanisms under
  intense investigation
• 1. Core accretion model. Coagulation of
  planetesimals that when exceeding 10 Earth
  masses, gravitationally captures gaseous
  envelope (eg. Bodenheimer Pollack 1986)
• 2. Gravitational instability model . GI in
  Toomre unstable disk produces Jovian mass objects
  in one go (eg. Boss 1998).
• For either 1 or 2 final mass determined by
  gap opening in face of disk viscosity.
• Terrestrial planet formation model 1 - do gaps
  open too?

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• Core accretion
• 3 phases rapid growth of rocky core, slow
  accretion of planetesimals and gas, runaway gas
  accretion after critical mass achieved (near 10
  ME)
• Problem formation time still uncomfortably long
  Jupiter at 5 AU forms in
• - 1Myr with 10 ME core
• - 5 Myr with 5 ME core

Hubickyj et al 2005, Icarus
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• GI rapid formation within few thousand yrs
• - disk must have Toomre Q lt 1
• - disk must cool quickly (less than ½
  orbital period Gammie 2001)
• Problem latter point not satisfied in detailed
  simulations (eg. Cai et al 2004)

Mayer et al 2002
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When do giant planets quit growing?
Gap opens in a disk when Tidal Torque
Viscous Torque
Protoplanet
Tidal Torque
Disk
Viscous Torque
Disk
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- Gap-opening mass Final mass of a planet
- Two competing forces (Tidal vs Viscous) -
Smaller gap-opening masses in an inviscid disk
Lin Papaloizou (1993)
Depends on disk physics! - disk flaring
(h/a) governed by heating of disk (ie central
star - disk viscosity very low in central
region or dead zone
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Migration of planets - by tidal interaction with
disk a planet moves in very rapidly (within a
million years!) but can be saved by dead zone
( Matsumura, Pudritz, Thommes 2006)
?10-3
?10-3
?10-5
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Detecting Jovian planets in other
disks...close-up view with ALMA
Mplanet / Mstar 0.5 MJup / 1 Msun Orbital
radius 5 AU Disk mass as in the circumstellar
disk as around the Butterfly Star in Taurus
50 pc
100 pc
Wolf DAngelo (2005)
astro-ph / 0410064
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Birth of a Solar System what ALMA can do..
ALMA band 7 300 GHz 1 mm resolution 1.4
to 0.015
Highest resolution at 300 GHz 1 mm (0.015)
100 AU 0.3 at d300pc
Highest resolution at 850 GHz 350 mm
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Condensation sequence accounting for
compositions of planets

• Temperature of disk drops as radius increases.
• All materials whose condensation temperatures are
  higher than disk temperature at that radius can
  condense out into solids
• - so hot innner region of disk has metals
  outer cool regions have ices

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Biomolecule formation organic molecules made
in protostellar disks

• Organic chemistry in molecular layer 3 layer
  vertical structure at r gt 100AU
• 2D, stellar ultra-violet irradiation of disks
• -molecules dissociated in surface layer,
• - abundant in gas phase in intermediate
  layer,
• - frozen out onto grains in densest layer.
  (Zadelhoff et al 2003, AA).
• Delivery system of biomolecules to Earth?
• Water, and biomolecules by asteroids?
  comets?
• Simulations Typically find a few Earth oceans
  worth delivered by asteroids from beyond 2.5 AU.

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Comets Dirty snowballs
Cometary nucleus few km in diameter passage
near Sun heats up coma of dust and gas coma can
be 100,000 in size hydrogen envelope extends
millions of km
Halleys comet as seen in May 1910 May 10 30
deg tail May 12 - 40 deg tail. Period of
comet 76 years
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Giotto images of Halleys comet
Evaporating dust and gas from Halleys nucleus
30 tons per second for comet inside 1AU
Halleys comet would evaporate in 5000 orbits In
general density 100 kg/ cubic metre
temperature, few 10s of Kelvins mass
composition, dust mixed with methane,
ammonia water ices
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Cometary orbits evidence for two distinct
reservoirs of comets
Isotropic distribution of comets at 50,000 AU
result of gravitational scattering? Oort cloud
Disk-like distribution of comets beyond Neptune
remnant of original disk? Kuiper Belt
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Origin of oceans. delivery of water by comets
or asteroids?

• Clue to origin of Earths water
• HDO/H2O 150 ppm ½ of cometary
  value
• Asteroids (carbonaceous chondrites) beyond ice
  line (2.5 AU) can have high water content
• No more than 10 of Earths water from comets
• Perturbations by Jupiter of asteroid system
  perturbs their orbits into ellipses that cross
  Earths orbit and collide, bringing in water.
• Do amino acids survive during this bombardment?
• Evidence for bombardment craters on Moon and
  elsewhere and formation of the Moon itself in
  late heavy bombardment

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Formation of the Moon Impact Model
1. Mars sized object collides with proto-Earth
which has already formed iron core much of
impactor and debris encounters Earth a 2nd time.
2. Collision tears off Earths mantle material
Moon ends up with composition similar to Earths
mantle

• Debris from collision in orbit around Earth
  collects together to form the Moon
• lt 10 of initial ejected material ends up
  accreting to form the Moon.

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Brief history of the Moon

• Just after the end of the major meteoritic
  bombardment
• b) Lunar vulcanism floods maria with lava ending
  3 billion years ago
• c) Original maria pitted with craters over last 3
  billion yr