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Chapter 6: Planetological
foundations for origins of life
2 Planet formation – magic in the
residue of stellar formation!
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?
Emmanual
Kant and
PierreSimon
Laplace:
18th century
giants
HH 30 (from HST)
Star formation
sets the stage
for planet
formation
Flared,
gaseous,
dusty disk
Gas Accretion & Gap-formation
Protoplanet
http://www.astro.psu.edu/users/niel/astro1/slideshows/class43/slides-43.html
Planet formation theories
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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).
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For either 1 or 2 – final mass determined by “gap opening”
in face of disk “viscosity”.
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Terrestrial planet formation; model 1 - do gaps open too?
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
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Hubickyj et al 2005, Icarus
GI: rapid formation within few thousand yrs
- disk must have Toomre Q < 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)
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Mayer et al 2002
When do giant planets quit growing?
Protoplanet
Gap opens in a disk
when
Tidal Torque ~
Disk
Tidal Torque
Viscous Torque
Disk
Viscous Torque
Planetary masses: determined by gap opening
- Gap-opening mass ~ Final mass of a planet
- Two competing forces (Tidal vs Viscous)
- Smaller gap-opening masses in an inviscid disk
Disk pressure scale height h [AU]
M Planet
h
 40   
M Star
a
5
Lin & Papaloizou (1993)
Disk Radius a [AU]
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
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
30
20
Dead
Zone
10
Disk Radius [AU]
Disk Radius [AU]
30
=10-3
20
10
=10-5
0
0 2×106 4×106 6×106 8×106 107
0
0 2×106 4×106 6×106 8×106 107
Time [years]
Time [years]
(w/o Dead Zone)
(w/ Dead Zone)
Detecting Jovian planets in other
disks...close-up view with ALMA
Mplanet / Mstar = 0.5 MJup / 1 Msun
50 pc
Orbital radius: 5 AU
Disk mass as in the circumstellar
disk as around the Butterfly Star
in Taurus
100 pc
Wolf & D’Angelo (2005)
astro-ph / 0410064
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”)
~ Highest
resolution at
850 GHz =
350 mm
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
Biomolecule formation:
organic molecules made in protostellar disks
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Organic chemistry in “molecular layer” – 3 layer vertical
structure at r > 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, A&A).
Delivery system of biomolecules to Earth?
Water, and biomolecules: by asteroids? comets?
Simulations: Typically find a few Earth ocean’s worth
delivered by asteroids from beyond 2.5 AU.
Comets: Dirty
snowballs
Halley’s comet as seen in May 1910:
May 10 – 30 deg tail; May 12 - 40 deg
tail. Period of comet: 76 years
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;
Giotto images of Halley’s comet
Evaporating dust and gas from Halley’s nucleus: 30 tons per
second for comet inside 1AU – Halley’s comet would
evaporate in 5000 orbits
In general: density 100 kg/ cubic metre; temperature, few 10s
of Kelvins; mass 1012  1016 kg ; composition, dust mixed with
methane, ammonia & water ices
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
Origin of oceans…. delivery of water
by comets or asteroids?
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Clue to origin of Earth’s 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 Earth’s water from comets
Perturbations by Jupiter of asteroid system perturbs their
orbits into ellipses that cross Earth’s 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…
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
Earth’s mantle
material – Moon ends
up with composition
similar to Earth’s
mantle
3. Debris from collision in orbit around Earth
collects together to form the Moon:
< 10% of initial ejected material ends up
accreting to form the Moon.
Brief history of the Moon
a) 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