ASTR 200 : Lecture 12
Planetary condensation and formation
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Basic properties needing explanation
• All planets orbit Sun in same sense (counter-clockwise
viewed from N)
- natural; the disk has a sense of rotation
• All planets orbit in almost same plane, with e~0
- The gas results in circular orbits, out of which
the planets then accrete
• Sun contains 99.8% of Solar System's mass
- Most of the gas gets to the star, that in the disk
gets blown away
• Inner planets rocky, outer planets/satellites icy or
heavily gas-rich
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- Must have to do with conditions in the disk...
Temperature structure
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Phase diagram of H20
~10­5
protoplanetary
disks
~150K
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The Lewis Model
-Chemical condensation sequence at low
pressure of a gas of solar composition
-As T drops, different chemical species can
condense, starting at about 1600 K
• Refractory Oxides and Metals first
• then Silicates (<1200 K)
• then water ice (<160 K)
• then ammonia and methane ice (<80K)
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Condensation sequence
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At each distance, what has 'come out' of
the gas phase is different.
• Where it is very hot, only get the metals (and
refractory oxides) solidifying.
• Somewhat cooler, get the above plus abundant
silicates, but no water ice or other ices
• Below 160K add water ice to that
– Because there is lots of H and O, get a LOT of water
ice (about as much mass as all previous phases)
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• Only at very cold places do methane and ammonia
ices condense to be added as solids
• THEN, the planets are assembled out of the local
solids that are around, everything else blown away
At each distance, planetesimals form
• Think of these as km-100 km bodies like modern
asteroids or comets
• What they are made out of depends on the solids
that locally condensed out of the nebula
• You then build larger objects out of these, all the
way up to planets, taking millions – 100 Myr.
• IF you can make very large planets (called 'critical
cores', about 10 times the mass of the Earth) those
cores can begin to attract H and He atmospheres
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– If NOT, then when the Sun ignites the gas will be
removed, leaving only solid material to accumulate
– Typical stars take 1-10 Myr to clear their gas
But how do you put planets together
from dust???
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the small 'planetesimals' clump
together to form planets
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These planets, having formed in the
spinning disk, have low e (nearly
circular) and all nearly in same plane.
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For the terrestrial planets, this can be
successfully simulated on a computer.
The planets form from planetesimals in
about 10-100 Myr.
There are puzzles with the giant planets.
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Lewis Model: Correct predictions
• Rocky bodies closer to Sun, icy bodies
farther out
• Mercury: Large metal content
• Venus/Earth/Mars sequence of more water
(bulk)
• 'Wet' asteroids in the outer main belt (near
the snow line)
• Icy satellites of the giant planets
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For the jovian planets, can get the sequence
of properties of their moons.
- formed in sub-nebula
- moons close to Jupiter: higher density
Jupiter
Sun
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Roughly, asteroids and comets are 'leftover
planetesimals' that were not incorporated into planets
Planetary migration
• Historically, people assumed that the planets formed
where we see them today...
• It is fairly likely that planets, once they become large
enough (Earth-mass scale) can move via interactions
with either :
• Other solid bodies
(planets, planetesimals)
• The gas in the disk,
if the gas has not yet been
blown out of the system
- at right: simulation of a
gas giant interacting with
the gas-rich disk
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Types of planet migration
The details of these are beyond the scope of ASTR 200
• TYPE I: planet drives 'density waves' in the disk and the
interaction back on the planet makes planet move
• (image on previous slide)
•TYPE II: giant planets open a gap
– Planet slowly migrates towards star
– 'Hot Jupiters' postulated to form
via this mechanism ----------------->
• Planetesimal driven migration
- happens as planets clear away the
many planetesimals
- planet moves in when it scatters a
planetesimal out, planet moves out
when planetesimal scattered in
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Warning: There are also DEBRIS disks
If you search online for 'disks', you often don't find images of
protoplanetary (that is, planet-forming) accretion disks
Instead, many are disks around gas-free 'mature' systems where
collisions have created dust that is reflecting the light of the star.
140 AU
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b) Optical light image of Fomalhaut. The radial streaks are an artifact of the technique which removes the central star's light.