Download Formation of Solar System

Formation of Solar System
• Nebular Hypothesis (Laplace, 18th century)
– Rotating sphere of gas flattens into spinning disk
– Contraction produces a protostar at the center
– Material in surrounding disk becomes the planets
• Problem: angular momentum…
– Sun is 99% of mass of solar system
– Planets have 99% of angular momentum
– Shouldn’t be that way… Sun should be spinning faster
than once every 25 days (because it’s at the center)
Formation of Solar System
• Where did all the stuff come from?
• How old is all the stuff?
Formation of Solar System
• Where did all the stuff come from?
– Stars come from clouds of molecular gas
and dust.
– Masses from 0.08 Ms to 90 Ms
– Solar systems like our own probably limited
to 1-3 Ms
• Too big – too violent and short-lived
• Too small – unlikely to ever form planets
Formation of Solar System
• How old is all the stuff?
Age of meteorites =
Age of solar system =
Age of earth
Formation of Solar System
• Star forming nebula and circumstellar disks
Formation of Solar System
• Dust to protostars
– Dense clouds litter the galaxy
– Low temperature (10 K)
– High density (1000 to 1 million times greater than
interstellar medium)
– Rich collection of molecules (but mostly H)
– 1015 km in diameter
– 2000 or so in Milky Way
– Favorable places for star formation (and, therefore,
perhaps planetary formation)
Formation of Solar System
• Dense clouds
associated with
young stars
(“stellar
nurseries”)
Formation of Solar System
• Problem…
– Gravity. What does it do? ATTRACTS
– So all clouds should eventually contract under
their own gravity.
– But they all don’t!
Formation of Solar System
• Solution…
– Jean’s Mass
• Gravity is counteracted by internal
pressure of the cloud.
• Internal pressure comes about by
temperature and density of a
spherical cloud
• There will be a balance between
pressure and gravity…
• Until you exceed a certain mass
(Jean’s mass)
Formation of Solar System
• Contracting dense cloud…
– Becomes a disk because of angular momentum
• In plane of rotation, centripetal and gravitational forces are
in equilibrium
• Normal to plane of rotation, gravity and pressure compete.
Gravity wins, pulling material into disk
– Heats up
• Gravitational energy converted to kinetic energy (and
temperature) via collisions
• After few kyr, edge of cloud ~2000-3000 K
• Cloud becomes opaque at center (due to density), radiation
is trapped, and gets hotter
• Star ignition temperature ca. 106 K achieved in 108 yr
Formation of Solar System
• Violent early phase…
– Young protostars go
through the “T-Tauri”
phase where strong,
bi-polar outflows of
50 km/s jets of
material form.
– protostar can loose
up to 0.5 Ms within
106 yr
Formation of Solar System
1. Dense cloud collapse (0.1 – 0.5 Ma)
2. Disc dissipation: some material transported
toward protostar (0.05 Ma)
3. T-Tauri phase (1-2 Ma)
4. Gas dissipation: planetary accretion &
residual nebula removed (3 – 30 Ma)
•
What about angular momentum problem…?
Viscous drag: particles in disc interact, stealing/transferring some
angular momentum from star (slowing its spin) towards
edges
Formation of Solar System
• Star with a disk around it… now what?
Formation of Solar System
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Condensation
Coagulation
Planetesimals
Planetary growth
Planetary migration
Satellites, rings, asteroids, comets
Formation of Solar System
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Condensation
Coagulation
Planetesimals
Planetary growth
Planetary migration
Satellites, rings, asteroids, comets
Formation of Solar System
• Condensation
– Away from star, disk will be relatively cool (400 K at 4.5 AU,
3000 K at 1 AU) and material will form
compounds/minerals (not necessarily solid, though)
H2O
CH4
FeS
Al2O3
CaMgSi2O6
Formation of Solar System
• Temperature
is key…
– Refractory:
condense
at high T
– Volatile:
condense
at low T
Formation of Solar System
Formation of Solar System
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Condensation
Coagulation
Planetesimals
Planetary growth
Planetary migration
Satellites, rings, asteroids, comets
Formation of Solar System
• Now we have particles of condensed stuff
colliding with one another. What happens?
Formation of Solar System
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They break.
– If collisions violent enough
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They bounce.
– Elastic interactions
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They stick.
– This is coagulation
– By magnetism? Static electricity? Friction? Fluffy
particles with lots of cavities (e.g. velcro effect)?
Formation of Solar System
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Coagulation will occur rapidly…
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10 mm particles at 1 AU in 2 kyr
15 mm particles at 5 AU in 5 kyr
0.3 mm particles at 30 AU in 50 kyr
What limits rate (so that takes longer to happen
further out)?
1. Density (e.g. column mass) – decreases outward from
Sun
2. Number of collisions/time (density dependent)
3. Thickness of disk (thicker away from Sun) determines
timescale for particles to get to midplane
Formation of Solar System
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•
Condensation
Coagulation
Planetesimals
Planetary growth
Planetary migration
Satellites, rings, asteroids, comets
Formation of Solar System
• Remember T-Tauri stage… it could blow
particles up to 10 m out of the solar system.
• So we have to get up to this size and larger
before ca. 1 Ma!
• Coagulation can’t do it, though.
Formation of Solar System
• How do you get 0.1 to 10 km planetesimals?
– Gather coagulants into turbulent knots in thinning
disk
– Gravity draws material together (focusing) in collisions
that have a net effect of accretion (rather than
dispersal)
– At given distance from Sun, runaway growth will
promote development of one or two dominant
planetary embryos that sweep up all material
Formation of Solar System
Formation of Solar System
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Condensation
Coagulation
Planetesimals
Planetary growth
Planetary migration
Satellites, rings, asteroids, comets
Formation of Solar System
• Planetary growth in inner solar system
– Embryo growth is self limiting as material used up
– Embryos every 0.02 AU formed by time of T-Tauri
stage
– Embryos about 0.1 mass of eventual planets
– Subsequent interactions (collisions) take 107-108
yr to complete planet formation
Formation of Solar System
Formation of Solar System
• Planetary growth in outer solar system
– Larger rock-ice embryos up to 5 Me
– Takes 10x as long to form as in inner solar system
– Embryos get large enough to gravitationally trap
volatiles e.g. H, He, etc.
– T-Tauri stage eventually begins, removing remaining,
unbound H and He
– Sizes of outer planets determined by amounts of H
and He present at those distances (and amount of
time available for accumulation before removed)
Formation of Solar System
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Condensation
Coagulation
Planetesimals
Planetary growth
Planetary migration
Satellites, rings, asteroids, comets
Formation of Solar System
• New, controversial idea… radii of planetary orbits can
change over time!
– By 2-30 % (in or out) over 0.1 Ga
• Why might this be true?
1.
2.
3.
4.
Discovery of icy, minor planets out to 60+ AU
Discovery of extrasolar planets size of Jupiter close to their
star
Volatiles in Jupiter atmosphere consistent with formation at
lower T (e.g. further from Sun) than present position implies
Orbits of satellites demonstrably change over time (e.g.
Moon), so why not that of planets around Sun?
Formation of Solar System
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Condensation
Coagulation
Planetesimals
Planetary growth
Planetary migration
Satellites, rings, asteroids, comets
Formation of Solar System
• Satellites, rings, asteroids, comets
– Presence of Jupiter and orbital resonance greatly
affected distribution of asteroids
– Jupiter (and other giant planets) affected orbits of
comets
– Gravitational focusing affected impact histories of
all planets
– Protosatellite disks, capture, and giant impacts
lead to formation of moons and ring systems