Download Comparing Earth, Sun and Jupiter

Formation of the Solar System
 Any model for the formation of the SS has to
explain many non-trivial general patterns:
i. oldest age of any SS body is <4.6x109y; many
meteorites are this age
ii. orbits of planets, and Sun’s equator, lie in a
single plane. Orbits are nearly circular.
iii. planets all revolve in the same direction that
the Sun rotates
iv. Sun has 99.9% of mass but planets (mostly
Jupiter) have 98% of angular momentum
v. inner planets are denser, made of “rare”
elements like silicates, metals; outer planets’
composition more like solar
vi. asteroid composition “transitional” with
inner more rocky/silicate and outer more
ices
vii. oldest meteorites have grains that cooled at
T ~ few hundred K
viii. comet composition dominated by ices
ix. volatiles present in inner SS
x. giant planets all have regular satellites and
ring systems orbiting in planet’s equatorial
plane
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The Solar Nebula
The Sun’s cocoon nebula extended at least 40 AU,
and probably several hundred AU from the forming
Sun
The protoplanetary disk condensed directly out of
this nebula: it was not (for example) ejected from
the Sun.
 If planets form from such disks, this explains
why orbits are all in the plane of the Sun, and
orbit in the same direction as the Sun’s
rotation.
Mostly composed of hydrogen and helium
 Also many volatile elements such as inert gases,
water vapour
 These volatiles have a high vapour pressure: in
a vacuum they will sublime from a solid state
Heated by the Sun, but also accretion of matter
onto the disk, friction, and magnetic flares.
 Expected to have a temperature of about
>1000 K within 2-3 AU, declining to ~100 K at
5-10 AU.
 The inner part of the disk will contain a rocky
component (small dust grains)
 Farther away, various types of ices will form.
Formation of planetesimals
 One like scenario for the formation of the planets
follows:
 Gas and dust in the disk (total mass ~0.01 MSun)
orbit the protostar in nearly circular orbits
 However, the gas is also partially supported by
pressure, so it orbits more slowly.
 Thus, the dust grains feel a headwind of ~10
m/s due to the gas.
 The smallest dust grains then orbit with the
gas; larger grains orbit more quickly (since
surface area increases less quickly than mass).
 Since dust grains are moving at different
speeds, they tend to collide with one another,
and will often stick. This could grow objects
~1 km across within ~10,000 years. The
composition will depend on the distance from
the protostar.
 Intermediate sized grains were sufficiently
slowed by the headwind that their orbits
decayed (they spiral inward) which caused
some mixing of solid material through the disk.
 Once sizable planetesimals have formed,
mutual gravitation speeds up the accretion
process (to approx. asteroid sizes)
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Formation of Giant Planets
Probably formed initially like the terrestrial
planets
 Sweeping up of planetesimals formed rock-ice
planets with masses 10-15 times larger than
Earth.
Once they became this massive, their gravitational
fields became strong enough to accrete the
surrounding gas and dust.
The accreting dust would have formed a disk,
analogous to the solar nebula disk, but smaller and
with a different temperature structure
 Moons could then form within these disks
 This explains why the gas giants have such rich
moon and ring systems.
In principle, gravitational collapse could also play a
role in the growth of terrestrial planets
 However, earth has very little heavy, inert
gases (Ne, Ar, Kr, Xe) relative to the cosmic
complement
 These are too heavy to escape (like H), and
won’t chemically react, so must never have
been accreted from the nebula
Ejection of planetesimals
 It is thought that interactions with Jupiter and
Saturn “kicked” planetesimals out to very large
radii (the Oort Cloud).
 Interactions with Uranus and especially Neptune
tended to populate the Kuiper Belt, but also
deflected some planetesimals inward to interact
with Jupiter and Saturn.
 As a result of the inward and outward “traffic,”
the orbits of all four giant planets were
significantly modified. Neptune was affected most,
and may have migrated outward by as much as 10
A.U.