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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 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) 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.