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fridges of SS. 100 mil bodies larger than 1km. Some comets. Planet: in orbit around sun, sufficient mass w nearly round shape, has cleared the neighborhood around its orbit. Dwarf planet:has NOT cleared the neighborhood of smaller objects. Solar System orbits and rotates in an orderly pattern.Solar Nebular theory:SS formed from a giant cloud of interstellar gas and dust that collapsed under the influence of their own gravity. As solar nebula contracted: rotation rate increased, flattened into a disk, temp increased.Protosun formed at center (accreted gas from the disk). Rotation rate increase: conservation of angular momentum says that as a rotating object contracts, its rate of rotation must increase. Flatten to disk: centrifugal effects caused solar neb to flatten into a disk while the core of the nebula formed the protosun. Observe disks of gas and dust around young stars (planetary systems forming in Orion nebula). Orderly motion of SS are a direct result of SS birth’s in a spinning, flattened cloud of gas. Two types of planets: dust grains are the “seeds” of planet formation. Molecules condensefrom the solar nebula into dust grains, which collide and stick and get larger by accretion. Grow into planetesimals, collect into protoplanets. Four categories of seeds that condense at different temperatures: hydrogen and helium gas- does not condense in the nebula-relative abundance of 98%. Ices-condenses at less than -120 deg C- 1.4% relative abundance. Rock- condensing temp of 250-1000C-relative abundance of 0.4%. Metal- condensing temp of greater than 1000C, 0.2% relative abundance. Temp controls what planets are made of.Inner nebula was hot, only metals and rock could condense. Cold outer nebula: ices could condense in addition to metal and rock. Terrestrial planetsform by accretion of rock/metal planetesimals. Small particles of rock/metal present inside the frost line, planetismals of rock/metal built up as particles collided, gravity assembled these into terrestrial planets. Jovian protoplanetsgrew big enough to capture gas from solar nebula. Ice could also form small particles outside frost line. Larger planetesimals and planets were able to form. Gravity of these larger planets was able to draw in surrounding hydrogen and helium gasses. Solar wind drives out gas. Sun begins nuclear reactions after ~10 mil years. Solar wind becomes strong enough to blow away remaining gas. Jovian planets stops accreting H/He gas, young planets and leftover planetsimals remain indisk. What happened to the planetesimals? Young SS would have been filled w them, many were swept up by forming planets. Leftover planetesimals bombarded other objects in the late stages of SS formation. Evidence by cratered surfaces of moon & mercury. Period of heavy bombardmentlasted for 800 mil years. Fate of planetesimals: ejection or collision. Outer planetesimals were scattered outward and inward; had gravitational interactions with the young Jovians; changed their orbits: totally ejected from SS or ejected them to the far reaches of the SS or sent them into the innter SS to collide w terrestrials. Cosmic Water