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‣Planets orbit Sun in the same direction and in nearly the same plane Two main planet types: Terrestrial and Jovian Smaller bodies: Dwarf planets, asteroids, comets, moons The defini‣‣‣-tion of a planet is in orbit around the Sun has sufficient mass so that it assumes a nearly round shape has cleared the neighborhood around‣‣‣‣What properties of our solar system must a formation theory explain? Motions of large bodies Two types of planets Asteroids and comets What theory best explains the features of ‣‣‣‣our solar system? Nebular theory states that solar system formed from a large interstellar gas cloud. What caused the orderly patterns of motion in our solar system? Solar nebula spun ‣‣‣faster as it contracted, flattened into a disk We have observed such disks around newly forming stars‣‣Why are there two types of planets? Only rock and metals condensed inside the frost line Rock, metals, and ices condensed outside the frost line Planetesimals then accreted into plan‣‣‣-ets Ice particles outside frost line made outer planets more massive Gravity of these massive planets drew in H, He gases Where did asteroids and comets come from? They are leftover ‣‣‣‣planetesimals, according to the nebular theory When did the planets form? Radiometric dating indicates that planets formed 4.6 billion years ago‣‣‣What are terrestrial planets like on the inside? Core, mantle, crust structure Denser material is found deeper inside What processes shape planetary surfaces? Cratering, volcanism, ‣‣‣‣tectonics, erosion What determines a world’s level of geological activity? Interior heat drives geological activity Big planets cool off slower than small ones Radioactive decay is cur‣‣‣‣-rently main heat source How does a planet’s surface reveal its geological age? Amount of cratering tells us how old the surface is‣‣‣What geological processes shaped our Moon? Early cratering still present Maria resulted from volcanism What geological processes shaped Mercury? Cratering and volcanism simi‣‣‣‣-lar to Moon Tectonic features indicate early shrinkage What are the major geological features of Venus?