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solar system: 1 star 8 planets 5 known dwarf planets, 160+ moonssmall solar system objects.Sun: 99.85% mass of solar system, 330,000*mass Earth, 109*diameter Earth.98% hydrogen&helium. Terrestrial planet: rocky surface, small, close together, mostly rock&metal. Mercury: smallest, covered in craters. Venus: closest planet in size&mass to Earth, thick&toxic atmos produces greenhouse effect->surface hot. Mars: frozen desert planet, has seasons, weather, polar ice caps, volcanoes, canyons.A planet: 1)in an orbit around a star 2)massive enough->spherical 3)have cleared its orbit (largestgravitational pull in the area). Terrestrial planets: closer together, smaller mass&size, higher density,solid surface (rock&metal), fewer/no moon, no ring. Jovian planets: bigger, space farther, lower density, colder temp, made of hydrogen, helium, ices, no solid surface, rotate faster, have more moons, all have ring. Asteroids: rocky, airless worlds that orbit Sun. majority orbit within Asteroid Belt, a ring between the orbits of Mars and Jupiter. asteroid belt is sparsely populated. largest asteroid, Ceres, is large enough for its gravity to make it spheroidal shape, and the only asteroid classified as dwarf.The Kuiper Belt andOort Cloud:beyond the orbit of Neptune, icy bodies, debris left over from the formation of the solar system. K-belt: disk-shaped region. Pluto and Eris: largest, best known of these icy worlds. O-cloud: spherical cloud of small icy bodies, contain billions of large object. dwarf planet: orbits the Sun, has enough mass to assume a nearly round shape, not cleared the neighbor around orbit, not a moon. planets sweep up or scatter objects near their orbits while dwarf planets tend to orbit in zones of similar objects. Dwarf generally smaller than Mercury. five dwarf planets: Ceres(Asteroid Belt), Pluto, Haumea, Makemake, and Eris (K-Belt). Solar nebula hypothesis: Sun&planets formed from a cloud of gas and dust 4.5b years ago.1)Motion of Planets: planets move around Sun in same direction, counterclock. Sun and most planets rotate in direction as orbit(venus rotates backwards, uranus rotates on its side, all moon orbit counterclock. suggest the planets all formed from the same flattened disk-like structure, condensing out of same material. 3)Asteroids: small rocky world. Comets: made of “dirty ice”. mostly in orbit beyond Neptune.Nebula theory: 1)The solar system began as a spinning cloud of gas and dust(nebula). nebula began to contract under the influence of its own gravity.2)As the spinning cloud contracted under gravity, most material fell into protosun at center, rotation rate increase as physical size decreased, flattened into a spinning disk(circumstellar disk), temp increased. 3)Within solar nebula, dust and ice particles collided and clumped together. Through accretion, these particles grew into objects (planetesimals).