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It Might Be a Planet If . . . Steve Case North Mississippi NSF GK-8 November 2006 What is a planet? • Until recently, there was no exact definition. • There were historically five planets. • Uranus, Neptune, and Pluto were discovered after the invention of the telescope. Our Nine-Planet Solar System Mercury • Closest planet to the Sun • Very hot and rocky Venus • About the same size as Earth • Covered with thick clouds • Farther from the Sun than Mercury, but hotter because of clouds Earth • Our home planet • Contains water necessary for life • Surrounded by a protective atmosphere Mars • Smaller than Earth • May have once had surface water • Planned destination for human exploration Jupiter • Largest planet in the Solar System • Storms appearing as streaks and bands • One large storm bigger than Earth: Great Red Spot • First of the “Gas Giants” Saturn • Known for beautiful rings of rock and ice debris • Least dense planet (would float in water) • Farthest planet known to ancient astronomers Uranus • Orbits on its “side” • Blue color comes from methane in atmosphere Neptune • Most distant of the “Gas Giants” • Very similar to Uranus in size and composition • Great Dark Spot similar to Jupiter’s Red Spot Pluto • Until recently, considered farthest planet from the Sun • Smaller than any other planet • Too distant to see surface features What About A 10th planet? • In 2005, Eris discovered • Larger and farther from the Sun than Pluto • Publicized at first as 10th planet • More and more objects discovered beyond Pluto A New Definition Needed • If there were many objects about the same size as Pluto located near Pluto’s orbit, should they all be considered planets? • If Pluto was a planet, why weren’t these new objects planets as well? So what is a planet? • In 2006 the International Astronomical Union came up with three rules for determining whether or not a body is a planet: – Must orbit the Sun – Must have spherical (ball-like) shape – Must clear orbit of other debris 1. Planets orbit their stars. • All planets in the Solar System orbit the Sun. • If a body in the Solar System orbits around something other than the Sun, it is not a planet. 2. Planets are shaped like a ball (spherical). • Planets have enough gravity that they were pulled into a spherical shape when they formed. • If the body does not have a generally spherical shape, it is not considered a planet. 3. Planets “sweep up” their area around the Sun. • Planets have enough gravity to clear their orbits of smaller debris. • This debris is either captured by the planet and becomes that planet’s moon or else it is pushed out of the planet’s orbit. What about Pluto? • Pluto is roughly spherically shaped, and it does orbit the Sun. • Pluto has not cleared its orbit of other debris. • Scientists have found many other Pluto-like objects near Pluto’s orbit. • Pluto was given a new classification as a dwarf planet. Introducing the Dwarf Planets • Dwarf planets only fulfill two of the three requirements to be a planet: – They orbit the Sun. – They are roughly spherically shaped. – They have NOT cleared their area of all other debris. Our Eight-Planet Solar System • • Scientists now say that our Solar System has only eight planets. Pluto is no longer considered a planet. Our Solar System now has three dwarf planets: Pluto, Eris, and Ceres. Pluto and Eris are located beyond Neptune. Ceres is the largest body in the asteroid belt between Mars and Jupiter. Other Members of the Solar Family • Besides planets and dwarf planets, there are many other objects in the Solar System. • Some of the major non-planet objects include moons, asteroids, and comets. Moons • Bodies that orbit other things in the Solar System besides the Sun. • Many of the planets have moons. Earth has one moon. Mars has two. Jupiter has over fifty. • Even some smaller objects like dwarf planets and asteroids have moons. Asteroids • Large pieces of rocky debris that orbit the Sun. • Many are found in the Asteroid Belt between Mars and Jupiter. • Thought to be debris from a planet that never formed. Comets • Large pieces of rock and ice that orbit the Sun beyond Neptune. • Occasionally are “bumped” out of orbit and fall in toward the Sun. • As they approach the Sun, ice and dust is burned off to form a tail stretching away from the Sun. Our Solar System: • Eight planets • Three dwarf planets • Moons • Asteroids • Comets