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Because it's census time here in the United States, it's a good time to talk about the census of the solar system as well.
Our home neighborhood consists of one star, eight planets, close to 170 known moons, and millions of rocky asteroids. But the great bulk of the population consists of comets -- balls of rock and ice in the deep-freeze of the outer solar system.
All of these are leftovers from the solar system's birth. They agglomerated from the disk of gas and dust that surrounded the newborn Sun. Countless numbers of them stuck together to form the planets. The gravity of the planets -- especially the giant planets Jupiter, Saturn, Uranus, and Neptune -- then flung the leftovers into deep space.
Many of them left the solar system entirely. Tens of millions formed a broad, flat disk beyond the realm of the planets known as the Kuiper Belt. And many more -- perhaps as many as a trillion -- formed an extended shell around the solar system known as the Oort Cloud.
The comets in the Oort Cloud are just barely bound to the solar system. The gravity of a passing star can give them enough "kick" to escape the solar system. Or it can give them a kick toward the Sun. As one of these bodies loops around the Sun, some of its icy surface vaporizes, surrounding it with a cloud of gas and dust. The Sun pushes some of that material outward to form a glowing tail: the signature of a comet.
We'll talk about the most famous comet tomorrow.
Script by Damond Benningfield, Copyright 2010