The Oort Cloud
The Oort cloud is a vast swarm of some 2 trillion comets orbiting our star in the most distant reaches of our solar system, extending from beyond the orbits of Neptune and Pluto out to 100,000 times the Earth-Sun distance. Almost one-third the distance to the nearest star. While the planets are confined to a flattened disk in the solar system, the Oort cloud forms a spherical shell centered on the Sun, which gradually flattens down to an extended disk in the inner region, called the Kuiper belt. Bright comets observed through telescopes or with the naked eye get thrown out of the Oort cloud or Kuiper belt, and become visible when they get close to enough so that the Sun's energy can transform the surface ices into gases. These gases drag off the embedded dust, and we see the light reflected from the dust as a tail. Comets are the leftover icy building blocks from the time of planet formation, which formed in the region of the outer planets. Essentially thesecomets are dirty snowballs, composed primarily of water ice, with some carbon monoxide and other ices, in addition to interstellar dust. When their orbits passed close enough to the giant planets to be affected, some were thrown toward the Sun and some were tossed outward toward the distant reaches of the solar system, the spherical swarm we now call the Oort cloud. Some of the comets sent inward hit the inner rocky planets, and probably contributed a significant amount of ocean water and organic material, the building blocks of life, to Earth. Comets that live in the Oort cloud are especially important scientifically because they have been kept in a perpetual deep freeze since the formation of our solar system 4.6 billion years ago. This means that they preserve, nearly intact, a record of the chemical conditions during the first few million years of the solar system's history, and can be used to unravel our solar system's origins much like an archaeologist uses artifacts to decipher an...
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