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The solar system consists of much more than just the Sun and planets. It contains billions of other objects and extends far beyond the outermost planets. Several hundred thousand asteroids revolve around the Sun in orbits mainly between Mars and Jupiter. Countless smaller meteoroids, including cometary debris and fragments from the collision of larger bodies, are also present, some of which approach Earth's orbit closely enough to be known as near Earth objects. In addition, as many as a billion objects, most the size of a speck of dust, cross through our atmosphere as meteors or micrometeoroids each day, though the vast majority are invisible to observers on the ground. Astronomers have recorded more than 800 comets passing through the inner part of the solar system. Billions more lie in the area surrounding the solar system, in the disk of debris known as the Kuiper belt and in the swarm of comets known as the Oort cloud. All of these objects orbit the Sun at high speeds. Some orbits, like those of the planets near the Sun, are almost circular. Other orbits, like those of comets that make their way in among the planets, are stretched out into long ellipses. As in most scientific fields, new discoveries are constantly changing our understanding and definitions. The objects in the Kuiper belt, for example, were discovered in the 1990s. When the new planetarium at the American Museum of Natural History opened in 2000, many visitors were shocked to find that Pluto, long known as the ninth planet, had been demoted. In 2006 the International Astronomical Union classified Pluto as a dwarf planet.