The four rocky planets make up the inner zone. The gas giants form a "middle solar system." Beyond them lies an enormous third zone composed of the Kuiper Belt and the Oort Cloud, both named for the astronomers who predicted their existence.
The Kuiper Belt, which was discovered in the 1990s, is a ring of dwarf planets, including Pluto, and smaller icy objects that range from 3 billion to 5 billion miles beyond the sun. More than 1,000 Kuiper Belt objects have been detected, and astronomers think there might be 50,000 to 100,000 more. Most are small, but some rival Pluto in size. Some have atmospheres and moons of their own, and some might have warm, wet interiors.
Far beyond the Kuiper Belt is the Oort Cloud, which Stern calls "the solar system's attic." The cloud, a gigantic sphere with an outer edge almost 5 trillion miles from the sun, nearly a quarter of the way to the next nearest star, Proxima Centauri,is thought to contain at least 1,000 planetary bodies, some as large as Earth or larger, and as many as a trillion comets.
"If the Oort Cloud were the diameter of a football stadium, the inner solar system would be the size of a washer one-eighth of an inch in diameter," said David Aguilar, a spokesman for the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Posted by Jason McManus.