The Kuiper Belt

From page 138 in "You and the Universe:" Pluto’s 2.6 to 4.6 billion-mile orbit from the Sun is so elliptical that it was actually closer to the Sun than Neptune from 1979 to 1999. With a 17° inclination to the ecliptic (the plane of Earth’s orbit around the Sun), Pluto’s orbit is the most inclined as well. Because of these anomalies and its extremely small size—2/3 the size of our Moon—some astronomers believe that Pluto, its moon Charon, and Triton (the largest moon of Neptune) are merely the largest members of the Kuiper Belt. (Charon was the river Styx ferryman who carried the dead to Hades/Pluto. The dead were buried with a coin in their mouth or on their eyelids to pay for the crossing.)

The Kuiper Belt is the frozen remains of a disk of primordial material that condensed into our present solar system five billion years ago. As many as 10 billion icy bodies 10 miles or more across orbit the Sun in this region, stretching from just beyond Neptune’s orbit to about 4.5 billion miles out. Many believe it to be the source of short-period comets like Halley’s that take less than 200 years to orbit the Sun. The Kuiper Belt is estimated to contain more than 35,000 objects greater than 60 miles in diameter, several hundred times the number and mass of similar sized objects in the asteroid belt. And there may be 100 million comets 12 or so miles across in the Kuiper belt still detectable by Hubble.

In June 2002 a KBO (Kuiper Belt Object) 750 miles across was discovered, the biggest solar system find since Pluto in 1930. Called Quaoar, it orbits the Sun once every 288 years from four billion miles out. Then in March 2004 Sedna (named for the Inuit goddess who created the arctic sea creatures) was announced. About 1000 miles in diameter and with an amazing 10,500-year orbit 8 to 84 billion miles from the Sun, Sedna was the largest KBO discovered. It may even be the first seen member of the Oort Cloud, though that would place it 10 times closer than the Oort Cloud’s theoretical inner limit. Since the Kuiper belt has a very sharp edge to it, just beyond the orbit of Pluto, and Sedna wanders in a no-man’s land between the Kuiper belt and the Oort cloud, many astronomers believe Sedna was pulled from a more circular orbit by a passing star about 4 billion years ago.