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8.9 Quaoar and Outer Solar System Bodies

Quaoar (pronounced kwah-o-wahr) was discovered in 2002. At half the size of Pluto, it was the largest object to be discovered in the solar system since Charon was found in 1978. Quaoar orbits 1.6 billion kilometers (1 billion miles) beyond Pluto, or 42 A.U. (4 billion miles) from Earth. This orbit puts it within the Kuiper Belt, a group of bodies that orbit beyond Neptune near Pluto. Like most of these objects, Quaoar is probably made of ice and rock in roughly equal portions. Water ice has been detected on the surface, and other ices are probably also present, such as frozen methane and frozen carbon dioxide (dry ice).

Although Quaoar was the largest Kuiper Belt Object (KBO) known at that time, new 1,000-km-scale objects in the outer Solar System are being discovered every year. Makemake and Haumea are both over 1000 km in diameter and Varuna and 2002 AW197 are both about 900 km in diameter. An object called 2004 DW may now be the largest known KBO — it may be even bigger than Quaoar. However, most KBOs are probably only about 100 km across. The exact sizes of these objects are uncertain because they are so far away. Most telescopes can't resolve them, so estimates on their sizes have to be made from measurements of how much light the bodies reflect and how much heat they emit. 

Most of these large bodies travel in orbits similar to Pluto's orbit. This indicates that Pluto is not alone in the outer regions of the Solar System, although it is still by far the largest of these objects. Pluto is also unique in that it has by far the highest albedo (or brightness) of any known KBO.

In 2003, another large body was spotted in the outer solar system. Eventually dubbed Sedna, this object is the most distant object known in our Solar System. At 86 A.U. (8 billion miles), it's currently three times as far from the Sun as Pluto. Its surface is the coldest in the Solar System: 33 K (-400 °F). Sedna is more than 2/3 the size of Pluto. It travels in a very elliptical orbit that takes it out to 942 A.U. every 11,500 years. This is far beyond the Kuiper Belt. But Sedna is also a lot closer than the Oort Cloud was thought to lie. It might have been scattered outward from the Kuiper Belt, or it might be the first known object in an "inner" Oort Cloud. Sedna is unusual in other ways, too — it is surprisingly bright compared to other outer Solar System bodies. And it's almost as red as Mars! This is another aspect of Sedna that baffles scientists.

These distant bodies are composed of some of the most primordial material in the Solar System. They have barely been heated by the Sun and they have probably escaped the impact processing that bodies in more heavily populated regions of the Solar System experience. Scientists hope that these cold, icy bodies will contain clues to the early Solar System and the formation of planets. More and more outer Solar System bodies like Sedna and Quaoar are being discovered as technology improves. For example, CCDs can capture faint light much more efficiently than the photographic plates used to search for Pluto. There are probably many more bodies like these beyond Pluto, and we will doubtless continue to extend the limits of the known Solar System.