Like the suburbs of a booming metropolis, the outskirts of the solar system keep getting more crowded. Two years ago astronomers found Quaoar, the largest known member of a remote fringe of asteroids called the Kuiper belt. This March, the same researchers announced the discovery of an even bigger object, three-quarters the size of Pluto, orbiting farther from the sun. Named Sedna, after the Inuit goddess who created the animals of the sea, it is by far the most distant known object in our solar system.
Sedna was detected by Caltech astronomer Mike Brown and his colleagues working at California’s Palomar Observatory. The 1,100-mile-wide planetoid lies 8 billion miles from the sun—three times as distant as Pluto—but its elongated 10,000-year orbit will eventually sweep it more than 80 billion miles out, deep into the mysterious Oort cloud where comets originate. At that distance, Sedna’s surface temperature will be a bitter –400 degrees Fahrenheit. Its surface appears unusually reddish, for reasons unknown. “I have a pet theory for almost everything, but this one has me baffled,” Brown says.
Sedna is the largest addition to the solar system since the discovery of Pluto in 1930, but another, unnamed planetoid recently found by Brown’s team may soon break the record again; researchers are still working to pin down its size.