A new giant in the Kuiper belt.

When it comes to the biggest bodies in the solar system beyond Neptune, Pluto tops the list. Charon, Pluto's moon, has always been designated the runner-up.

Astronomers now have for the first time found an object that's slightly bigger than Charon. It's an icy body in the Kuiper belt, a reservoir of comets in the outer solar system.

Dubbed 2001 KX76, this object has an estimated diameter of 1,270 kilometers, making it the largest known member of the belt. Charon, in comparison, has a diameter of about 1,200 km, and Pluto is 3,300 km across.

The newly discovered object lies an estimated 43 astronomical units from the sun--43 times the Earth-sun distance--and appears to have the largest intrinsic brightness of any Kuiper belt resident, researchers report in a July 5 circular of the International Astronomical Union.

The find usurps the title of Kuiper belt king from another recently discovered resident, 20000 Varuna, which measures 900 km across. Some astronomers argue, however, that Pluto is the true behemoth of the belt, rather than the pipsqueak of planets (SN: 6/9/01, p. 360).

Robert L. Millis, director of the Lowell Observatory in Flagstaff, Ariz., and his colleagues discovered 2001 KX76 in images from a survey of the Kuiper belt taken by a 4-meter telescope at Cerro Tololo Inter-American Observatory in La Serena, Chile.

"Until the Kuiper belt has been thoroughly explored, we cannot pretend to know the extent or the content of the solar system," says Millis. "We have every reason to believe that objects ranging up to planets as large or larger than Pluto are out there [in the Kuiper belt] waiting to be found."