In recent years, scientists have debated whether or not they should demote tiny Pluto from full-fledged planet status to mere "Transneptunian Object." Its fate remains to be seen. But now Pluto's only moon, Charon, faces a challenge of its own: researchers have found a new space rock that threatens to take the satellite's place as the second-largest object in the outer fringes of our solar system, a region referred to as the Kuiper Belt.

Astronomers from Lowell Observatory, the Massachusetts Institute of Technology and the Large Binocular Telescope Observatory spotted Charon's icy rival, a planetary body they dubbed 2001 KX76, on May 22, 2001. "This object is intrinsically the brightest Kuiper Belt Object (KBO) found so far," says Robert Millis, director of Lowell Observatory. The team used albedoor the fraction of sunlight reflected, in this case, by 2001 KX76to approximate its size. Even the most conservative estimates suggest that 2001 KX76 is about 1,270 kilometers in diameter, or larger than Charon by at least 5 percent, Millis says.

Astronomers have discovered about 400 KBOs since 1992, when they detected the first. The new object orbits the sun in a plane tilted 20 degrees from that of the major planets. Researchers presume 2001 KX76's orbit is linked to Neptune's orbit; the KBO makes three trips around the sun for every four the planet makes. "2001 KX76 is so exciting because it demonstrates that significant bodies remain to be discovered in the Kuiper Belt," Millis adds. "We have every reason to believe that objects ranging up to planets as large or larger than Pluto are out there waiting to be found." Poor Pluto.