Astronomers discover giant Kuiper Belt object

Posted: July 3, 2001

Astronomers announced Monday that they have discovered an object in the distant Kuiper Belt that could rival Pluto's moon in size.

A team of astronomers from Lowell Observatory, MIT, and the Large Binocular Telescope Observatory said that the new object, 2001 KX76, is the brightest Kuiper Belt object (KBO) yet discovered and thus likely the largest. The object was discovered in images taken in late May using a 4-meter telescope in Cerro Tololo, Chile.

"This object is intrinsically the brightest Kuiper Belt object found so far," said Robert Millis, director of Lowell Observatory and head of the survey team. "The exact diameter of 2001 KX76 depends on assumptions that astronomers make about how its brightness relates to its size."

Those assumptions center on the reflectivity, or albedo, of KBOs: the brightness of an object is a function of both its size and albedo. One object could be brighter than another because it is larger, more reflective, or some combination of both. To determine the size of a KBO requires astronomers to determine, or at least estimate, its albedo.

In the past, astronomers have used an albedo estimate of 4 percent to estimate the size of KBOs, based on their compositions and the effect radiation exposure would have to darken their surfaces. That value gives 2001 KX76 a diameter of 1,270 kilometers, larger than both the largest asteroid, Ceres (diameter: 932 km) and Pluto's moon Charon, 1,200 km across.

Earlier this year another team of astronomers was able to directly measure the albedo of another large KBO, Varuna. Using that larger albedo value -- 7 percent -- gives 2001 KX76 a diameter of 960 km, still larger than both Ceres and Varuna, which is 900 km across.

The discovery of 2001 KX76 is more ammunition in the debate regarding the classification of Pluto, the smallest and most distant planet. As new discoveries close the gap in size between the largest KBOs and Pluto, 2,275 km across, some planetary scientists have argued that Pluto should either be jointly classified as a planet and KBO or be stripped of planet status altogether.

Pluto appears to share a number of characteristics with objects in the Kuiper Belt, a disk of icy bodies beyond the orbit of Neptune. Of the more than 400 KBOs discovered since the first, 1992 QB1, was found nine years ago, a sizeable fraction have similar orbits to Pluto, locked in an orbital resonance with Neptune such that they complete two orbits of the Sun in the time it takes Neptune to make three. Pluto also appears to have a similar composition to many KBOs based on spectroscopic studies of the belt.

Millis believes that the gap between Pluto and the largest KBOs will continue to shrink, if not disappear outright, as studies of the belt using larger ground- and space-based telescopes continue. "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," he said. "Until the Kuiper Belt has been thoroughly explored, we cannot pretend to know the extent or the content of the solar system."