New world found beyond Pluto – 2001 KX76

Discovery image for the new planetary body, 2001 KX76. The picture was made by combining two images taken at separate times. 2001 KX76 is shown by the pair of coloured dots – it had moved from one image to the next, indicating that it is part of our solar system, not a distant star. (Image: Deep Ecliptic Survey Team/NOAO/AURA/NSF). Astronomers from Lowell Observatory, the Massachusetts Institute of Technology, and the Large Binocular Telescope Observatory in the USA have discovered an icy planetary body, roughly equal in size to Pluto’s moon Charon, orbiting the Sun beyond Neptune, in a region of the solar system known as the Kuiper Belt.

The new object is designated 2001 KX76, and was discovered in the course of the Deep Ecliptic Survey, a search for Kuiper Belt Objects (KBOs) being conducted by the Lowell-MIT-LBT team using telescopes at Kitt Peak National Observatory in Arizona and Cerro Tololo Inter-American Observatory in Chile. Astronomers have estimated its size from its brightness, by estimating how much sunlight it reflects towards the Earth. Assuming a reflectivity (or albedo) of 4 percent, 2001 KX76 has a diameter of approximately 1,270 kilometers, making it bigger than Ceres, the largest known asteroid. For comparison, Pluto’s moon Charon has an estimated diameter of 1,200 kilometers.

The existence of the Kuiper Belt was postulated by J. A. Fernandez and by M. Duncan, T. Quinn, and S. Tremaine in the 1980s to explain the origin of short-period comets. These comets move around the Sun in the same direction as the planets, and are found in orbits that are tipped only modestly with respect to the ecliptic plane. These researchers showed that short-period comets could not have originated from the more distant spherical Oort Comet Cloud as originally believed. They predicted that a second, more flattened reservoir of 'proto-comets' must lie beyond the orbit of Neptune.

The first Kuiper Belt Object was found in 1992 by David Jewitt and Jane Luu of the University of Hawaii. Since then, astronomers have found over 400 KBOs, but tens of thousands likely remain to be discovered. These objects are believed to be remnants from the formation of the Solar System, and consequently are among the most primitive and least-evolved objects available for study by planetary astronomers.

'2001 KX76 is so exciting because it demonstrates that significant bodies remain to be discovered in the Kuiper Belt. 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. Until the Kuiper Belt has been thoroughly explored, we cannot pretend to know the extent or the content of the Solar System.' Dr Robert Millis, Director of Lowell Observatory.