New object dethrones Ceres as largest minor planet

Posted: August 24, 2001

European astronomers have confirmed that an object in the outer solar system discovered earlier this year is the largest minor planet in the solar system, bigger than both the asteroid Ceres and Pluto's moon Charon.

A team of German, Finnish, and Swedish astronomers used a unique "virtual telescope" to study 2001 KX76 and found that the Kuiper Belt object (KBO) is at least 1,200 kilometers in diameter, significantly larger than Ceres, the largest asteroid. The results were announced Thursday by the European Space Agency and the European Southern Observatory.

This image is a colour composite image, based on three exposures with the Wide Field Imager (WFI) at the MPG/ESO 2.2-m telescope at the La Silla Observatory. Photo: ESA, ESO, Astrovirtel & Gerhard Hahn (German Aerospace Center, DLR)
2001 KX76 was discovered in May by a group of American astronomers, who found the object in images taken by a four-meter telescope in Chile. When the discovery was announced July 2, astronomers estimated the object to be between 960 and 1,270 kilometers in diameter, based on the object's brightness, estimates of its albedo, or reflectivity, and the initial orbit calculated for it.

In an effort to refine the object's orbit, European astronomers used Astrovirtel, a new virtual telescope that allows astronomers to search through archived images from several observatories to see if 2001 KX76 could be located in any of its images. The object was located in several of Astrovirtel's images, dating back to 1982.

Those images allowed astronomers to calculate a more accurate orbit for 2001 KX76. With that new orbit -- which shows that the KBO is slightly more distant from the Sun than the previous calculations -- astronomers recalculated the size of the object. If 2001 KX76 has an albedo of 7 percent, the same as that measured earlier this year for another KBO, Varuna, then 2001 KX76 has a diameter of 1,200 kilometers. If it has a slightly darker albedo of 4 percent, a common value for comets, then the object is 1,400 kilometers across.

By comparison, Ceres, which has been considered the largest minor planet in the solar system, has a diameter of only 932 kilometers. 2001 KX76 is also at least as large as Charon, Pluto's only moon, which is no more than 1,200 kilometers across.

In this schematic diagram the relative sizes of the largest Kuiper Belt Objects (KBO) are illustrated. The newly discovered object, 2001 KX76 (diameter about 1200 km), is the largest known KBO and is even larger than Pluto's moon, Charon. For comparison Pluto's diameter is about 2300 km. Photo: ESA, ESO, Astrovirtel & Gerhard Hahn (German Aerospace Center, DLR)
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.

The recalculation of the size and orbit of 2001 KX76 also marks a major step forward for virtual observatories, which are only now beginning to harness the vast archives of data collected by telescopes for additional uses such as this. "These observations were originally made for a completely different project," noted team leader Gerhard Hahn.

The research also involved collaboration between amateur and professional astronomers, with the key calculations of the object's orbit performed by German amateur astronomer Arno Gnaedig on his home computer. "The Web and the access to 'virtual observatories' means that amateur astronomers -- located far from any 'real' professional telescopes -- can also make important contributions," he noted.