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Planet X Found To Be Larger Than Pluto

Size comparison of UB313 to Pluto, our moon and Earth.
by Staff Writers
Bonn, Germany (SPX) FEB 1, 2006
German astrophysicists have measured the planetary body called UB313, that orbits beyond Pluto, and have found it is the bigger of the two. By measuring its thermal emission, the scientists were able to determine its diameter is about 3000 kilometers, or about 700 km larger than Pluto.

The discovery makes it the largest solar system object found since Neptune was found in 1846. Like Pluto, UB313 constitutes another of the icy bodies orbiting in the Kuiper belt beyond Neptune.

The planet also is the most distant object ever seen in the solar system, traveling in an elongated orbit that takes it out to 97 times farther from the Sun than Earth, or almost twice as far as the most distant point of Pluto's orbit.

A team led by Frank Bertoldi at the University of Bonn and the Max Planck Institute for Radio Astronomy, and MPIfR's Wilhelm Altenhoff, measured the amount of heat UB313 radiates to determine its size, which when combined with the optical observations also allowed them to determine its reflectivity.

"Since UB313 is decidedly larger than Pluto," Bertoldi said, "it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status."

The planet was discovered in January 2005 by astronomer Mike Brown and colleagues at the Californian Institute of Technology in Pasadena.

They were conducting a sky survey using a wide field digital camera that searches for distant minor planets at visible wavelengths, and discovered a slow-moving object, whose distance and orbital shape they were able to determine. The instruments could not determine the object's size, however, although from its optical brightness they estimated it was larger than Pluto.

In optically visible light, the apparent brightness of objects depends on their size as well as on surface reflectivity, which is known to vary between 4 percent for most comets to over 50 percent for Pluto, thereby rendering impossible any accurate size determinations from optical light alone.

Reporting in the Feb. 2 issue of Nature, the Bonn group said it overcame this difficulty by using the IRAM 30-meter telescope in Spain, which is equipped with the sensitive Max-Planck Millimeter Bolometer (MAMBO) detector developed and built at the MPIfR, to measure specific characteristics of UB313's heat radiation that could pinpoint both its diameter and distance from the Sun.

The team also discovered that UB313's surface reflects about 60 percent of incoming sunlight, which is very similar to Pluto's reflectivity.

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