W.M. Keck Observatory

Pluto plus.
The object 2003 UB313--shown in a ground-based image with its moon--is barely bigger than Pluto, according to a new Hubble photo.

Downsizing the "Tenth Planet"

LOS ALTOS HILLS, CALIFORNIA--A new Hubble Space Telescope image reveals that a bright object in the outer solar system is Pluto's near-twin in size, not a big brother. The so-called "tenth planet," announced last July to much fanfare, is a "smidge" bigger than Pluto rather than earlier estimates of 25% to 50% larger, a planetary scientist reported here on 25 January. The downsizing puts a new focus on an ongoing struggle among astronomers to define the lowest size limit for planets.

Astronomers found the body, designated 2003 UB313, as a dot of light moving slowly among the background stars. Although more than twice Pluto's distance from the sun, the object appears so bright that planetary scientist Michael Brown of the California Institute of Technology in Pasadena and colleagues claimed that 2003 UB313 was markedly larger than Pluto. This sparked a debate about whether to call it a planet (ScienceNOW, 1 August 2005). However, Brown noted that he needed better data to gauge its true size.

NASA's Hubble Space Telescope has provided that measure with the best possible image from space, taken in December 2005. Brown showed the picture for the first time to about 1000 people at a public talk here. The blob of light--spanning several pixels on Hubble's detector--looks fuzzy but still had enough resolution for Brown's team to determine that 2003 UB313 is barely bigger than Pluto. That's surprising, Brown said, because it means that the object reflects a remarkable 92% of the light that hits it--compared with roughly 60% for Pluto. "I had expected it to be darker and considerably larger," Brown said. Geysers may continually coat the surface with fresh frost, he speculated, although how that occurs on such a frigid body is unknown.

Brown declined to provide the calculated size, stating that his team would reveal that number at a NASA press conference. However, a chart of 2003 UB313's projected size on Brown's Web site indicates that with a reflectivity of 92%, the object would be roughly 1% larger than Pluto's assumed diameter of 2280 kilometers. The team's previous estimates had ranged from 25% larger (on its Web site) to 50% larger (at NASA's announcement in July).

"Its reflectivity is much higher than expected," comments astronomer Alex Filippenko of the University of California, Berkeley. "It does indicate a fresh, icy surface." Others noted that committees of the International Astronomical Union, which have debated for years about whether Pluto and its cousins are bona fide "planets," now face the starkest possible distinction. "It's almost cosmic justice" that 2003 UB313 and Pluto are a near-match, says astronomer Andrew Fraknoi of Foothill College, which hosted the talk.

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