At the Edge of Solar System, a 10th Planet May Lurk

By Guy Gugliotta
Washington Post Staff Writer
Thursday, February 2, 2006

So what, exactly, is a planet? Astronomers have been deadlocked over this for years, but a decision may finally be forthcoming, because a thing they discovered last year in the solar system's outer reaches has turned out to be bigger than Pluto.

So if Pluto hangs on to its status -- by no means a slam-dunk -- then the fetchingly named 2003 UB313, which is about 30 percent fatter, probably would have to be a planet, too. The 10th.

But if Pluto gets demoted, the solar system will have just eight. Either way, textbooks that refer to nine planets are doomed to obsolescence.

"It's only fair," said astronomer Frank Bertoldi of Germany's University of Bonn. "It's my view that Pluto for historical reasons should remain a planet -- otherwise school kids will be confused. Any object that's bigger than Pluto should also be a planet."

Bertoldi, reporting today in the journal Nature, led a research team that measured surface temperatures of 2003 UB313 to confirm that this "ice dwarf," more than 9 billion miles from the sun at its farthest point, is about 1,800 miles in diameter, whereas Pluto's diameter is about 1,380 miles.

Neither one is "big." Earth's moon is 2,160 miles across.

Astronomers suspected that 2003 UB313 was at least as big as Pluto and probably bigger when its discoverers measured its brightness last summer. Any object that far away could not be as bright as it was and still be smaller than Pluto.

"But they didn't know how big it was, or whether it could have been as big as Mercury," said Carnegie Institute of Washington astronomer Scott S. Sheppard, author of an article accompanying the Nature paper.

To pin down 2003 UB313's size, astronomers needed to know not only its brightness but also its reflectivity -- how much of the sunlight illuminating the distant body was bouncing off its surface.

"A small body with high reflectivity can be as bright as a large body with low reflectivity," Sheppard said. Brightness in the visible spectrum is a function of both the reflectivity of the body and its size, expressed as the area illuminated by the sun.

To determine reflectivity, Bertoldi's team used a radio telescope in the mountains of southern Spain to examine 2003 UB313 in thermal wavelengths close to infrared and not visible to the naked eye. There its "heat signature" could be measured and subtracted from the total sunlight that reached it. The difference was the reflectivity.

"We took two measurements and got a consistent solution," Bertoldi said in a telephone interview. "We've done this many times over the years, and it's a very secure way of measuring the size of an object" when the object is either too small or too far away to be analyzed easily by a traditional optical telescope.

A team of researchers led by the California Institute of Technology's Michael E. Brown reported the discovery of 2003 UB313 last summer. It was the most distant object ever seen in the solar system, a chill sphere cloaked in water and methane ice, and circling the sun in a highly eccentric orbit 44 degrees off the plane where most of the planets dwell.

Brown nicknamed the new discovery "Lila," after his baby daughter, but permanent naming awaits a decision by the International Astronomical Union, which is also trying to decide how to define a planet.

This, it appears, is heavy going. One school agrees with Bertoldi that the Pluto tradition should be upheld, but another suggests that Pluto, 2003 UB313 and other denizens of the Kuiper Belt beyond Neptune are not planets but a new class of smaller objects -- ice dwarves unlike either the terrestrial planets of the inner solar system, among them Earth, or the outer "gas giants" such as Jupiter and Neptune. There is no clear leader in the debate.

"Until they decide whether it's a planet or not, we don't know how to name it," Brown said in a telephone interview. "This could go on for years."

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