Newly observed "planet" is larger than Pluto.

Credit: Max Planck Institute for Radio Astronomy

Pull out your old grade-school models and dioramas of the solar system. They may need to be fixed.

In a paper published in the February 2nd issue ofNature, astronomers from the University of Bonn found that UB313—an object discovered on the outskirts of the solar system last July—is larger than Pluto. And it may not be the last large object discovered in the distant reaches of the solar system.

Frank Bertoldi, lead author of the study, said that observations made by the team that discovered UB313 indicate that we can “expect to find now a few more of these objects—not 10 or a 100, but maybe a few more.”

“Now we have a consistency problem,” he added. “Either we take Pluto out, or put these things in.”

Bertoldi and his colleagues, from the University of Bonn and the Max Planck Institute for Radio Astronomy, determined the diameter of UB313 to be 3,000 km—about 700 km larger than Pluto. The astronomers measured the wavelength of radiation from the object to be 1.2 mm. At this wavelength, only the surface temperature of the object and its size—not the reflection of the sunlight— contribute to its brightness. Since surface temperature can be estimated as a function of distance from the sun, radiation at this frequency provides a reasonable estimate of size.

The confirmation of UB313’s size is fueling the debate about what exactly makes an object a planet. In September, 2005 a panel organized by the International Astronomical Union (IAU) made the radical suggestion that the word planet should only be used with a modifier—such as terrestrial or gaseous. Bertoldi said that such changes in nomenclature confuse, rather than clarify, the issue for the general public.

“Scientifically, we know there are different kinds of planets,” he said. “There are Earth-like planets, gaseous planets, big icy planets, then there is Pluto—part of the Kuiper belt.”

Because UB313 and Pluto are so similar, it’s difficult to justify bestowing the title of planet on only one of them. Both reside in the region beyond Neptune called the Kuiper belt; they have a similar absorption spectrum, suggesting that UB313, like Pluto, has an icy surface consisting mostly of frozen methane. Each has a single moon.

Bertoldi is in favor of setting the size of Pluto as the lower size-limit for planets.

“Just for consistency, [we should] also call any object that is larger than Pluto that is found in the solar system a planet,” he said.

W. J. Althoff, a coauthor of the paper, suspects that Pluto will retain its place in the planet pantheon for historical reasons. But, he points out, when Pluto was initially discovered and classified, its size was substantially overestimated.

“I wonder what the classification would have been if the real size was known,” he said via e-mail.

UB313, sometimes referred to in the press as “Xena,” was discovered through observations made at the Palomar Observatory’s Samuel Oschin Telescope.  It is 97 times farther away from the sun than the Earth, and is the largest new body found in the solar system since the discovery of Neptune in 1846.

Mike Brown, one of the team that originally discovered UB313, is currently analyzing data about the object recorded by the Hubble Space Telescope. He thinks that data will provide a very precise measurement of UB313, but that the measurements made by Bertoldi and colleagues are the best estimate to date.

The debate over Pluto’s status as a planet has particular relevance because of New Horizon&mdadsh;NASA’s Pluto-Kuiper Belt mission that launched on January 19th, 2006. The eight-year cruise is designed to study the last unexplored planet in our solar system. Bertoldi suggests that astronomers at NASA might prefer that Pluto remains a planet, preserving the dramatic significance of the mission.

“Because the American congress spent half a billion dollars to send a mission to Pluto, if you tell them now it’s not a planet any more they may just think astronomers are crazy,” he said.

Originally published February 3, 2006

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