When astronomers announced the discovery of UB313, the so-called tenth planet, a little more than a year ago, they had a hunch it might be bigger than Pluto because of its brightness. But despite several attempts to observe more closely the mysterious object orbiting the sun at a distance of more than 14 billion kilometers, accurate estimates of its size remained elusive. Now German astronomers working in Spain have determined that UB313 has a diameter of roughly 3,000 kilometers--roughly 700 kilometers larger than Pluto's.

Frank Bertoldi of the University of Bonn in Germany and his colleagues used the IRAM 30-meter telescope in the Sierra Nevada mountains of southern Spain to observe UB313 in the infrared range. Because visual brightness alone is not an accurate indicator of size--it could result from the body's surface being either actually large or mirrorlike--the researchers made observations in wavelengths longer than those of visible light. Outside the range of visible light, the scientists could measure the amount of light the object absorbs and then radiates back as heat. By combining the infrared and visible measurements, they could then determine the object's size and its overall reflectivity, or albedo. Based on observations made over nine nights in August 2005, the team reports, UB313 appears to have a diameter of between 3,094 and 2,859 kilometers. Even the smallest size in that range would make the candidate planet's diameter more than 500 kilometers larger than Pluto's.

These calculations rely on several assumptions, however, such as UB313 lacking an atmosphere that would either reflect more light or trap more heat even though Pluto has such a covering. The astronomers estimate that the planet reflects roughly the same amount of light as Pluto, perhaps thanks to an icy methane surface. Their research appears in today's issue of Nature.

The finding adds impetus to the debate surrounding what constitutes a planet. The International Astronomical Union is currently working on a definition based on minimum size so as to keep Pluto a planet, but this could open the possibility of even more planets in the outer reaches of the solar system. After all, UB313 is one of more than 1,000 objects discovered beyond the orbit of Neptune since 1992. In an accompanying commentary, Scott Sheppard of the Carnegie Institution of Washington suggests it might be time for a new term for such trans-Neptunian objects, much like "asteroid" was coined to refer to the many inner solar system objects discovered in the mid-19th century. If it is size that matters, however, it will be difficult to keep UB313 out of the exclusive planetary club. Says Bertoldi: "Since UB313 is decidedly larger than Pluto, it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status."