Claims that our solar system has a 10th planet beyond Pluto are supported by a new finding by German and French astronomers. They calculate that it is bigger than Pluto. In the debate over whether this object is a planet or not, size might be the determining factor.
The definition of a planet is a hot topic among astronomers these days after last year's discovery of an icy body beyond Pluto's orbit known only by its scientific designation 2003 UB313.
Many, like Scott Sheppard of the Carnegie Institution in Washington, argue that this new object should not be considered a planet, and for that matter, nor should Pluto. He says both are typical, but larger members of the thousands of icy bodies circling beyond Neptune in the so-called Kuiper Belt.
Unlike the eight major planets, their orbits are not independent, but are attuned to that of Neptune, and their gravitational fields do not dominate their local environments.
"Pluto and 2003 UB313 and the rest of the thousands of Kuiper Belt objects beyond Neptune, since they formed and evolved in a very similar manner, should be grouped together," he said. "So I would personally consider Pluto and the rest of the Kuiper Belt objects not really planets, but the third domain of the solar system or something like that."
The third domain refers to the fact that Pluto and its Kuiper Belt cousins are ice covered, in contrast to the inner planets with rocky surfaces like Earth and the outer gas giants Jupiter and Saturn.
But Sheppard admits there are other ways to classify planets that that would not exclude Pluto and its newly discovered distant neighbor. They would include anything orbiting the sun, spherical in shape, and larger than a certain minimum size.
Here is where the new measurement of the object 2003 UB313 comes in. A team led by astronomer Frank Bertoldi of the University of Bonn reports in the journal Nature that it is bigger than the tiny Pluto, which is smaller than Earth's moon.
The body is too far away to measure by visible means. It has a very elongated orbit that takes it up to 97 times farther from the Sun than Earth is. So Bertoldi's team calculated the diameter based on the amount of heat the object emits using a sensitive infrared radiation detector on a telescope in Spain.
"The result we have is significant," he said. "Our best estimate now is 3,000 kilometers. There is an uncertainty of about 300 to 400 kilometers. It could be lower, it could be higher, but it makes it definitely larger than Pluto, which has a diameter of 2,300 kilometers."
The finding could be considered a victory for the man who discovered 2003 UB313, Michael Brown of the California Institute of Technology. He always believed it was bigger than Pluto and maintains that size matters in determining whether something is a planet.
"Pluto has been called a planet for so long that I think we are never going to not call Pluto a planet," he said. "If Pluto is going to be called a planet, then anything larger than Pluto is a planet. Things that are smaller I think we just call them typical members of the Kuiper Belt and they do not join this very special class of things that are planets."
The researchers who measured 2003 UB313 agree. Frank Bertoldi says if it is not considered a planet, it would be increasingly hard to justify calling Pluto one.
"It is sort of a matter of taste and I guess one has to distinguish between two considerations," he said. "There is a scientific one and there is a cultural one. As scientists, we know what we are talking about. It is not of any consequence whether we call it a planet or not. Culturally, why not? Let us call all the objects larger than Pluto that we find out there also planets."
The International Astronomical Union, the discipline's governing body, is debating the issue. It has formed a committee to define what a planet is.
Scott Sheppard at the Carnegie Institute says its 20 members are divided, but predicts they are likely to keep Pluto a planet and name the new body one, too. Yet he is not sure that will settle the issue.
"I think no matter what the International Astronomical Union decides, it will still be a debate for years to come," he said.