NASA

The solar system beyond Neptune is a dark and mysterious place. It is also crowded. Besides Pluto and its moon Charon, there are planetesque chunks of rock and ice like Sedna and the recently discovered 2003UB313 as well as a host of asteroids and comets in the Kuiper belt and beyond. Determining which of these objects constitute new planets and which do not remains controversial work currently under review by the International Astronomical Union (IAU). Some astronomers have even argued that Charon deserves to share the planet title since it is roughly half the ninth planet's size and might have a similar atmosphere.

But new observations, reported today in Nature by Amanda Gulbis of the Massachusetts Institute of Technology and her colleagues prove that Charon lacks an atmosphere and therefore lacks one potential criterion for planet status. "I think having an atmosphere is a key component," Gulbis says. "Our findings show that it doesn't have an atmosphere. I would say that Charon is definitely not a planet."

The astronomers took advantage of a rare moment when Charon crossed in front of a distant star and blocked its light to determine whether it has an atmosphere. If it had one, the star's light would be gradually blocked as opposed to the more precise cutoff that would be expected for a nearly gas-less moon. On July 11, 2005, Charon passed in front of UCAC2 26257135, revealing less of an atmosphere than either Pluto or even Earth's own moon has. "It's astounding that our group could be in the right place at the right time to line up a tiny body three billion miles away," says team member Jay Pasachoff of Williams College.

This occultation, the first to be observed since 1980, also allowed the scientists to come up with a measurement of Charon's radius, roughly 605 kilometers or "about twice the size of Massachusetts," Gulbis notes. Using this radius and mass measurements from the Hubble Telescope, the astronomers calculate that Charon is only roughly 63 percent rock. This means it may have formed in the same way as our own natural satellite: when a large object hit the parent planet and ejected a plume of lighter materials that coalesced into a moon.

Similar occultations could provide data on Sedna and 2003UB313. That, in turn, could decide whether they are planets, because the IAU is working on a definition based on minimum size. If Pluto continues to qualify as a planet, then 2003UB313 may indeed become the 10th planet because it appears to be roughly the same size, meaning that Charon--the ferryman of the Kuiper belt--may help a new member of the solar system cross a river of doubt.