David Biello in Scientific American:
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.”