News of another “planet” beyond Pluto may become common. First came Quaoar in 2002 and then Sedna in 2003. Unlike these two worlds, the latest candidate, announced in July, is actually bigger than Pluto, by 50 percent. Designated 2003UB313 and unofficially nicknamed Xena, the mass of ice and rock currently lies three times farther out than Pluto. Investigators originally photographed it in 2003 at Palomar Observatory near Los Angeles, but its strange orbit, tilted nearly 45 degrees off that of nearly all other planets, delayed its discovery until this past January. Near-infrared images reveal a surface of mostly methane ice, remarkably similar to Pluto’s. One or two more planets of like size might dwell within the same distance, says planetary scientist Michael Brown of the California Institute of Technology. An untold number of worlds might lurk beyond that, perhaps deriving from the Kuiper belt or the hypothesized Oort cloud. “No one has really probed out to that distance,” he remarks.
This article was originally published with the title "Worlds without End" in Scientific American 293, 4, 32 (October 2005)