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Planetary News: Pluto (2008)Meet the Plutoids: IAU Makes Amends for Pluto's DemotionBy Amir AlexanderJune 12, 2008
It has been nearly two years since the members of the International Astronomical Union (IAU) decided that Pluto, known since its discovery as the "9th planet," was not a planet at all. Instead this longstanding favorite of children around the world was designated as a founding member of a new class of "dwarf planets." According to the IAU, the exclusive and now permanently closed club of "true" planets includes only the eight inner planets, from Mercury to Neptune. No new space rocks need apply. Now, in an apparent effort to assuage angry Pluto supporters, the IAU has decided to honor the former planet with a new distinction: Dwarf planets, at least those residing at the edge of the solar system beyond the orbit of Neptune, will henceforth be known as "plutoids." To be precise, according to the IAU's definition a plutoid is a celestial object that satisfies three conditions: it orbits the Sun at an average distance greater than that of Neptune; it is massive enough that its own gravity brings it to hydrostatic equilibrium, making it near-spherical; and it has not cleared the neighborhood around its path from other orbiting debris. This last condition is crucial for distinguishing plutoids from true planets, which orbit in regal isolation after having cleared all debris from their path in the early days of the solar system. In addition to Pluto itself, the IAU has so far designated only one other object as a plutoid: that is Eris, the giant Kuiper belt object (KBO) whose discovery upset the old planetary order. Unlike other large solar system objects detected in recent years, Eris was found to be larger than Pluto. It followed that if Pluto was a planet, then Eris should certainly be one as well, and other large KBO's might qualify for the honor as well. But if Eris was not a planet, then Pluto certainly should not be one either. It is this option that was ultimately adopted by the IAU, in effect permanently limiting the number of planets to eight. In addition to Pluto and Eris, several other objects are being considered for the "plutoid" designation. The candidates include giant KBO's 2005FY9, known informally as "Easterbunny," and 2003EL61, nicknamed "Santa." Other large objects, such as Sedna and Quaoar, may qualify as well, but they are too dim for their shape to be determined with confidence. Their induction into the ranks of plutoids will therefore have to wait until better images are available. Will Pluto's many fans around the world accept plutoids into their hearts, and acknowledge them as true heirs of their favorite (former) planet? Only time will tell. Did you like this story? Send
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