PLUTO鈥檚 status as a planet is in danger. Astronomers have found a similar object more than half Pluto鈥檚 diameter in the Kuiper Belt, a collection of primordial icy bodies beyond the orbit of Neptune. The object, dubbed 鈥淨uaoar鈥�, is the biggest thing found in the Solar System since the discovery of Pluto.
Many researchers feel that because Quaoar isn鈥檛 counted as a planet 鈥� it鈥檚 more like a big dirty snowball 鈥� then Pluto, which also orbits in the Kuiper Belt, shouldn鈥檛 be called a planet either. 鈥淭his helps to displace the opinion that Pluto is a planet,鈥� says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics.
Astronomers have found several hundred Kuiper Belt objects besides Pluto since the first was discovered in 1992, but Quaoar is by far the largest. Mike Brown and Chad Trujillo of Caltech first spotted it in June this year, after months of surveying the belt with a 1.2-metre telescope. At the Division of Planetary Sciences meeting this week in Birmingham, Alabama, they report that Quaoar is in a circular orbit about 6.3 billion kilometres from the Sun (see Graphic). 鈥淭hey鈥檝e done a nice job on it,鈥� says Marsden.
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Follow-up observations with the Hubble Space Telescope suggest that Quaoar is around 1250 kilometres across, about the same size as Pluto鈥檚 moon Charon. Independent infrared observations confirmed that estimate, and showed that the object reflects 10 per cent of the sunlight that reaches it. Brown believes the object is a mixture of rock and ice, with a dark surface due to faint ultraviolet light scouring away its surface ice during the 4.5 billion years since the Solar System formed.
What鈥檚 worse for Pluto, Brown and Trujillo expect to find bodies even larger than Quaoar, since their search of the Kuiper Belt is not yet complete. 鈥淚鈥檇 bet we find something as big as Pluto,鈥� says Brown.
Like Marsden, Brown regards Pluto as simply the largest object in the Kuiper Belt, although he says whether it should be called a planet is 鈥渘ot a scientifically interesting question鈥�. He鈥檚 more interested in seeing if any objects have formed further out, more than 7.5 billion kilometres from the Sun. Several groups have looked, but 鈥渟o far everybody has come up blank,鈥� he says.