It's not quite as big as Pluto, but it's still one amazing find. Two groups of astronomers at the Keck Observatory in Hawaii and the Sierra Nevada Observatory in Spain, have discovered a large Kuiper Belt object — with its own little moon — about 52 astronomical units (Sun-Earth distances) from the Sun. The object, temporarily designated 2003 EL61 for when it was first spotted two years ago, is as bright as magnitude 17.5. It could be the second largest Kuiper Belt object (KBO) known to date — about 70 percent the diameter of Pluto, which is currently 31 a.u. from the Sun.

Most important are the measurements made for the past six months by Michael E. Brown (Caltech) and colleagues at Keck. Using adaptive optics, the team located and tracked a small satellite orbiting 2003 EL61. The tiny body revolves around the KBO every 49 days in a nearly circular orbit some 49,500 kilometers (30,760 miles) from it.

By determining the companion's orbit, Brown and colleagues calculated the mass of the 2003 EL61 system at around 28 percent of the Pluto-Charon system. From that they inferred the size. Contrary to what false news reports have said in the past 24 hours, "It's definitely not bigger than Pluto," says Brown. It's probably more like 1,500 kilometers across." Pluto, for reference, is about 2,250 km across.

Brown and his team have also looked at 2003 EL61 with the Spitzer Space Telescope. Those observations are still being analyzed, but, Brown notes, "The spectra are dominated by water ice. It looks much like [Pluto's moon] Charon."

More information about 2003 EL61 will be released in the coming months. But the object might be given a far more palatable name sooner than usual. The Minor Planet Center of the International Astronomical Union gives official designations to discoveries after their orbits have been confirmed. In this case, confirmation is imminent. Already astronomers have spotted the KBO on old Palomar Sky Survey photographic plates shot in 1955. Gareth V. Williams of the Minor Planet Center is currently looking through the Harvard College Observatory plate collection to see if it can be spotted as far back as 1890. "This object was 17th magnitude back in the 1920s," says Williams. "We may find dozens of images of it."


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