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New Object in Solar System Defies Categories
By Kenneth Silber
Staff Writer
posted: 07:00 pm ET
11 November 1999
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centaur_discA recently discovered object could make astronomers rethink how they categorize the numerous small bodies in the outer solar system. Conventionally, objects that orbit the sun in the neighborhood of the giant gas planets are called "centaurs." "Scattered disc objects," by contrast, are bodies found beyond the orbit of Neptune. On October 3, Jim Scotti, a planetary scientist at the University of Arizona's Lunar and Planetary Laboratory, detected a body about 60 miles (100 kilometers) long, located slightly beyond the orbit of Saturn. The object, called 1999 TD10, initially seemed a suitable candidate to be called a "centaur." But after further observations by astronomers, and calculations by Gareth Williams, associate director of the Minor Planet Center, a different picture has emerged: Object 1999 TD10 appears to have an orbit that takes it far into the realm of "scattered disc objects." Indeed, Scotti tells space.com, at its farthest point, or aphelion, the object may be about 300 astronomical units (AU) from the sun. (One AU is the average distance between the sun and Earth.) By contrast, Neptune's average difference from the sun is about 30 AU. Brian Marsden, director of the Minor Planet Center, says this is the first observational evidence that "there's really no difference between scattered disc objects and centaurs." They are all "basically comets," he says, and as they approach Neptune they are pushed either outward past the planets or inward toward the sun. Such a finding, says Marsden, is "not an enormous surprise" -- it accords with theoretical models -- but "it's always nice to have the first one you see." Scotti says 1999 TD10 is "unusual in terms of objects we've seen in the past. But that doesn't mean there aren't lots of these out there." He notes that the object would be un-detectable from Earth during most of its orbit.
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