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TRICKY ORBITThe orbit of the newly discovered solar system object SQ372 (blue) is shown in comparison to the orbits of Neptune, Pluto and Sedna (white, green, red). The sun is marked by yellow dot. The inset panel shows an expanded view, including the orbits of Uranus, Saturn and Jupiter inside the orbit of Neptune.Kaib

A tiny lump of ice and rock now near Neptune appears to have some very special properties. Astronomers have discovered that it came in from the cold — the frigid outermost limits of the solar system — and is now on its way back out there.

The roughly 40-kilometer-wide object, dubbed 2006 SQ372, may be the first known visitor to the planetary neighborhood that still makes return trips home to the remote Oort Cloud. This cloud is a proposed reservoir of long-period comets — those that visit the inner solar system no more than once every 200 years — and was first hypothesized to exist in 1950. It is likely thousands of times more distant from the sun than is Earth.

“We believe SQ372 is the first detected member of a comet population in the outer solar system that comes from the, up-until-now, unobserved inner Oort Cloud,” says codiscoverer Nathan Kaib of the University of Washington in Seattle. “Comets like SQ372 have the potential to tell us what the entire Oort Cloud looks like, which will test theoretical models of the cloud's formation as well as provide clues about the environment that the solar system first formed in.” Kaib's team reported the findings August 18 in Chicago at a meeting on the latest findings from the Sloan Digital Sky Survey.

Researchers have determined that 2006 SQ372 is now on the return leg of a 22,500-year, highly elongated orbit that will take it back to a region 240 billion kilometers from the sun. That’s nearly 1,600 times as far from the sun as Earth is, making 2006 SQ372 the most distant known tourist to visit the outer planets.

“It is pretty nifty to have found something that goes such a distance from the sun,” says codiscoverer Andrew Becker of the University of Washington.

Becker, Kaib and their colleagues found the object with the aid of the Sloan Digital Sky Survey, which is a mammoth study of half the northern sky using a 2.5-meter telescope at Apache Point Observatory in Sunspot, New Mexico. Although the survey primarily scans the northern sky to find galaxies and quasars, solar system objects stand out in the foreground, noticeably moving over days or weeks relative to the fixed stars. The object was first spotted in Sloan images taken in 2006 but follow-up detections in 2007 were needed to more accurately determine its elongated orbit.

Even at its farthest point from the sun, 2006 SQ372 is only a tenth as far as the main part of the proposed Oort Cloud. Simulations by Kaib suggest that this distance is far enough for the body to have been a resident of the inner part of the Oort Cloud.

It turns out, he notes, that only comets from the outer part of the Oort Cloud can quickly “hop” over the orbits of Jupiter and Saturn and make it near Earth. Therefore, the long-period comets that have so far been observed can only reveal the structure of the outer portion of the Oort Cloud.

Theoretical models of the formation of the Oort Cloud predict that it should also host a massive inner part, but comets from this region never make it near Earth. To see the long-period comets from the inner region of the Oort Cloud requires observing comets whose orbits always stay well outside Saturn's orbit — like 2006 SQ372.

The gravity of a passing star could have flung the object out of the inner Oort Cloud and toward the inner part of the solar system. Other icy refugees from the cloud come much closer to the sun’s warming rays, suddenly venting pockets of ice and dust and flaunting the signature dusty tails of comets.

A much larger, Pluto-sized object called Sedna, codiscovered by Mike Brown of the California Institute of Technology in Pasadena in 2003, might also be a remote refugee from the inner Oort Cloud, Becker and Kaib suggest. Sedna doesn’t venture nearly as far out as 2006 SQ372 does, but it also doesn’t come as close to the sun, likely preserving more of the materials it acquired from the solar system’s outer reaches.

Brown says he would have thought that 2006 SQ372 is an escapee from a less-remote reservoir of frozen bodies, called the Kuiper Belt, which lies just beyond Pluto. 2006 SQ372 has a relatively short lifetime of about 180 million years, due to its gravitational interactions with Neptune and Uranus, while aloof Sedna has a stable orbit “and has been there for a long time,” notes Brown. The relatively unstable orbit of 2006 SQ372 means “it’s more or less impossible to predict where it was last time around,” he notes, but a large population of similar though less extreme objects has been identified as refugees from the belt.

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