Planetoid on the Fringe: Solar system record breaker

Lurking more than 13 billion kilometers from Earth in the coldest, remotest part of the solar system, a newly discovered body lies three times farther from the sun than Pluto does. It’s the most distant object ever found to orbit the sun and the largest denizen of the solar system discovered since Pluto in 1930.

DISTANT DENIZEN. Artist’s depiction of Sedna, the most distant object known in the solar system. To the right of Sedna, about halfway to the white dot that indicates the sun, lies a faint image of the remote object’s proposed moon, which the Hubble Space Telescope will soon look for. R. Hurt/Caltech
Arrows mark the spots in telescope images that led to Sedna’s discovery. Brown, et al.

Almost as red as Mars, the body may also be unchanged since shortly after the sun’s birth and so may provide rare clues about the solar system’s earliest history, says codiscoverer Michael E. Brown of the California Institute of Technology in Pasadena. His team announced the discovery in a March 15 circular of the International Astronomical Union.

“Awesome!” exclaims planetary scientist David Jewitt of the University of Hawaii in Honolulu.

Brown and his colleagues found the so-called planetoid by using a small telescope at Palomar Observatory in Escondido, Calif. In a sequence of images taken on Nov. 14, 2003, the astronomers spied “the slowest-moving object we have ever seen,” says Brown. The motion indicated that the body is part of the solar system, rather than the fixed background of stars and galaxies, and its slowness showed that the planetoid resides at the solar system’s edge.

After culling additional data from several telescopes, Brown and his colleagues estimated that the body is about three-quarters the size of Pluto, but it’s larger than the planetoid Quaoar, which Brown’s team found in 2002. Until now, Quaoar had been the solar system’s largest known object beyond Pluto (SN: 10/12/02, p. 228: Hefty Discovery: Finding a Kuiper belt king). The new object appears to rotate slowly, suggesting that it has a moon.

The planetoid’s discoverers have dubbed it Sedna after the Inuit goddess who lives in an icy cave at the bottom of the Arctic Ocean, and the distant object is already proving puzzling. Although Sedna appears to be unusually reflective, its surface does not contain ices, which render other bodies of the solar system highly reflective in sunlight. Moreover, Sedna’s extreme redness is perplexing. “We’re completely baffled” about the composition of the planetoid’s surface, says Brown.

Sedna’s orbit poses yet another riddle. Using archival images to trace the path of Sedna back to 2001, Brown’s team found that the body, now 120 times more distant from the sun than Earth is, has the most elongated orbit known in the solar system. The group calculates that Sedna’s 10,000-year orbit takes it as far from the sun as 900 times the Earth-sun distance and as close as 76 times that distance.

“The question is, ‘How the hell did [Sedna] end up in this orbit?'” says Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Brown’s team suggests that Sedna is part of the cloud of frigid debris that the Dutch astronomer Jan Oort predicted 54 years ago to be a source of long-period comets with orbits that span millennia. However, the hypothetical Oort cloud (SN: 7/29/00, p. 72: Available to subscribers at A comet’s chilly origin) is situated about 10 times as far out as Sedna’s greatest distance from the sun.

To reconcile that apparent discrepancy, Brown and his colleagues speculate that the sun was born among a cluster of stars, one of which passed close enough to tug on Sedna early in the history of the solar system. Such a tug could have deflected Sedna, along with a group of similar, as yet unseen bodies, into orbits closer to the sun. Some planetary scientists call this proposed assemblage the inner Oort cloud. Sedna may be its first known inhabitant.

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