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03/15/2004 08:00:00

Most distant object in solar system discovered; could be part of never-before-seen Oort cloud

PASADENA, Calif.--A planetoid more than eight billion miles from Earth has been discovered by researchers led by a scientist at the California Institute of Technology. The new planetoid is more than three times the distance of Pluto, making it by far the most distant body known to orbit the sun.

The planetoid is well beyond the recently discovered Kuiper belt and is likely the first detection of the long-hypothesized Oort cloud. With a size approximately three-quarters that of Pluto, it is very likely the largest object found in the solar system since the discovery of Pluto in 1930.

At this extreme distance from the sun, very little sunlight reaches the planetoid and the temperature never rises above a frigid 400 degrees below zero Farenheit, making it the coldest known location in the solar system. According to Mike Brown, Caltech associate professor of planetary astronomy and leader of the research team, "the sun appears so small from that distance that you could completely block it out with the head of a pin."

As cold as it is now, the planetoid is usually even colder. It approaches the sun this closely only briefly during the 10,500 years it takes to revolve around the sun. At its most distant, it is 84 billion miles from the sun (900 times Earth's distance from the sun), and the temperature plummets to just 20 degrees above absolute zero.

The discoverers---Brown and his colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University--have proposed that the frigid planetoid be named "Sedna," after the Inuit goddess who created the sea creatures of the Arctic. Sedna is thought to live in an icy cave at the bottom of the ocean--an appropriate spot for the namesake of the coldest body known in the solar system.

The researchers found the planetoid on the night of November 14, 2003, using the 48-inch Samuel Oschin Telescope at Caltech's Palomar Observatory east of San Diego. Within days, the new planetoid was being observed on telescopes in Chile, Spain, Arizona, and Hawaii; and soon after, NASA's new Spitzer Space Telescope was trained on the distant object.

The Spitzer images indicate that the planetoid is no more than 1,700 kilometers in diameter, making it smaller than Pluto. But Brown, using a combination of all of the data, estimates that the size is likely about halfway between that of Pluto and that of Quaoar, the planetoid discovered by the same team in 2002 that was previously the largest known body beyond Pluto.

The extremely elliptical orbit of Sedna is unlike anything previously seen by astronomers, but it resembles in key ways the orbits of objects in a cloud surrounding the sun predicted 54 years ago by Dutch astronomer Jan Oort to explain the existence of certain comets. This hypothetical "Oort cloud" extends halfway to the nearest star and is the repository of small icy bodies that occasionally get pulled in toward the sun and become the comets seen from Earth.

However, Sedna is much closer than expected for the Oort cloud. The Oort cloud has been predicted to begin at a distance 10 times greater even than that of Sedna. Brown believes that this "inner Oort cloud" where Sedna resides was formed by the gravitational pull of a rogue star that came close to the sun early in the history of the solar system. Brown explains that "the star would have been close enough to be brighter than the full moon and it would have been visible in the daytime sky for 20,000 years." Worse, it would have dislodged comets further out in the Oort cloud, leading to an intense comet shower, which would have wiped out any life on Earth that existed at the time.

There is still more to be learned about this newest known member of the solar system. Rabinowitz says that he has indirect evidence that there may be a moon following the planetoid on its distant travels--a possibility that is best checked with the Hubble Space Telescope--and he notes that Sedna is redder than anything known in the solar system with the exception of Mars, but no one can say why. Trujillo admits, "We still don't understand what is on the surface of this body. It is nothing like what we would have predicted or what we can currently explain."

But the astronomers are not yet worried. They can continue their studies as Sedna gets closer and brighter for the next 72 years before it begins its 10,500-year trip out to the far reaches of the solar system and back again. Brown notes, "The last time Sedna was this close to the sun, Earth was just coming out of the last the last ice age; the next time it comes back, the world might again be a completely different place."

Written by Robert Tindol