Astronomers have identified an icy, red, planet-like object orbiting the sun 8 billion miles away, a distance that is nearly three times as far as Pluto and stretches the limits of the solar system far beyond anything yet discovered.
Scientists said the object may be the first visible evidence of the Oort cloud, a massive spherical shell of comets thought to be loosely orbiting the sun and extending outward almost halfway to the nearest star.
Should the object prove to be part of the Oort cloud, it could also provide new evidence supporting the theory that the early solar system was formed when the sun was part of a closely knit cluster of stars.
The discovery has set off a debate about whether the object is a planet. "In my opinion [a planet] has to be more massive than any other object in a similar location," said astronomer Michael Brown, of the California Institute of Technology, who led the team that observed the object at California's Palomar Observatory. "Our prediction is that there are going to be many, many more of these."
Regardless of the final determination, the new object substantially enlarges the solar system and adds evidence that its nether reaches may be sprinkled with countless dwarf planets, planetoids and would-be comets far beyond the gas giants Saturn, Uranus and Neptune.
"There's absolutely nothing else like it in the solar system," Brown said in a telephone news conference from NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We didn't think we'd ever find something in the Oort cloud this close."
Brown said the team has provisionally named the object "Sedna," after the Inuit goddess of the sea. "We knew it would be the coldest, most distant object in the solar system," he said, "and we had decided that it would be right to name it after an Arctic deity."
He said Sedna is 800 to 1,100 miles in diameter -- at most two-thirds the size of Pluto, or nearly one-seventh the size of Earth.
The team identified Sedna from a series of three photographs taken with Palomar's Samuel Oschin telescope in a three-hour period on Nov. 14, 2003. Sedna appeared as a tiny red point of light, moving across the photograph in each successive exposure, a giveaway that it was a solar system object.
Further observations determined that Sedna was moving in a pronounced elliptical orbit, and at its current distance of 8 billion miles is probably as close to Earth as it will ever be, Brown said. At its extreme, Sedna will be 84 billion miles from Earth, taking 10,500 years to complete an orbit.
"We don't know what it's made of," Brown said, noting that Sedna is one of the reddest objects in the solar system, rivaling Mars, and very bright. "We have been unable to figure out what sort of material is red and reflective," he said. "Our hypothesis is that it is half-rock and half-ice." Brown also said Sedna rotates slowly, an indication that it probably has a moon.
Sedna was observed well beyond the farthest objects so far discerned in the Kuiper Belt, a shell of small icy bodies, including Pluto, between 2.8 billion and 4.7 billion miles from the sun. Brown suggested Sedna could thus be the first object ever seen within the Oort cloud.
"Not so fast," said planetary scientist Alan Stern of the Southwest Research Institute in Boulder, Colo. "This is quite an exciting find, but the jury is out on whether this is the Oort cloud or the Kuiper Belt. We don't know much about it yet. We need to do the work."
Stern, lead scientist for NASA's New Horizons mission to Pluto and the Kuiper Belt, praised the Sedna discovery for demonstrating the outer solar system's large population of dwarf planets, of which Sedna appears to be one. "It's big enough for gravity to make it round," Stern said. "You can argue whether it's major or minor, but it's a planet by any definition."
But whether it's a planet, or planetoid, as Brown describes it, Sedna's possible link to the Oort cloud could provide a glimpse of the forces that shaped the early solar system. Dutch physicist Jan Oort in 1950 theorized that comets began as bits of debris flung outward by planetary gravity during the solar system's formation, only to be arrested in space by gravity and other forces from nearby stars.
"We believe that the stars had to be a lot closer than they are today" for the debris to stay in place, Brown said. Scientists say the Oort theory, which has stood the test of time, shows that the sun was part of a more closely knit "cluster" of stars during the solar system's early days.
Even now, however, forces from the now-distant stars can still "perturb" the orbits of Oort cloud objects enough so they reenter the inner solar system. These are comets.
Although Sedna has an elliptical orbit, it is not a comet, Brown said, "because it doesn't enter the solar system."
Reported By TechNews.com, http://www.TechNews.com
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