Caltech astronomers have discovered the most distant known object in the solar system, a frigid, near-planet-sized body of rock and ice that is now three times as far from the sun as Pluto.
The mysteriously red planetoid, named Sedna by its discoverers, was probably formed when the solar system was created, then flung out to the distant regions of the sun's gravitational pull, where it has been unaffected by celestial impacts and solar warming for 4.5 billion years.
"This opens up a fossil window into the early solar system," said astronomer Michael E. Brown of Caltech, leader of the team that discovered it.
"Very little has happened to this object" since it was formed, and further study could provide valuable information about conditions at the time the sun and its companions were created, he said at a NASA-sponsored news conference Monday announcing the discovery.
If estimates are correct, the massive rock is the largest object found in the solar system since astronomer Clyde Tombaugh discovered Pluto in 1930. It may also be the first object to be discovered in the Oort cloud, the hypothesized but never observed halo of frozen rubble and debris that is thought to be the source of most of the solar system's comets.
Sedna, named after the Inuit goddess of the ocean who created the sea creatures of the Arctic, is not a comet because its orbit does not carry it into the inner solar system. It is not quite big enough to be a planet, but is too big to be considered an asteroid.
"There is absolutely nothing else like it in the solar system," Brown said.
Brown emphasized that Sedna is not the 10th planet of the solar system. It is simply too small. In fact, many astronomers no longer consider Pluto to be a planet because it is so small and possesses an orbit, like Sedna's, that lies at an angle to the orbital plane of the other planets.
He conceded, however, that there really wasn't a good definition for what a planet is. "There hasn't had to be a definition" before now, he said. "Everybody knew what was a planet and what wasn't. But with the discovery of new objects, it has become somewhat confusing.... In my opinion, this is not a planet."
He said he expected to find many more objects like it, some potentially even bigger.
The new object was discovered by the same team that two years ago identified the planetoid Quaoar, a chunk of rock that is about 780 miles in diameter, about the size of Pluto's moon Charon. Quaoar is much closer to the sun, orbiting in the Kuiper Belt, a swath of icy debris that stretches from Neptune to beyond the orbit of Pluto.
Sedna is much farther out -- so far, in fact, that the team initially didn't believe the results. "It took us a few weeks before we were convinced that we had stumbled on something big," Brown said.
The new object is between 800 and 1,100 miles in diameter, intermediate between Quaoar and Pluto. Right now, Sedna is 8 billion miles from the sun, about 90 times the distance of Earth from the sun. But at the far point of its elliptical 10,500-year orbit, Sedna will be 84 billion miles from the sun.
To the surprise of astronomers, Sedna is red -- almost as red as Mars -- and shiny. Other objects discovered in the outer reaches of the solar system, like Quaoar, are also red, but they are very dark, like coal or asphalt. Sedna, however, reflects 20% to 25% of the sunlight that reaches it.
"Quite frankly, we are baffled as to why that is," Brown said.
The planetoid is very cold, about minus 400 degrees Fahrenheit. At the farthest reaches of its orbit, it will be colder still, only about 20 degrees above absolute zero (minus 459 degrees Fahrenheit). It has no atmosphere because all common gases freeze at those temperatures.
Brown and his colleagues chose the Inuit name for the planetoid because Sedna, the goddess, also lived in a very cold environment.
Sedna rotates slowly, once every 40 days. "There are very few celestial objects that rotate that slowly," Brown said. The team believes that the rotation is locked to the orbital period of a moon, just as Pluto's nine-day rotation is locked to the orbital period of Charon.
Researchers are planning to turn the Hubble Space Telescope toward Sedna in an effort to image its possible moon. That would also give researchers more precise information about Sedna's diameter and density.
Brown and his colleagues -- Chad Trujillo of Caltech's Gemini Observatory in Hawaii and David Rabinowitz of Yale University -- found the planetoid on the night of Nov. 14, 2003, using a 48-inch telescope at Caltech's Palomar Observatory east of San Diego. The team has been repeating the sky survey carried out when Tombaugh discovered Pluto, but they are using a digital camera that is 100 times more sensitive to light than the photographic plates used earlier.