Scientists say they have found the first example of a new breed in the solar system's menagerie: a planetoid that spends all its time far beyond Pluto, in a chilly region that was once thought to be empty.
The object could be three-quarters the size of Pluto — leading to speculation in early news reports that it represented a "10th planet" in our solar system. But the head of the discovery team, astronomer Michael Brown of the California Institute of Technology, told journalists Monday that he wouldn't consider the newfound mini-world to be a planet.
Then again, he doesn't consider Pluto to be a major planet either.
Size and distance
The planetoid has not yet been given an official name by the International Astronomical Union, only a number: 2003 VB12. However, Brown and his colleagues have provisionally named it Sedna, after the goddess in Inuit mythology who created sea creatures.
Sedna is thought to be 800 to 1,100 miles (1,200 to 1,700 kilometers) in diameter. That would make it one of the largest objects found in the solar system since Pluto was first spotted in 1930.
What's most distinctive about Sedna, however, is its distance.
"There's absolutely nothing else like it known in the solar system," Brown said.
The mini-planet has an eccentric 10,500-year orbit that ranges between 8 billion and 84 billion miles (12.8 billion and 134 billion kilometers), which is much farther away than the planets and an outlying ring of frozen cosmic leftovers known as the Kuiper Belt.
This led Brown and his colleagues to conclude that Sedna is the first object ever observed in the Oort Cloud, a zone of comets that stretches halfway to the next star.
"If this object were to come into the inner solar system, we would classify it as a comet, and it would be the most spectacular comet anyone had ever seen in their life," he said. "But because it never comes into the inner solar system, it's in that region where comets live before they become comets. That's what the Oort Cloud is."
Puzzling cloud
As far away as Sedna is, scientists didn't expect that anything in the Oort Cloud would be nearly that close to the inner solar system — and that may require a change in theories about the origin of the solar system 4.5 billion years ago, Brown said.
He speculated that the sun was created within a star cluster, and that the gravitational effects of the other stars knocked Sedna and other infant worlds out of their original orbits. Today, that star cluster has dispersed, leaving the Oort Cloud as the result of all that ancient interaction.
“Very little has happened to this object since the beginning of the solar system.” Brown said. Thus, Sedna could open "a new fossil window into the solar system."
Brian Marsden, who heads the International Astronomical Union's Minor Planet Center, agreed that Sedna's location was a "puzzle," but wasn't sure he agreed with Brown's hypothesis. Marsden said the planetoid could instead have been pushed into its current orbit by an as-yet-undiscovered object on the very fringe of the solar system.
"We need to go perhaps a lot further beyond the orbit of Neptune," Marsden told MSNBC.com. "Are there in fact perturbers there in the plane of the ecliptic, like — I hesitate to use the word — planets? ... It's tempting to think there might be more Earth-size planets out there."
Cold and dark
Sedna was found as the result of a systematic search for slow-moving objects at the edge of solar system, Brown said. Researchers first detected the object last Nov. 14, using a 48-inch (1.2-meter) telescope at the Palomar Observatory near San Diego. Three hours' worth of observation told them that the object was extremely distant, and within days, telescopes in Chile, Spain, Hawaii and Arizona were put on the case, along with NASA's orbiting Spitzer Space Telescope.
Members of the research team, including the Gemini Observatory's Chad Trujillo and Yale University's David Rabinowitz, combined data about the object's temperature, spectral signature and motion to estimate how far away and how big Sedna was.
At Sedna's distance, temperatures would not get higher than 400 degrees below zero Fahrenheit (-280 degrees Celsius), making it the coldest known celestial body in the solar system, Brown told journalists.
"If you were standing on the surface of Sedna today, and you held a pin at arm's length, you could cover up the entire sun with the head of that pin," he said.
He said Sedna appears to be made up of equal portions of ice and rock. Although the object is too small and faraway to be seen as anything more than a speck, it appears to have an unusually red and shiny appearance.
"We're quite frankly baffled as to why that is," he said.
Sedna also seems to rotate every 40 days, leading researchers to speculate that a moon may be circling the planetoid in that period of time. "Observations from the Hubble Space Telescope should put that question to rest very quickly," Brown said.
Why not a planet?
So if Sedna turns out to be almost as big as Pluto, and possess a moon as well, why shouldn't it be considered the 10th planet?
"The reason it's difficult to answer that question is because astronomers don’t have an official definition of what is and what isn't a planet, and the reason ... is because for most of the history of humans there hasn't had to be an official definition," Brown said. It's only been since the discovery of Kuiper Belt objects similar to Pluto that astronomers have had to deal with the controversy. Pluto currently ranks as the biggest known object in the Kuiper Belt.
Brown said he would define a planet as a celestial body that is considerably more massive than other objects traveling in similar orbits around their parent star. In contrast, some of the objects in the Kuiper Belt are more than half Pluto's size, and it may be only a matter of time before astronomers find a Kuiper Belt object about as big as Pluto. "So by my definition, Pluto is not a planet," Brown said.
By the same token, he expected Sedna to fail the planet test.
"Our prediction is that there will be many, many more of these objects found over the next five years or over the next decade," Brown said, "and it will turn out that Sedna in fact is also not the most massive object in its orbit out there."
Marsden agreed that if more objects like Sedna are discovered, that would rule out planethood. But he said his view might be different if Sedna turned out to be the only world of its kind at that distance.
"What would I say it is then?" he asked himself. "Would I use the P-word? That's a tricky one. With a circular low-inclination orbit at, say, 90 AU (Sedna's current distance), I might be more inclined to use the P-word than I would with Pluto."
Marsden said the latest discovery demonstrated the importance of studying the solar system's fringes, with increasingly sensitive telescopes as well as space missions such as New Horizons, which would target Pluto and the Kuiper Belt in 2015. Sedna would be worth visiting as well, Marsden said.
"I'd much rather send an unmanned mission to this object than a manned mission to Mars," Marsden said. "And you can quote me on that."