EMBARGOED for UPDATE: Story first posted 9 a.m. EST, March 15, 2004
The most distant object ever seen orbiting the Sun is nearly as large as Pluto, expanding astronomers notions of how the solar system formed and what resides in its outskirts.
The round world is currently three times farther away than Pluto from the Sun, a distance that expands even further on its 10,000-year orbit. It sits in a part of the solar system that some astronomers had thought empty. It is redder and brighter than anything astronomers have seen in the outer solar system, and scientists don't know why.
The object may even have its own little moon.
"There's absolutely nothing else like it known in the solar system," said Michael Brown of the California Institute of Technology in Pasadena, Calif.
Other researchers say they're not even sure how to classify the object, and the puzzling discovery is just the beginning of many years of investigation that will be needed to figure out the nature of space beyond Neptune.
Meet Sedna
The object is catalogued as 2003 VB12 and has unofficially been dubbed Sedna, goddess of the sea for Arctic dwellers. Brown thought that appropriate given the frigid conditions under which the solar system body has probably always existed. The International Astronomical Union would have to approve the name.
The discovery was led by Brown, who discussed it today at a NASA press conference.
Brown does not consider Sedna to be a planet. He and many other astronomers maintain that Pluto should not have ever received planet status, either, since astronomers are now finding myriad round objects beyond the orbit of Neptune, and several of them are quite large.
Pluto is about 1,413 miles (2,274 kilometers) wide. Sedna is estimated at no more than 1,100 miles (1,770 kilometers) in diameter. It may be the largest object in the solar system after Pluto, but more observations are needed to pin that down.
Sedna is some 8 billion miles away, or 86 astronomical units (AU) from the Sun. One AU is the distance from Earth to the Sun (93 million miles). Pluto is, on average, 39.5 AU from the Sun. But Sedna's orbit, tracked since November when it was first spotted, can bring the object out to some 84 billion miles. It is a very elliptical orbit.
"If you were stand on the surface of Sedna today and you held a pin at arms length, you cold cover the entire Sun with the head of that pin," Brown said. Even the largest backyard telescopes would have a hard time spotting Sedna from Earth, he said.
The region beyond Pluto is commonly called the Kuiper Belt. It is loaded with icy objects large and small.
Most primordial object
Brown said Sedna may be the most primordial object ever detected, having undergone very little heating by the Sun and having had few collisions in the sparse region of space where it resides. Other objects in the solar system, according to the latest thinking, have typically been transformed significantly since their formation.
The next two largest Kuiper Belt Objects were also discovered by Brown's group. Both were much closer.
Last month, the team announced 2004 DW, which was estimated to be between 520-1,170 miles wide (840 to 1,880 kilometers). The best estimate is that 2004 DW is 994 miles across (1,600 kilometers). It is nearly 47 AU from the Sun.
In 2002, the group found 2002 LM60, also named Quaoar (KWAH-o-ar). It is roughly 780 miles (1,250 kilometers) wide, about half as big as Pluto. Quaoar is 42 AU from the Sun.
Eugene Chiang, an astronomer at the University of California at Berkeley, told SPACE.com that the discovery of Sedna adds to the case that Pluto is more like a Kuiper Belt Object than a planet. He called Pluto's discovery, in 1930, an accident of history.
"If Pluto were discovered after all these discoveries, would we have called it a planet? No," said Chiang, who was not involved in the Sedna discovery.
Strange origins
Brown said Sedna occupies a region of space beyond the Kuiper Belt but inside the theorized Oort Cloud, a distant reservoir of icy comets that are detected only when they zoom through the inner solar system on occasion. The Oort Cloud is thought to extend halfway to the next known star, but scientists know almost nothing about its scope, density or composition.
Brown said the discovery suggests the Oort Cloud might be more dense -- containing more objects -- than was previously thought.
"It is very likely that there are more inner Oort cloud objects like Sedna," Brown says, noting that only 15 percent of the sky has been surveyed for objects so dim as this.
Sedna probably was formed nearer to the Sun, in what's now the Kuiper Belt. Like other objects there, it would have been gravitationally booted outward by the giant planets early in the 4.6-billion-year history of the solar system. Many such objects should have been ejected from the solar system. But interactions with very distant stars could have forced some to remain in the Oort Cloud.
Sedna's presence suggests, Brown said, that the Sun might have formed in a neighborhood more tightly packed with stars than what's evident today. Other theories of planet formation have suggested this, too, holding that the Sun was long ago booted out of the star cluster in which it was born.
Another view
It is not certain, however, that Sedna should be considered part of the Oort cloud, said Alan Stern of the Southwest Research Institute (SwRI).
"I think it's a really cool find," Stern said in a telephone interview, but he added that it was "not unexpected."
Stern heads up NASA's New Horizons mission that will launch in 2006 and explore Pluto and the Kuiper Belt. He expects the spacecraft to be functional at least out to 50 AU -- short of Sedna's distant location.
Stern and others have long theorized that there would be many objects beyond the orbit of Neptune. He said Sedna might turn out to be a Kuiper Belt Object that has been scattered outward. The problem, he said, is that scientists don't know enough about either region to say for sure what belongs where and what is or isn't in between.
"I think the jury is out on whether it's a scattered Kuiper Belt Object or an Oort Cloud object," Stern said. "There's a fair chance Mike [Brown] is right." Stern said it is not even clear whether there is actually a gap between the two regions.
Stern said only multiple missions to the Kuiper Belt and beyond would answer all the questions he has about the far reaches of the solar system.
Composition unknown
Scientists don't know what Sedna is made of, but they presume it is about half ice and half rock, like other distant solar system bodies. But Sedna appears redder than all but Mars, Brown said, and observations over the next six months or so aim to learn why.
Observations suggest Sedna has a satellite -- a small moon, but further study is needed to determine if that's the case, Brown said. He and other astronomers would not be overly surprised, as several Kuiper Belt Objects, as well as Pluto, have satellites.
Sedna's surface temperature is about minus 400 degrees Fahrenheit (-240 Celsius), the coldest known place in the solar system.
Sedna was found using the 48-inch Samuel Oschin Telescope at Caltech's Palomar Observatory near San Diego. The discovery was confirmed with other observatories, and the object's size was pinned down using NASA's new Spitzer Space Telescope.
Brown's team includes Chad Trujillo of the Gemini Observatory in Hawaii and David Rabinowitz of Yale University.
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