New Planetoid Found in Solar System—Most Distant Yet

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Sedna, the researchers say, lies beyond the Kuiper Belt in a region of space previously thought empty.

"One of the things we've learned about the Kuiper Belt in the last decade is that it is has a very sharp edge to it, just beyond the outer extent of Pluto, and this object is well beyond that edge, so it makes much more sense to say it's something other than a Kuiper Belt object," Brown said.

According to Brown, Sedna resembles objects predicted to lie in the hypothetical Oort Cloud, which is believed to be a zone of early comets that extends around the sun halfway to the nearest star.

But Sedna is much closer to the sun than the predicted distance of the Oort Cloud. Brown said Sedna may have been formed by gravity from a rogue star near the sun early in the history of the solar system.

Not a Planet

Brown and his colleagues are not calling Sedna a planet. Like many other researchers, he believes that Pluto should never have been considered a planet either. Part of the confusion is that astronomers have never properly defined what a planet is.

"In my opinion, to be considered a planet you need to be considerably more massive than any other object in a similar location," Brown said.

By this definition, Brown does not consider Pluto to be a planet, because it sits well inside a group of other objects in the Kuiper Belt that are nearly as massive as Pluto. And while Sedna is currently all by itself, Brown expects to find many more similar objects in the region over the next decade. He prefers to call Sedna an Oort Cloud object.

According to Stern, the astronomer with the Southwest Research Institute, the broadest definition of a planet is anything massive enough for gravity to make round.

"When an object is tiny, its strength controls its shape, but if you keep piling on mass the strength [eventually] means nothing. Be it cotton or steel, if it is thousands of kilometers across, the gravity will take over — it is rounded by gravity," said Stern.

Stern prefers to classify objects such as Sedna, which is thought to be about 1,000 miles (1,600 kilometers) across, as "dwarf planets," in the same vein as dwarf stars and dwarf galaxies.

Regardless of what Sedna is called, both Brown and Stern expect many more of these bodies to be found in the years to come.

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