Object Bigger than Pluto Discovered, Called 10th Planet


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Astronomers have discovered an object in our solar system that is larger than Pluto. They are calling it the 10th planet, but already that claim is contested.

The new world's size is not at issue. But the very definition of planethood is.

It is the first time an object so big has been found in our solar system since the discovery of Pluto 75 years ago.

The announcement, made today by Mike Brown of Caltech, came just hours after another newfound object, one slightly smaller than Pluto, was revealed in a very confusing day for astronomers and the media.

The new object, temporarily named 2003 UB313, is about three times as far from the Sun as is Pluto.

"It's definitely bigger than Pluto," said Brown, a professor of planetary astronomy. The object is round and could be up to twice as large as Pluto, Brown told reporters in a hastily called NASA-run teleconference Friday evening.

His best estimate is that it is 2,100 miles wide, about 1-1/2 times the diameter of Pluto.

One of many?

The object is inclined by a whopping 45 degrees to the main plane of the solar system, where most of the other planets orbit. That's why it eluded discovery: nobody was looking there until now, Brown said.

Some astronomers view it as a Kuiper Belt object and not a planet. The Kuiper Belt is a region of frozen objects beyond Neptune.

Pluto is called a Kuiper Belt object by many astronomers. Brown himself has argued in the past for Pluto's demotion from planet status, because of its diminutive size and eccentric and inclined orbit.

But today he struck a different note.

"Pluto has been a planet for so long that the world is comfortable with that," Brown said in the teleconference. "It seems to me a logical extension that anything bigger than Pluto and farther out is a planet."

Offering additional justification, Brown said 2003 UB313 appears to be surfaced with methane ice, as is Pluto. That's not the case with other large Kuiper Belt objects, however.

"This object is in a class very much like Pluto," he said.

NASA effectively endorsed the idea in an official statement that referred to 2003 UB313 as the 10th planet.

Yet in recent years, a bevy of objects roughly half to three-fourths the size of Pluto have been found.

No definition for 'planet'

Brian Marsden, who runs the Minor Planet Center where data on objects like this are collected, says that if Pluto is a planet, then other round objects nearly as large as Pluto ought to be called planets. On that logic, 2003 UB313 would perhaps be a planet, but it would have to get in line behind a handful of others that were discovered previously.

"I would not call it the 10th planet," Marsden told SPACE.com.

Way out there

The new world is about 97 astronomical units from the Sun. An astronomical unit is the distance between the Sun and Earth. It becomes the farthest-known object in the solar system, and the third brightest of the Kuiper belt objects.

It is colder than Pluto and "not a very pleasant place to be."

It was found using the Samuel Oschin Telescope at Palomar Observatory.

Backyard astronomers with large telescopes, some experience and a map may be able to spot 2003 UB313.

Brown said it will be a very exciting object to explore since professionals and amateurs both have access to it.

"My first reaction was, 'aw, I lost the bet by seven days,'" he said.

Brown's team has submitted a name proposal to the International Astronomical Union and has chosen not to divulge it until that body makes a decision.

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