Volume: 03, Issue: 16 08/10/2005 
This artist concept shows the planet catalogued as 2003UB313 at the lonely outer fringes of our solar system. Our Sun can be seen in the distance. Image courtesy NASA/JPL/Caltech.
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These time-lapse images of 2003UB313. The planet, circled in white, is seen moving across a field of stars. The three images were taken about 90 minutes apart.  Image courtesy Samuel Oschin Telescope/Palomar Observatory.
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Other Articles in This Issue:
Discovery Returns to Earth
Cassini Continues to Explore the Unknown
Spitzer Finds Hungry Black Holes
Put Your Astronomy Skills Into Practice

Scientists Discover Tenth Planet

Astronomy teachers have an interesting new finding to share with their students: a planet larger than Pluto has been discovered in the outlying regions of the solar system. Currently about 97 times farther from the sun than Earth, the planet is the farthest-known object in the solar system.

The planet is a typical member of the Kuiper belt, but its sheer size in relation to the nine known planets means it can only be classified as a planet, according to Dr. Mike Brown of the California Institute of Technology. The new planet is the third brightest of the Kuiper belt objects.

"It will be visible with a telescope over the next six months and is currently almost directly overhead in the early-morning eastern sky in the constellation Cetus," Brown said.

Brown made the discovery with colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University. The three first photographed the new planet with the 48-inch Samuel Oschin Telescope on Oct. 31, 2003. The object was so far away, however, that its motion was not detected until they reanalyzed the data in January 2005. Over the last seven months, the scientists have been studying the planet to better estimate its size and its motions.

Scientists can infer the size of a solar system object by its brightness, just as one can infer the size of a faraway light bulb if one knows its wattage. The reflectance of the planet is not yet known. Scientists can not yet tell how much light from the sun is reflected away, but the amount of light the planet reflects puts a lower limit on its size.

"I'd say it's probably one and a half times the size of Pluto, but we're not sure yet of the final size, Brown said. [But] we are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system.

A name for the new planet has been proposed by the discoverers to the International Astronomical Union. They are awaiting the decision of this body before announcing the name.

For more information on the discovery and to view images, visit http://www.nasa.gov/vision/universe/solarsystem/newplanet-072905-images.html .

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