10th Planet

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Click to see full-size image.Image used by permission of the author.
Click to see full-size image.
Image used by permission of the author.

An object that appears to be larger than Pluto has been discovered beyond Pluto on the Kuiper Belt. It was discovered by Dr. Mike Brown of the California Institute of Technology and his colleagues using the Samuel Oschin Telescope at Palomar Observatory near San Diego. It is currently about 97 times farther from the sun than Earth, or 97 Astronomical Units (AU). For comparison, Pluto is 40 AU from the sun.

Using a large telescope, amateur astronomers should be able to see the planet but it will look just like a spec against the starry background. But it is possible to image it using a 4" telescope as proven by Dennis Simmons of Brisbane, Australia. The composite of images on the right was taken with the following equipment:

  • Vixen 4" f/9 refractor
  • SBIG ST7E ccd camera
  • EM200 Temma2 GoTo mount driven by SkyTools RealTime software
  • 4 images acquired via CCDSoft, each of 15 mins duration autoguided by the ST7E.
    • Images flat fielded and dark subtracted in MIRA AP6.
    • Images median combined in MIRA AP6.
  • Other details:
    • FOV 26 x 18 arc mins.
    • Image 1 start 01:29 AEST
    • Image 4 start 02:14 AEST.
  • Location details:
    • Brisbane, Australia.
    • 4th August 2005.
    • Mag 4 suburban skies.

From NASA:
The planet was discovered by, in addition to Brown, Chad Trujillo, of the Gemini Observatory in Mauna Kea, Hawaii, and David Rabinowitz, of Yale University, New Haven, Connecticut. They first photographed the new planet with the 48-inch Samuel Oschin Telescope on October 31, 2003. The object was so far away, however, that its motion was not detected until they reanalyzed the data in January of this year. In the last seven months, the scientists have been studying the planet to better estimate its size and its motions.

"We are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system," Brown adds.

Telescopes have not yet revealed the planet's disk. To estimate how big it is, the astronomers must rely on measurements of the planet's brightness. Like all planets, this new one presumably shines by reflecting sunlight. The bigger the planet, generally speaking, the bigger the reflection. The reflectance, the fraction of light that bounces off the planet, is not yet known. Nevertheless, it is possible to set limits on the planet's diameter:

"Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto," says Brown. Pluto is 1400 miles (2300 km) wide. "I'd say it's probably [about] one and a half times the size of Pluto, but we're not sure."

The size of the planet is further limited by NASA's Spitzer Space Telescope, which has already proved its mettle in studying the heat of dim, faint, faraway objects such as the Kuiper-belt bodies. Because Spitzer has been unable to detect the new planet, the overall diameter must be less than about 2000 miles (3200 km), says Brown.

The planet's temporary name is 2003 UB313. A permanent name has been proposed by the discoverers to the International Astronomical Union, and they are awaiting the decision of this body before announcing the name.