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Tenth planet discovered in outer solar system

  • 10:35 30 July 2005
  • news service
  • Jeff Hecht
These images of the newfound planet were taken in October 2003, each about 90 minutes apart. The planet, circled in white, moves across a background of stars (Image: Samuel Oschin Telescope, Palomar Observatory)
These images of the newfound planet were taken in October 2003, each about 90 minutes apart. The planet, circled in white, moves across a background of stars (Image: Samuel Oschin Telescope, Palomar Observatory)

Astronomers have found a tenth planet, larger than Pluto and nearly three times farther from the Sun as Pluto is today.

Temporarily designated 2003 UB313, the new planet is the most distant object yet seen in the solar system, 97 times farther from the Sun than the Earth is. It also is the largest body yet found orbiting in the Kuiper belt, the group of icy bodies including Pluto which orbit beyond Neptune.

Like Pluto, 2003 UB313 is covered by methane ice, and at its present distance is chilled to just 30°C above absolute zero, says Mike Brown, the Caltech astronomer who announced the discovery on Friday. The sleep-deprived father of a three-week-old daughter, Brown said the discovery was "almost as exciting as having a new baby".

Pluto was the only object known in the Kuiper belt until 1992, but since then astronomers have spotted hundreds more faint, icy bodies with orbits beyond Neptune. Five years ago, Brown's group began a systematic search for big Kuiper belt objects, which earlier yielded Quaoar - about 800 miles in diameter - and Sedna, previously the most distant object known at 91 times the Earth's distance from the Sun.

The survey also yielded the new planet and two other objects only slightly smaller than Pluto, which Brown kept quiet as he analysed the survey data and made new observations to learn more about 2003 UB313.

Tilted orbit

The survey first spotted the new planet in October 2003, but it was not until 8 January 2005 that Brown realised the object was so distant that its brightness meant it had to be very big. Calculations showed it was near the most distant point of its 560-year orbit - in 280 years it will be only 36 times as far from the Sun as the Earth is. (A graphic of its orbit can be viewed here.)

Its orbit is unusual in being tilted 44° from the orbital plane of the Earth and most other planets. Brown suspects the planet's orbit was warped by a series of encounters with Neptune.

Although the planet’s brightness is known, estimating its size requires knowing what fraction of incident light it reflects. Infrared observations could provide that information, but the planet was too faint and cold for the Spitzer Space Telescope to spot.

From that failure, Brown concluded the planet must reflect 50% to 100% of incident light - and thus must be larger than 2300-kilometre-wide Pluto but no larger than 3000 km in diameter. Pluto reflects 60% of visible light, and if the new planet does the same it would be near the large end of that size range.

Hacked data

Brown had hoped to have time to make more observations to pin down more details including the new planet's size and brightness. But Spanish astronomers independently discovered one of the two other big new Kuiper Belt objects, and on 28 July the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Massachusetts published an orbit based on their data for the object it designated 2003 EL61.

The following morning, Brown received a phone call claiming that unknown hackers had stolen some of his data and planned to publish it as their own. (Update, August 1, 2005: The data was in fact accessed via logs of telescope activity that Brown did not realise were made publicly available on the web.) That led him to announce the planet and a third object - temporarily designated 2005 FY9 by the Minor Planet Center.

The discovery is sure to heat up the debate over how to define a planet. Some astronomers claim Pluto is just an overgrown Kuiper-belt object, but Brown thinks it should remain a planet. The International Astronomical Union has avoided a formal definition, but the new object may force the issue. Brown has already proposed a name, but would not disclose it.

The discovery shows "the raw power of the new all-sky surveys" that examine huge areas looking for things that move, says Brad Schaefer, an astronomer at Louisiana State University. The survey has now covered most of the sky, but Brown said a few more big objects may await discovery.



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