- Astronomers have found a tenth planet, larger than Pluto
and nearly three times farther from the sun as Pluto is today.
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- 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. It also is the largest body yet found orbiting
in the Kuiper belt, the group of icy bodies including Pluto which orbit
beyond Neptune.
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- 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."
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- 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.
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- 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
the 2003 UB313.
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- Tilted orbit
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- 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 it 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.
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- 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.
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- 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.
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- From that failure, Brown concluded the planet must reflect
50% to 100% of incident light - and thus must be larger than the 2300-kilometre
Pluto but no larger than 3000km 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.
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- Hacked data
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- 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.
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- The following morning, Brown received a phone call claiming
that unknown hackers had stolen some of his data and planned to publish
it. That led him to announce the planet and a third object - temporarily
designated 2005 FY9 by the Minor Planet Center.
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- 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.
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- 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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- http://www.newscientistspace.com/article.ns?id=dn7763
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