for National Geographic News
Update: On April 11, 2006, astronomers announced that the so-called tenth planet is not as large as previously speculated, though they maintained that it is bigger than Pluto. UB313, nicknamed Xena, is about 1,490 miles (2,400 kilometers) wide, according to new measurements made using the Hubble Space Telescope, the scientists say.
Scientists have measured the size of a solar system object discovered last year and confirmed that it is larger than Pluto.
The icy object, called 2003 UB313, is located in the far reaches of the solar system. It measures 1,900 miles (3,000 kilometers) in diameter. Pluto, by contrast, measures 1,400 miles (2,300 kilometers).
The finding has fueled the debate over what constitutes a planet. Pluto is traditionally considered the ninth planet in our solar system (see interactive map).
Some scientists argue that 2003 UB313 should now be considered the tenth planet of our solar system. Others say Pluto's status as a planet should be reconsidered.
"Since UB313 is decidedly larger than Pluto, it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status," said Frank Bertoldi, an astronomer at the University of Bonn and Max Planck Institute for Radioastronomy in Germany.
Bertoldi led the study, which will be reported in tomorrow's issue of the journal Nature.
Like Pluto, 2003 UB313 is one of the icy bodies found in the Kuiper belt, a ring of some 100,000 objects on the fringes of our solar system beyond Neptune.
These objects are remnants of the clouds of gas and dust from which the sun and the planets are believed to have formed 4.5 billion years ago.
2003 UB313 is the most distant object ever seen in our solar system. It orbits up to 97 times farther away from the sun than Earth doesalmost twice as far as the most distant point of Pluto's orbit.
Mike Brown, an astronomer at the California Institute of Technology in Pasadena, discovered 2003 UB313 a year ago.
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