London, Sept. 15 (Guardian News Service): A bloated alien world orbiting a star on the fringes of the Milky Way has baffled astronomers, who believe it is the largest planet discovered. At 16 times the size of the Earth, the planet dwarfs the celestial bodies in our solar system, even the gas giant Jupiter, which is more than 140,000 km across at the equator.

Early measurements of the planet suggest it is about 1.4 times the size of Jupiter, but unusually it has a density just one quarter that of water.

"We could be looking at an entirely new class of planets,'' said GasparBakos, an astronomer at the Harvard-Smithsonian Centre for Astrophysics inMassachusetts. "It's lighter than a giant ball of cork ... It would floatin a bathtub if you could find a tub big enough to hold it in.''

The planet was spotted by Dr Bakos's team using a network of automatedtelescopes in Arizona and Hawaii called Hat. The telescopes tracked theplanet as it moved in front of its parent star, causing a mini-eclipse thatmade the star appear 1.5% dimmer for more than two hours.

The planet, named Hat-P-1, orbits one of a pair of stars 450 light yearsaway in a constellation called Lacerta, whose northern tip lies at the edgeof our home galaxy.

Astronomers are perplexed by the discovery because it appears to bepuffed up to around 24% larger than planet formation theory predicts. Thestudy is due to appear in the Astrophysical Journal.

The announcement came as the International Astronomical Union finallydecided upon an official name for the rock which triggered the recentdemotion of Pluto to dwarf planet status. Formerly known as Xena, thedistant object, which was discovered in 2003 and is larger than Pluto, wasnamed Eris - after the Greek goddess of chaos and strife.

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