The largest planet ever found orbiting another star is so puffy it would float on water, astronomers said Thursday.

The newly discovered planet, dubbed HAT-P-1, is both the largest and least dense of the nearly 200 worlds astronomers have found outside our own solar system.

HAT-P-1 orbits one of a pair of stars in the constellation Lacerta, about 450 light-years from Earth.

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"This new planet, if you could imagine putting it in a cosmic water glass, it would float," said Robert Noyes, a research astrophysicist with the Smithsonian Astrophysical Observatory.

HAT-P-1 is an oddball planet, since it orbits its parent star at just one-twentieth of the distance that separates Earth from our own sun.

While Earth takes a year to orbit the sun, the newly found planet whips around its star once every 4.5 days.

Astronomers believe HAT-P-1 may belong to an entirely new class of planets, along with a second, smaller distant world that's also puffier than theories would have predicted, Noyes said.

Astronomers used a network of telescopes in Arizona and Hawaii to discover the planet. Its parent star is too faint to see with the naked eye but can be spied with binoculars.