WASHINGTON · 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.
"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. The planet, a gas giant, is probably a puffed-up ball of hydrogen and helium.
To their astonishment, the astronomers found HAT-P-1 had only half the mass of Jupiter. And its density was about a quarter that of water, and even less dense than cork.
It's not unusual to find a giant planet less dense than water. Unlike the rocky planets Mercury, Venus, Earth and Mars, our own solar system's giant planets, such as Jupiter and Saturn, are composed largely of light gases, such as hydrogen and helium. Saturn, like HAT-P-1, would float if one could set it down in a huge tub of water.
But HAT-P-1 appears to be the fluffiest planet ever discovered. In that tub of water, Noyes said, "the new planet would float very high up, barely sinking in."
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 think HAT-P-1 may belong to an entirely new class of planets, along with a second, smaller distant world that is 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 seen with binoculars.
