Cosmic Puffball

Another star, another planet, and this one’s an oddball.

Astronomers have announced that they’ve found a puffball of a planet (TrES-4) orbiting a star some 1,400 light-years from Earth in the constellation Hercules. The planet, about 70 percent larger than Jupiter, is the largest exoplanet yet found. It orbits its sun once every 3.5 days at a distance of about 4.5 million miles. And it’s hot – about 2,300 degrees F.

But if the planet is big, it’s also a relative fluffball. The team calculates that the planet’s gravity is so weak that its atmosphere may trail away from it like a comet’s tail. The researchers estimate that the orb has the density of balsa wood or volcanic pumice.

And therein lies a mystery, notes Edward Dunham, a researcher at the Lowell Observatory in Flagstaff, Ariz., and a member of the team. The planet falls into a category known as superheated gas giants, or “hot Jupiters.” Current theories about how these kinds of planets form can’t explain how something so voluminous could have such a low mass, and hence, low density. The results are set to be published later this year.

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