Correction Appended

A planet around a distant star appears inexplicably puffy, astronomers reported yesterday.

The planet is wider than Jupiter. But with half the mass, it is less dense than cork. In a cosmic bathtub, it would float.

The astronomers said they were baffled about how the planet formed and how it remained so puffy.

“The short answer is, I have no idea,” said Dimitar Sasselov, a professor of astronomy at Harvard and a member of the research team. “It’s a very strange planet.”

The planet, designated HAT-P-1, was found orbiting a star 450 light-years away in the constellation Lacerta. It is the second of its kind that has been located. The first was found circling the star HD 209458 in 1999.