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-1-P, 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.

At that time, astronomers speculated that perhaps an unusual event -- for example, that the star had somehow been knocked on its side, generating tidal forces that heat the interior -- led to its puffiness.

With the discovery of a second low-density planet, astronomers are wondering whether they are flukes or there is a whole class of puffy planets in the universe formed by a common process not yet understood.

''Now we have two of them, and we wonder if we should go back to the drawing board,'' said Robert W. Noyes, a senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and another member of the research team.

The findings have been submitted to The Astrophysical Journal and were announced at a news conference at the Smithsonian Institution.

An automated network of telescopes noticed that in the double-star system ADS 16402 in Lacerta, one star dimmed about 1.5 percent for two hours every four and a half days. The data suggested that something was periodically passing in front of it and blocking some light.

The astronomers took a closer look with larger telescopes. They were also able to verify the presence of HAT-1-P by detecting four-and-a-half-day wobbles in the frequency of the star's light caused by the gravitational pull of the planet.

Many planets detected around other stars have been ''hot Jupiters,'' gas giants like Jupiter or larger, in very close orbits around the stars. The new planet falls into that category, circling at a distance one-twentieth that between Earth and the Sun.

While gas giants like Jupiter and Saturn are primarily hydrogen and helium, they also possess rocky cores and crushing pressures within that squeeze the hydrogen and helium to higher densities.

Jupiter's average density is 133 percent greater than of water. Saturn's is 70 percent that of water. The density of HAT-1-P is one-quarter that of water.

''Both Saturn and this planet would float on water, but this one would float a lot higher,'' Dr. Noyes said. ''The low densities would suggest very few rocks at the core.''

Astrophysicists now have two problems to solve: how a planet that has almost no elements heavier than hydrogen and helium can form and how it stays hot.

''Our lack of understanding is worse than we thought it was,'' said Alan P. Boss, a planetary theorist at the Carnegie Institution of Washington who was not involved in the research.

Almost certainly, higher-than-expected temperatures within HAT-1-P cause the gases to expand and the planet to puff. Its diameter, 38 percent wider than Jupiter, is 24 percent bigger than theories predicted. HAT-1-P's proximity to the star is insufficient explanation.

Gas giants actually do pass through a puffy period in their youth when they are much hotter and then contract as they cool. Neither star, however, is young, and neither planet is likely to have formed recently.

''From the point of view of planetary theory, they're in a region we would call Neverland,'' Dr. Sasselov said.

Like Peter Pan, the two planets seem to have found a way to maintain their youthful appearance.

One idea that Dr. Sasselov is exploring is that hydrogen and helium separate in a puffy planet, forming layers like oil on water. The energy from the heavier helium's falling to the center of the planet might be enough to explain the heating, he said.

''That's an exotic scenario,'' Dr. Sasselov said. ''It's a possibility.''

Another possibility is that a second planet is pulling HAT-1-P into an eccentric orbit that would heat the interior through friction caused by the tides. That cannot be the case for the HD 209458 planet, however, because years of observations have shown the orbit to be almost circular.

Dr. Sasselov said he thought it unlikely that the two large planets had been slammed in a way that tipped them on their sides, adding, ''It's like an impossible billiards ball hit.''

Chart: ''Strange New Planet''
Smithsonian astronomers have discovered a new planet, designated HAT-P-1, in the distant constellation Lacerta. It is now the largest measured planet, although is has only half the mass of Jupiter.

HOST STAR: Slightly younger than Earth's Sun.

Less dense than cork, the planet takes only 4.5 days to orbit its host star and orbits at 1/20th the distance of Earth from the Sun.

MERCURY is the closest planet to the Sun, but is still 6 to 9 times farther from the Sun than HAT-P-1 is from its host star.

(The New York Times; planet rendering by David A. Aguilar)

(Source by Harvard-Smithsonian Center for Astrophysics)