A newly discovered alien planet has a record-breaking low density - about the same as that of balsawood. Astronomers say the planet, called TrES-4, could be losing grip of its puffed-up atmosphere.
"TrES-4 is the largest known exoplanet," says Georgi Mandushev from Lowell Observatory in Arizona, US. "Because of the planet's relatively weak pull on its upper atmosphere, some of the atmosphere probably escapes in a comet-like tail."
Mandushev's team discovered TrES-4 in a project called the Trans-Atlantic Exoplanet Survey (TrES), which uses a network of small automated telescopes in the Canary Islands and in Arizona and California in the US. They look for the slight dimming of stars when planets pass in front of them, blocking out some starlight.
The periodic dimming of a star 1400 light years away in the constellation Hercules revealed the presence of TrES-4, which orbits the star every 3.5 days. Follow-up observations using large telescopes in Hawaii and Arizona showed that the planet is just 0.84 times as massive as Jupiter.
But the amount of dimming makes sense only if the planet is about 1.7 times as wide as Jupiter. Astronomers calculate that its average density is only about 0.2 grams per cubic centimetre - less dense than a wine cork. "It's way lower than the density of water," says Mandushev.
TrES-4 is the least dense planet to date, but several other puffed-up planets are known (see Third 'puffed-up planet' discovered).
Why they're so bloated is still unclear, however. Part of the explanation is that they are very close to their parent stars and are therefore blazing hot. TrES-4 is only about 7.2 million kilometres from its star and its surface temperature is about 1300º Celsius.
But even accounting for the heat, TrES-4 is still far larger than theory predicts. "We just cannot explain why it's so puffed up," Mandushev told New Scientist. "I discussed it with Travis Barman, a theoretician here at Lowell Observatory, and he literally said, 'This planet cannot exist.'"
One possibility is that internal heat inflates the planets. A poorly understood mechanism might separate their hydrogen and helium, with the heavier helium sinking into the core. The contraction could release gravitational energy as heat.
However, it is not clear why this would happen to some giant planets and not others.
Mandushev adds that TrES-4's days are numbered. Its parent star, which is 1.2 times as massive as the Sun, has run out of hydrogen fuel in its core and has begun to grow into a red giant star.
"In a very short time - probably less than a billion years - the planet will be engulfed by the expanding host star," he says.
Journal reference: Astrophysical Journal Letters (forthcoming)
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