鈥淚F SATURN was to float in water, it would be like an iceberg, with most of the planet under the water,鈥� says Dimitar Sasselov. 鈥淏ut our new planet would stick out on top like a beach ball.鈥�
This is not some fantasy planet. Sasselov, of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and his colleagues reported last week that they have found an extrasolar planet that has an average density about a quarter that of water, making it about as dense as cork. The planet is the second of its kind to be found, suggesting such strange, puffed-up planets are quite common.
鈥淭he planet has an average density about a quarter that of water, making it about as dense as cork鈥�
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Astronomers, led by G谩sp谩r Bakos of the CfA, spotted the planet using the Hungarian Automated Telescope (HAT) 鈥� a network of six small telescopes in Arizona and Hawaii. Computer analysis of HAT鈥檚 observations revealed that one of a binary pair of sun-like stars 450 light years away 鈥� in the direction of the constellation Lacerta 鈥� dims by about 1.5 per cent every 4.5 days. Follow-up observations using large telescopes in Hawaii confirmed that the dimming is due to a planet half as massive as Jupiter 鈥渢ransiting鈥� across the star鈥檚 face during its orbit.
However, the amount of dimming only makes sense if the planet, named HAT-P-1, is 38 per cent wider than Jupiter, making it very bloated and about 24 per cent bigger than theories of planetary structure can explain. A similar planet, HD 209458b, was found in 1999. It too is about 20 per cent larger than expected. Since only 11 transiting planets are known, the fact that two are bloated suggests that they are common.
鈥淲hatever the process that makes planets like this, it is not particularly rare,鈥� says Tim Brown, an extrasolar planet expert at the National Center for Atmospheric Research in Boulder, Colorado. 鈥淏ut I don鈥檛 think anybody understands why [they] are so big.鈥�
One idea is that some kind of internal energy source puffs them up, such as tidal heating: the gravity of the parent star might distort and stretch the planet, warming and swelling it up in the process. However, the theory would work only if the planets have either unlikely orientations for their axis of rotation or highly elongated orbits. Astronomers know that HD 209458b鈥檚 orbit, at least, is not very elongated.
Sasselov says there is another possibility 鈥� that some poorly understood mechanism has allowed hydrogen and helium to separate out. 鈥淏eing heavier, helium would then settle to the centre of the planet,鈥� he says. 鈥淭hat contraction could release gravitational energy as good old-fashioned heat.鈥� This would puff up the planet. If so, astronomers would have to explain why it doesn鈥檛 happen in all giant planets.
