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Super-hot giant exoplanet spills into host star
by Tushna Commissariat
Posted: 2 March 2010

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A gas-giant planet orbiting a star in the Auriga constellation is slowly having its atmosphere stripped off and siphoned onto the surface of its host star.

A recent paper in the journal Nature, by Shu-lin Li from Peking University and three others, details the intense and eventually fatal interaction between the star and its companion. The host star, WASP 12, is comparable to our Sun in terms of its mass and size; but its companion planet, WASP 12b is the anomaly.

An artist's impression of WASP 12b and its host sun WASP 12. Image: NASA/Fredrick Pont.

The 1.4 Jupiter-mass planet orbits its host some 75 times closer than Earth orbits our Sun, completing an orbit once every 26 hours. It also has an unusual orbital eccentricity for a planet with such a short period – most planets have circularized orbits if they are close to their hosts. Li and colleagues suggest that during the star's evolution tidal forces were so strong that the planet's orbit could not evolve significantly. It is thought that WASP 12b will ultimately spiral into its host in approximately 10 million years.

Due to WASP 12b's proximity to its star, it is one of the most intensely heated planets known at over 2,000 degrees Celsius and it is subjected to immense tidal forces that distort the planet's surface. These tidal forces also cause the planet's atmosphere to escape its gravitational pull and fall towards the host star, forming a disc of material around the star. The star is siphoning off material from the planet at a rate of about 10-7 Jupiter masses per year, equivalent to nearly 190 million billion tonnes.

Li and colleagues speculate that one or more super-Earths – planets with masses about six times that of Earth – might be embedded within the disc formed around the star, which could directly affect the eccentricity of WASP 12b's orbit.

"It's important to study the interactions within this system to discover the processes that affect the evolution of planets," comments Lewis Dartnell, an astrobiologist at University College London who was not involved in Li's work. He goes on to explain that all the atmosphere of WASP 12b will be blown away at some point and it would be interesting to study the remnant and see if it becomes volcanically active like other smaller terrestrial planets that have been found. The Corot 7b planet, discovered in February last year, with a diameter slightly larger than Earth and a 20 hour orbital period, is also located very close to its host. It would have high temperatures, between 1,000 and 1,500 degrees Celsius, and possibly rivers of magma and liquid lava covering the surface. "It was a type of object whose existence had been predicted but not seen before. It will be intriguing to see how Corot 7b and WASP 12b will evolve in time," says Lewis.

WASP 12b was discovered by the SuperWASP planetary transit survey and is located 600 light years away in the constellation Auriga.

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