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Destiny of Hot Jupiters TEXT SIZE: A A A

The atmosphere of a planet WASP-12b, which orbits a nearby star, is reported to pull away by the gravity of its Sun at a staggering rate of six billion metric tons per second.  This is the first time astronomers have witnessed the ongoing disruption and death march of a planet. The discovery was made by an international group of astrophysics, primarily working at the recently established Kavli Institute for Astronomy and Astrophysics (KIAA), Peking University. The research findings have been published in Nature (Li, S. L., Miller, N., Lin, D. N. C. & Fortney, J. J., 2010, Nature 463, 1054-1056).

Until 15 years ago, the only known planets are those in our own Solar system. Today, more than 400 planets have been detected around other stars. Because they are easier to detect, most of the so-called exo-planets are gaseous and have mass comparable to that of Jupiter.  Unlike Jupiter and Saturn one in five of these exo-planets are on orbits very close to their host stars. For some of these systems, the planet regularly passes between their Sun and the Earth and undergoes annular eclipses. By accurately measuring how much of the star's light is blocked out, and how long the occultation lasts, it is possible for astronomers to measure planets' size.  Astrophysicists then use this information to figure out the internal make up of these planets and construct scenarios on how they may have acquired these properties.

The planet WASP-12b has puzzled astronomers, since it is one of the few planets that is significantly larger than astrophysicists have predicted. The planet's mass is almost 50% larger than that of Jupiter, the most massive planet in our own Solar System. Although theoretical models had predicted that WASP-12b should have about the same size as Jupiter, it turned out to be 80% larger in size (or almost six times larger in volume). Some mechanism therefore has to be responsible for “puffing up” the planet.

WASP-12b revolves around its host star every single day, at a distance 75 times closer than the distance between the Earth and our Sun. The small separation between WASP-12b and its host star makes it among the most intensely heated planets known. Its day-time temperature is more than 2500 degrees centigrade.

On Earth, tidal forces between the Earth and the Moon cause local sea levels to modestly rise and fall twice a day. WASP-12b, however, is so close to it's host star, that the gravitational forces are enormous. The tremendous tidal forces acting on the planet completely change the shape of the WASP-12b into the shape similar to that of a rugby or American football.

Figure caption:The WASP-12 system.  The massive gas giant WASP-12b is shown in purple with the transparent region representing its atmosphere. The gas giant planet's orbit is somewhat non-circular. This indicates that there is probably an unseen lower mass planet in the system, shown in brown, that is perturbing the larger planet's orbit. Mass from the gas giant's atmosphere is pulled off and forms a disk around the star, shown in red.

 The continuous deformation of a planet causes friction in the planet's interior, and produces heat. Prof. Doug Lin, director of the Kavli Institute for Astronomy and Astrophysics mentions: “This is the first time there is direct evidence that the internal heating (or “tidal heating”) is responsible for puffing up the planet to it's current size”.

The research team at the Kavli Institute for Astronomy and Astrophysics at Peking University, have shown that the combination of the strong tidal forces and the puffed-up size of WASP-12b, causes the planet to lose mass to the host star.
The head author of the Nature article, Shulin Li, who carried out the bulk of the research work as part of her PhD in Beijing, commented: “WASP-12b is losing it's mass to the host star at a tremendous rate of six billion metric tons each second. At this rate, the planet WASP-12b will be completely destroyed by its host star in about ten million years”. She continues: “This may sound like a long time, but for astronomers it's nothing. This planet will live less than 500 times shorter than the current age of the Earth”.

The material that is stripped off WASP-12b does not directly fall onto the host star. Instead, it forms a disk around the star, and slowly spirals inwards. A careful analysis of the orbital motion of WASP-12b suggests circumstantial evidence of the gravitational force of a second, lower-mass planet in the disk. This planet is most likely a massive version of the Earth, a so-called super-Earth.

The disk of planetary material and the embedded super-Earth are detectable with the currently available telescope facilities. Their properties can be used to further constrain the history and fate of the mysterious planet WASP-12b.

Author and contact information:(Author list)

The leading author, Shulin Li, was a graduate student at the KIAA, and is currently working at the National Astronomical Observatories of China.

Douglas Lin is the Founding director of the Kavli Institute for Astronomy and Astrophysics, Peking University. He and Jonathan Fortney are professors whereas Neil Miller is a graduate student at the Department of Astronomy and Astrophysics, University of California, Santa Cruz.

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