Saturday, February 27, 2010

Dissipating Planet

A paper by Shu-lin Li, et al. (2010) entitled “WASP-12b as a Prolate, Inflated and Disrupting Planet from Tidal Dissipation” describes some peculiar properties of an extrasolar planet. WASP-12b is an extrasolar gas giant planet that has 1.4 times the mass of Jupiter and it is located so remarkably close to its parent star that it takes only a little over an Earth day to orbit the star. In fact, WASP-12b orbits its parent star at a distance of just 2 stellar radii from the star’s surface and the planet is distorted by the star’s gravity into a prolate shape, similar to that of a rugby ball.

The nonzero orbital eccentricity of WASP-12b makes it very susceptible to the strong tidal effects from its parent star which heats the planet’s interior and causes the planet to expand. WASP-12b has an orbital eccentricity of about 0.05 and this is odd because orbital circularization should have already circularized its orbit into a zero eccentricity orbit. Another planet with a mass of a few Earths is probably responsible for perturbing WASP-12b to maintain its nonzero orbital eccentricity. WASP-12b is extremely “bloated” and it has a diameter that is 80 percent larger than Jupiter’s. The atmospheric temperature of WASP-12b is estimated to be a scorching 2500 degrees Kelvin.

WASP-12b has ballooned so much that its own gravity is unable to retain its mass from the gravitational pull of its parent star. Gas from WASP-12b is flowing towards the parent star through a nozzle that is located at the L1 Lagrangian point, a region that is situated between the planet and the star. WASP-12b is losing mass to its host star at a rate of a few billion metric tons each second. The material that is pulled off from WASP-12b forms an accretion disk around the parent star and gradually spiral inwards into the star.