Friday, February 11, 2011

Puffy Planet

A remarkable property of gas giant planets and brown dwarfs is that even though their individual masses can range from less than half the mass of Jupiter up to over 80 times the mass of Jupiter, they all have roughly the same diameters as Jupiter as their sizes do not increase with mass. This is because the volumetric size of a gas giant planet is governed by Coulomb pressure while the volumetric size of a brown dwarf is governed by electron degeneracy pressure and both of these forces neatly compensate for gravitational compression, giving approximately the same diameter as Jupiter for objects ranging in mass from gas giant planets to brown dwarfs. A significant number of extrasolar planets are known with diameters that are much larger than the diameter of Jupiter, making these planets larger than what is theoretically expected for them. Attempts have been made to explain the anomalously large diameters of these extrasolar planets by a number of proposed heating mechanisms that can potentially deliver enough thermal energy into the interiors of these planets to inflate them to their observed diameters.

The largest extrasolar planet currently known is a transiting planet called WASP-17b. This planet has 49 percent the mass of Jupiter and its orbit around its host star happens to be orientated in such a way that the planet is observed to periodically pass in front its host star. WASP-17b is located at a distance of 7.7 million kilometres from the centre of its host star and it takes 3.74 Earth-days to make one orbit around its host star. The host star of the WASP-17b is a spectral type F6V star with 1.3 times the mass of our Sun, an estimated surface temperature of 6650 degrees Kelvin and a luminosity that is over 4 times greater than our Sun’s luminosity. Looking from WASP-17b, its host star will appear almost two thousand times brighter than our Sun as seen from the Earth.

Measuring the fraction of starlight blocked by WASP-17b as it passes in front of its host star gives the planet an estimated size that is twice the diameter of Jupiter. Such a diameter also means that WASP-17b is up to 0.2 Jupiter diameters larger than the next largest planet and up to 0.7 Jupiter diameters larger than the theoretical diameter predicted using the standard cooling theory of irradiated gas giant planets. With twice the diameter of Jupiter and just under half the mass of Jupiter, WASP-17b has a density of just 6 percent the mean density of Jupiter or 8 percent the density of water, making it the least dense planet known. For such an inflated and low density planet, the surface gravity of WASP-17b is less than one-third the surface gravity of the Earth even though WASP-17b has slightly less than half the mass of Jupiter which already translates to 155 times the mass of the Earth.

The orbit of WASP-17b around its host star is slight non-circular with a very small non-zero orbital eccentricity. This means that WASP-17b is a little nearer to its host star than at other times. It has been suggested that a planet can be inflated to twice the diameter of Jupiter or more during a transient phase of heating caused by tidal circularization from a highly eccentric orbit to an almost circular one. In this scenario, the planet can persist in an inflated state for over a billion years after its orbit has circularized considerably. However, such a planet is expected to cool and contract significantly prior to complete orbital circularization where its orbit is still noticeably non-circular. Therefore, under the scenario of transient heating, the near-zero orbital eccentricity of WASP-17b and its highly inflated size means that a transient phase of tidal heating from orbital circularization alone is insufficient to have inflated the planet to its current observed size.

An enhanced atmospheric opacity for WASP-17b will enable its internal heat to be radiated off at a much lower rate and this can slow the contraction of the planet from a previous transient phase of tidal heating. However, even this is still insufficient to account for the current inflated diameter of WASP-17b. Ongoing tidal heating can still occur for WASP-17b if its orbit is being kept non-circular by interaction with another planet in the system. However, this is unlikely to be the primary source of heating to account for the inflated diameter of WASP-17b because its orbital eccentricity is too small for sufficient tidal heating to occur. Finally, the kinetic energy from the strong winds generated in the atmosphere by the large day-night temperature contrasts of the tidally-locked WASP-17b can be transported into the deep interior of planet and be deposited as thermal energy. However, a means to convert the kinetic energy into thermal energy will still be necessary and turbulent dissipation is one of the proposed mechanisms. To conclude, no current theory is able to account for the remarkable inflated diameter of WASP-17b.