Fiery World

Well over 500 extrasolar planets have been discovered so far and this number of set to increase ever more rapidly. A large fraction of these extrasolar planets are known to transit their host stars and transiting extrasolar planets allow much more to be known about them than would have been otherwise. An extrasolar planet which passes in front and transits its host star is also likely to pass behind and get occulted by its host star. The occultation of the planet as it goes behind its host star allows the total thermal emission and total reflected light from the planet to be measured, thereby allowing the temperature of the planet to be determined. A recently published paper entitled “Thermal Emission from WASP-33b, the Hottest Known Planet” describes observations of the thermal emission from an extrasolar planet called WASP-33b and this planet is the first known to orbit an A-type star.

WASP-33b orbits its host star at a distance of just 3.82 million kilometres, making it 15.2 times closer to its host star than the planet Mercury is from our Sun. At such a close distance, WASP-33b takes just 1.22 Earth days to complete one orbit around its host star. The host star of WASP-33b is a spectral class A5 star that has 1.44 times the diameter of the Sun, 1.50 times the mass of the Sun, almost 6 times the luminosity of the Sun and a surface temperature of 7430 degrees Kelvin. WASP-33b is expected to be tidally locked whereby one hemisphere is locked to perpetually face its host star while the other hemisphere faces away from its host star. The mass of WASP-33b is deduced to be less than 4.1 times the mass of Jupiter while the diameter of WASP-33b is estimated to be almost 1.5 times the diameter of Jupiter.

Assuming an albedo of zero and uniform heat redistribution to the night-side, the equilibrium temperature of WASP-33b is estimated to be around 2700 degrees Kelvin. In comparison, the planet WASP-12b which was previously the hottest known extrasolar planet has an estimated equilibrium temperature of 2500 degrees Kelvin. Furthermore, the hottest temperature measured for WASP-33b stands at a blistering 3466 degrees Kelvin, making it the hottest temperature ever recorded for an extrasolar planet. At this temperature, metals such as iron, gold and aluminium will not be able to condense into a liquid from their gaseous forms. This huge temperature suggests that the heat transport from the day-side of the planet to its night-side is inefficient and this enables a very high temperature to persist on the planet’s day-side.

The sub-stellar point on the tidally locked WASP-33b is basically a spot on the planet where its host star is always directly overhead. On this blazingly hot spot, the host star of WASP-33b will appear around ten thousand times brighter than our Sun would appear on a clear day on the Earth. The amount of irradiation received by WASP-33b is so enormous that the total amount of energy incident on an effective area of just one square meter on the sub-stellar point on WASP-33b for a period of one year will equal the amount of energy released in a 90 kiloton nuclear explosion.