Monday, July 5, 2021

WASP 12b, a strange exoplanet

WASP 12b is one of the strangest known exoplanets. Discovered in 2008, the exoplanet is a gas giant with a mass 1.4 times greater than that of Jupiter and a diameter twice that of the giant of the Solar System. This makes it one of the least dense exoplanets discovered to date.

WASP 12b orbits its star in just one day, at a distance equal to only 1/44 of that which separates the Earth from the Sun. This proximity makes the exoplanet a world with extreme environmental conditions.

From the moment of its discovery it has been noted that the period between one transit and another decreases each time by an amout of the order of milliseconds. This means that the orbital period of the exoplanet is getting shorter: WASP 12b is getting closer and closer to its star. Within a few million years the planet will be either destroyed by gravitational attraction or engulfed by the star.

Being located in such a narrow orbit, WASP 12b always shows the same hemisphere to the star. On the daytime side, temperatures are very high, above 2600 K. Contrary to what one might think, the planet is very dark: in fact it has an albedo of just 0.06. Its color is therefore even blacker than that of fresh asphalt. We are still trying to understand what this aspect could have been: it has been hypothesized that such a low albedo could be linked to atmospheric composition and high temperatures. The latter would in fact make the formation of clouds impossible and would ionize most of the atmospheric elements, in particular metals. The atmosphere of WASP 12b would therefore tend to behave more like a stellar atmosphere than a planetary one.

Credit: NASA, ESA, G. Bacon.

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