WASP-12b is a pitch-black exoplanet

Artist's impression of the exoplanet WASP-12b and its star (Image NASA, ESA, and G. Bacon (STScI))
Artist’s impression of the exoplanet WASP-12b and its star (Image NASA, ESA, and G. Bacon (STScI))

An article published in the journal “The Astrophysical Journal Letters” describes a research on the exoplanet WASP-12b. A team of astronomers from the McGille University, Canada, and the University of Exeter, UK, used the Space Telescope Imaging Spectrograph (STIS), an instrument of the Hubble Space Telescope discovering that the planet reflects only a very little percentage of the light it receives from its star. In a word, this strange planet is darker than fresh asphalt: in technical terms, its albedo is 0.064.

The planet WASP-12b was discovered in 2008 within the SuperWASP research program. Approximately 1,400 light years from the Earth, it orbits the star WASP-12, whose mass is about 35% higher than the Sun’s. Its size is about twice Jupiter’s and its mass is almost 40% higher than Jupiter’s. It orbits very close to its star, so much that its year lasts little more than an Earth’s day, with the result that it gets heated up a lot by WASP-12.

This type of planet is called hot Jupiter exactly for those characteristics but WASP-12b seems extreme even among them. Because of its proximity to its star, it’s heavily affected by its gravitational influence as well so its shape is distorted becoming similar to a football. The surface temperature on the side exposed to the star is estimated at around 2,600° Celsius (about 4,600° Fahrenheit).

This situation has huge consequences on what is happening in the planet WASP-12b’s atmosphere and they’re the most plausible explanation for the very low reflexivity of its starlight. Taylor Bell, the article’s lead author, explained that among the known hot Jupiters there are others very black but they’re much less hot than WASP-12b. One hypothesis is that clouds and alkaline metals can absorb much of the light received from their stars but this can’t happen on WASP-12b because the temperature is so high that clouds can’t form and alkaline metals are ionized.

A measurement of the planet WASP-12b’s albedo was performed in October 2016 during an eclipse, when it passed behind its star. This allows to understand how much light the planet reflects when it’s not eclipsed but to make a measurement with the required accuracy it was necessary to use the Hubble Space Telescope’s STIS instrument. This measurement allowed to compare the albedo with spectral models of those of possible atmospheric compositions but none of the proposed ones matched.

The collected data indicate that the atmosphere of WASP-12b is composed of atomic hydrogen and helium. Generally, hydrogen exists in the form of molecules but in this case the very high temperatures have an influence breaking those molecules. The consequence is that the atmosphere acts more like that of a small star than that of a planet.

The planet WASP-12b is the second whose albedo was measured by spectroscopic measurements after HD 189733b, another hot Jupiter that should have a deep blue color. According to Taylor Bell, the differences in the results of the first measurements of this type prove the importance of these spectral observations and the diversity among hot Jupiters. In essence, this is a type of research useful to better understand this type of exoplanet.

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