An unexpected metal in a superheated exoplanet’s atmosphere

Since 1995 and the discovery of 51 Pegasi b, which won the Nobel Prize for Michel Mayor and Didier Queloz in 2019, our knowledge of exoplanets has grown exponentially. There are now over 5,000 of them and these strange worlds sometimes have very surprising properties. On Wasp-76b, for example, it rains iron. This exoplanet belongs to the family of superheated Jupiters, which are gas giant planets of similar or greater mass than Jupiter, and are so close to their star that their temperature exceeds 2000 degrees Celsius. With its synchronous rotation, Wasp-76b always presents the same face to its star. In the luminous hemisphere there are clouds of red-hot iron that fall like rain when they come to the face immersed in darkness. And this is not the only one advantage of this exoplanet. Tomas Azevedo Silva of the University of Porto, Portugal, and his colleagues detected barium in the upper atmospheres of this exoplanet and another planet, Wasp-121b.

This detection was made possible by the precision Espresso spectrometer installed on the VLT (very large telescope), in Chile. Using this instrument, astrophysicists analyze the frequency spectrum of the light coming from the star.

During the transit of an exoplanet, that is, when it passes in front of its star, part of the light from the latter crosses the planet’s atmosphere before reaching us. During this transit, the elements in the gaseous layer absorb part of the radiation at specific frequencies. Then analysis using a spectrometer makes it possible to determine the composition of the exoplanet’s atmosphere. In the case of Wasp-76b and Wasp-121b, Tomas Azevedo-Silva and colleagues were surprised to learn of the barium signature, the existence of which had not been considered until then.

It is surprising to find barium in the atmospheres of these planets. This element is 2.5 times heavier than iron and therefore must sink into the bowels of gas giants. It is the largest element ever identified on an exoplanet.

The mechanism behind this oddity is not yet clear, but it is likely related to the action of unknown atmospheric phenomena capable of driving heavy elements into Jupiter’s superheated upper atmosphere. Future analyzes will be necessary to clarify this conundrum.

From the planet Jansen and its possibly diamond-like interior structure to Osiris’s infernal storms (with winds gusting at 7,000 kilometers per hour) to Kepler-16-b’s solar system, observations have revealed worlds that are more diverse than inhospitable. However, the representations we have of exoplanets are still often extrapolated from scant data and there is still much to discover and understand. These worlds can be counted in the billions in the Milky Way, and they certainly haven’t finished surprising us.