WASP-76b, the exoplanet where it rains iron

Artist's concept of iron rain on the exoplanet WASP-76b (Image ESO/M. Kornmesser)
Artist’s concept of iron rain on the exoplanet WASP-76b (Image ESO/M. Kornmesser)

An article – link to the file in PDF format – published in the journal “Nature” reports a study on the conditions existing on the exoplanet WASP-76b, an ultra-hot Jupiter where on the side illuminated by its star it’s so hot that metals vaporize and then condense on the dark side, where it rains iron. A team of researchers led by David Ehrenreich of the University of Geneva, Switzerland, used the ESPRESSO instrument mounted on the VLT in Chile to study the processes underway in the atmosphere of WASP-76b with the winds that carry the iron vapor across this gas giant.

About 640 light years from Earth, the exoplanet WASP-76b was discovered in 2013 orbiting a star with a mass almost 1.5 times the Sun’s and a diameter about 1.7 times the Sun’s. WASP-76b has a mass slightly lower than Jupiter but its size is almost twice Jupiter’s because it’s inflated by heat. In fact, its close to its star, about 5 million kilometers from it, to the point that its year lasts about 1.8 Earth days and a consequence is that it receives a huge amount of heat. Gas giant planets with characteristics of this type are called hot Jupiters.

Another consequence of its proximity is that it’s geolocked, which means that it always shows the same face to its star, like the Moon with the Earth. This means that the illuminated face is much hotter, with temperatures estimated at around 2,400° Celsius, than the dark face, where temperatures are estimated at around 1,500° Celsius. These are extreme conditions even for hot Jupiters, enough to include it in the subgroup of ultra-hot Jupiters.

The exoplanet WASP-76b is already an interesting research object due to the fact that it’s an ultra-hot Jupiter, however, a team of researchers also discovered processes taking place in its atmosphere that affect metals, and this makes it truly peculiar. To carry out this research they used the ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) instrument, mounted on ESO’s VLT (Very Large Telescope) in Chile. It’s meant to hunt Earth-like exoplanet that orbit Sun-like stars, however, Pedro Figueira, one of the scientists of the ESPRESSO team and part of the team that carried out this research, explained that it proved to be much more versatile. For that reason, it offers the possibility to study the atmospheres of exoplanets, including exotic ones such as ultra-hot Jupiters.

The remarkable differences in temperature between the day and night sides of the exoplanet WASP-76b generate very strong winds that transport elements and compounds present in the atmosphere even to the opposite side. On the day side the atmosphere is cloudless and temperatures are high enough to transform iron into vapor in atomic form. When this vapor is transported to the night face, it condenses into molecules that form droplets that literally rain and descend into the atmosphere.

Stefano Cristiani of the Italian National Institute of Astrophysics, another of the ESPRESSO instrument’s scientists and another member of the team who carried out this research, stressed the role of ESPRESSO’s precision and sensitivity in the reconstruction of the processes taking place in the atmosphere of the exoplanet WASP-76b. It’s a truly peculiar weather that shows how alien ultra-hot Jupiters can be.

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