WASP-76b — a hard rain’s gonna fall
The team have been able to determine the characteristics of WASP-76b’s relationship with its parent star which give rise to the extraordinary atmospheric phenomena observed. WASP-76b is tidally locked, much as the moon is around the Earth, meaning the planet constantly orbits its host star with the same side facing it. This leaves one side constantly bombarded by harsh radiation — thousands of times that received from the Sun by the Earth. Contrasting this, the side facing away from the parent star falls into a perpetual night.
“The temperature in the day-side is so high that molecules are split into atoms, including iron which evaporates into the atmosphere while in the night side, molecules are stable,” Allart says. “This big difference in temperature between the day and night-side makes that strong winds carry iron atoms from the day- to night-side and these atoms condense into molecules which we are calling an iron rain.”
The intense temperature of WASP-76b’s day-side is also driven by its proximity to its parent star, a distance so small, it takes only 1.8 earth days to complete an orbit.
The fact that ESPRESSO detected iron vapor in the atmosphere of WASP-76b, but only on one side of the planet, was the crucial information that tipped the team off as to the extraordinary iron rains that half the planet experiences. Simply, the iron vapor isn’t visible in the atmosphere of the night side because it has condensed and fallen to the surface.
“This system is somehow similar to the first system discovered to possess an exoplanet — 51 Pegasi. The star is orbited by a single giant close-in planet which is absolutely different from our Solar system. Moreover, the star is also bigger and hotter than our sun,” Allart says, but this doesn’t mean, that WASP-76b isn’t unique. “This is not the first planet for which we can see the presence of gaseous iron in its atmosphere but this is the first planet that exhibits such a contrast between day- and night-side.”
With regards to the WASP system, Allart says that he and his team intend to gather more data to complete their view of the system, in turn, perhaps leading them to a better understanding of both cooler and smaller systems.