Scientists find extreme exoplanet raining with iron

Rain is not necessarily synonymous with water on other planets. Astronomers working with the European Southern Observatory’s Very Large Telescope (VLT) have come across a bizarre exoplanet where it rains iron in the evening.

Artist illustration showing the night-side of WASP-76b, where it rains iron. Credit: ESO/M. Kornmesser.

The exoplanet, known as WASP-76b, is located about 6400 light-years away from Earth in the constellation Pisces. The ultra-hot gas giant orbits so close to its parent star that temperatures regularly climb above 2,400°C — but only on the planet’s day-side.

Just like the moon, WASP-76b is tidally locked, meaning it only shows one face to its parent star, since the planet takes just as long to rotate around its own axis as it does to orbit around the star. As a result, the night side is shrouded in perpetual darkness and is much cooler.

The exoplanet receives thousands of times more radiation than Earth does from the Sun, making the surface of Wasp-76b’s day side so hot it vaporizes metals like iron. Vigorous winds generated by the extreme temperature difference between the planet’s two sides carry a fraction of this iron vapor to the cooler side, where temperature decreases to 1,500°C. That’s still very hot, yet cool enough for iron vapor to condense and rain down.

“One could say that this planet gets rainy in the evening, except it rains iron,” says David Ehrenreich, a professor at the University of Geneva in Switzerland, who led the new research published in the journal Nature.

“Surprisingly, however, we do not see the iron vapor in the morning,” says Ehrenreich, adding that “it is raining iron on the night side of this extreme exoplanet.”

The discovery was made possible thanks to a new instrument equipped on ESO’s VLT in the Chilean Atacama Desert. Known as the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations — or ESPRESSO — the instrument was originally designed to hunt for Earth-like planets around Sun-like stars. However, ESPRESSO has proven itself much more versatile than originally thought, allowing astronomers to detect a strong signature of iron vapor at the evening border that separates Wasp-76b’s two sides.

“We soon realised that the remarkable collecting power of the VLT and the extreme stability of ESPRESSO made it a prime machine to study exoplanet atmospheres,” says Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile.

This crazy planet is not just some curious oddity. The insight gained by studying its atmosphere will help scientists better fine-tune and test climate and global circulation models. Ultimately, outlier planets like WASP-76 b will better our understanding of exoplanet atmospheres in general.

“What we have now is a whole new way to trace the climate of the most extreme exoplanets,” concludes Ehrenreich.

If you found an iron-raining planet weird, this exoplanet is actually not that peculiar. On Venus, it rains sulfuric acid, while on Neptune rainfall is in the form of diamonds.

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