An artist's illustration of the WASP-76b exoplanet's nightside, which faces away from its sun (Credit: M. Kornmesser/ ESO/CC-By-SA-4.0)

The search for an Earth-like planet that can support life has led to the discovery of many strange worlds, including one with two suns, a lava land, and a shimmering "sapphire" planet. However, none are as extreme or as bizarre as WASP-76b, which boasts 4,352-degree Fahrenheit (2,400-degree Celsius) temperatures, wind gusts of over 10,000 mph, and a steady pelting of iron rain!

"It's a kind of world we can't imagine easily because we don't have anything like that in our solar system," said Christophe Lovis, an exoplanet researcher at the University of Geneva in Switzerland and co-author of the study, published in the journal Nature on March 11, 2020.

Located 640 light-years away from Earth in the constellation Pisces, WASP-76b belongs to a category of giant planets called "ultra-hot Jupiters." The largely gaseous exoplanet's orbit around its sun takes less than two Earth days to complete — a journey so quick that it is tidally locked to its star. This means that only one of the exoplanet's sides gets exposed to its sun, while the other remains in perpetual darkness.​ This is similar to the moon's orbit around Earth, which is why we never see our satellite's "dark side."

The tidal lock causes the temperature between the exoplanet's two sections to vary widely, with the "dayside" creeping past 4,350 degrees F (2,400 degrees C), and the perpetually dark "nightside" hovering closer to 2,732 degrees F (1,500 degrees C). The blistering dayside heat causes the iron molecules present on the planet to vaporize, while the strong winds — the result of the disparity in temperature — transports the metallic gas to the "cooler" nightside. There, it condenses and falls as iron rain.

The rain could consist of compounds such as iron sulfide or iron hydride. But, "Given the conditions, the most likely [scenario] is that iron condenses into liquid droplets of pure iron," says study leader and University of Geneva professor David Ehrenreich. The researcher believes the iron eventually makes its way back to the dayside via atmospheric circulation, perpetuating the cycle.

An artist's illustration of the pelting iron rain on exoplanet WASP-76b (Credit: Frederik Peeters/ ESO/ CC-BY-SA/4.0)

Though scientists have known about WASP-76b and its extreme temperature variations since 2013, the unusual precipitation was only recently uncovered by the University of Geneva team using the European Southern Observatory's Very Large Telescope in Chile, South America. The scientists think the new information about the unusual exoplanet may help researchers finesse and test climate and global circulation models, and lead to a better understanding of exoplanetary atmospheres in general.

"Exoplanets are a real treasure trove full of surprises," Ehrenreich said. "The more you look, the more you find." The University of Geneva team plans to continue surveying other exoplanet atmospheres to see if WASP-76b is an outlier or a member of a peculiar class of worlds, where iron rain is the norm!