On This Scorching-Hot Exoplanet, a Forecast of Molten Iron Rain

Winds on WASP-76b blow gaseous iron into cooler regions, where it condenses and falls to the planet’s surface as liquid

An illustration showing iron rain showering down on the exoplanet WASP-76b, where a permanent temperature gradient pushes gassy iron into cooler regions, where it turns into liquid ESO/M. Kornmesser

On a planet some 640 light-years outside our solar system, the forecast is scorching, with a chance of liquid iron rain, new research suggests.

The goings-on of this distant world, called WASP-76b, are absolutely metal—literally. WASP-76b showcases “probably one of the most extreme planetary climates we’ve ever seen,” David Ehrenreich, an astronomer at the University of Geneva in Switzerland, tells Gege Li at New Scientist. Ehrenreich and his colleagues describe their observations, made with the Very Large Telescope (VLT) in Chile, in a paper published this week in Nature.

A gas giant like Jupiter, WASP-76b consists of a small, rocky core shrouded in a giant swath of gas. But it orbits its star in a much closer loop, taking less than two Earth days to complete each lap—a traverse so tight that only one side of the exoplanet ever faces inward. (A similar situation holds true for the moon’s orbit around Earth, which never sees “the dark side” of its satellite.)

The incessant exposure to the star’s heat keeps WASP-76b’s “day side” constantly roasting at about 4,350 degrees Fahrenheit (2,400 degrees Celsius)—hot enough to vaporize metals—while its other face remains perpetually plunged into darkness, staying a whopping 1,600 degrees Fahrenheit cooler.

More bizarre still is the physical boundary where the exoplanet’s light zone transitions to dark—and where Enrenreich and his colleagues predict the existence of pelting drops of iron rain. On the day side, heat evaporates iron into the atmosphere, generating metallic clouds. Buoyed by WASP-76b’s rotation, as well as ferocious winds that blow at speeds around 11,000 miles per hour, the gaseous iron then swirls nightward, smacking into a sharp temperature gradient that rapidly condenses it into molten liquid.

“One could say that this planet gets rainy in the evening, except it rains iron,” Ehrenreich tells Hannah Devlin at the Guardian.

But winds blowing from night to day on the other side of the planet don’t create the same phenomenon, since the comparatively cooler iron on the dark side stays liquid. Too heavy to be wafted dayward on a plume of gas, the metal rains down to WASP-76b’s unlit surface. Close enough to the exoplanet’s hot core, though, the droplets may eventually vaporize back into gas that simply stays put, reports Maria Temming for Science News.

The team couldn’t see the iron rain directly, instead inferring its presence by studying signals of the metal present in starlight illuminating the planet from behind. But the theory is a probable one, David Armstrong, an exoplanet expert at the University of Warwick who wasn’t involved in the study, tells New Scientist.

Though WASP-76b was first discovered in 2013, these observations represent a first for the exoplanet, thanks to observations made through the VLT’s new instrument, ESPRESSO. And as more observations pour in, the far-flung world may soon find itself in good company: With more than 4,000 exoplanets now identified, researchers are sure to find an even wider array of oddball traits.

“Exoplanets are a real treasure trove full of surprises,” Ehrenreich tells Mike Wall at Space.com. “The more you look, the more you find.”

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