Newly discovered exoplanet has very unusual weather, astronomers say

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On a giant alien planet located 640 light-years away, scientists suspect it regularly rains molten iron.

During the day, temperatures on the exoplanet, which circles a star in the constellation of Pisces, rise to upwards of 2,400 degrees Celsius -- hot enough to vaporize metal. When strong winds carry the metal vapor to the dark side of the planet, the vapor condenses and falls as rain.

"One could say that this planet gets rainy in the evening, except it rains iron," David Ehrenreich, a professor at the University of Geneva in Switzerland, said in a news release.

Scientists discovered the iron rain phenomenon with the help of observations from the European Southern Observatory's Very Large Telescope in Chile. They published their results this week in the journal Nature.

An illustration shows iron vapor condensing and falling as rain on the giant exoplanet WASP-76b. Photo by ESO/M. Kornmesser

Because the planet WASP-76b is tidally locked with its host star, one hemisphere is always facing its sun and the other remains in darkness. The temperature difference fuels strong winds. In addition to its stark day-night temperature divide, WASP-76b's two hemispheres feature distinct day-night chemistry.

The spectral readings made by VLT's ESPRESSO instrument revealed the presence of iron vapor at edge of the divide between night and day. However, scientists found little evidence of iron in the atmosphere of the night side of the exoplanet.

"The observations show that iron vapor is abundant in the atmosphere of the hot day side of WASP-76b," said María Rosa Zapatero Osorio, an astrophysicist at the Center for Astrobiology in Madrid, Spain. "A fraction of this iron is injected into the night side owing to the planet's rotation and atmospheric winds. There, the iron encounters much cooler environments, condenses and rains down."

Osorio is chair of the science team for the ESPRESSO, short for the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations. The instrument was intended to find Earth-like planets around stars similar to our sun, but it has proven adept at analyzing the chemistry of exoplanet atmospheres.

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