Meet The Exoplanet Where It Appears To Rain Iron

This illustration shows a night-side view of the exoplanet WASP-76b. To the left of the image, we see the evening border of the exoplanet, where it transitions from day to night. ESO/M. Kornmesser

The last decade has shown us there are a huge number of planets out there, and many of them are completely different from what we see in our Solar System. WASP-76b is one of them, and new observations suggest that on this peculiar world it rains iron.

WASP-76b, located 640 light-years away in the constellation of Pisces, weighs 284 times the Earth and orbits its star in just over 43 hours. It's a gas giant slightly smaller than Jupiter, but it orbits so close to its star it's become tidally locked. One side of the planet always faces the star, the other is in perpetual darkness. A new study published in Nature reveals the two faces of the planet have significantly different properties.

The daylight side has a temperature of 2,400°C (4,350°F), which is hot enough to vaporize metals in its atmosphere. The signature from these metals is exactly what the astronomers studied. In particular, they looked at the terminators (no, not Arnie), the region that separates the two sides.

In the day-to-night terminator, the “evening” one, they reported the presence of iron in the atmosphere that moved with quite a speed, possibly due to winds and the planet's rotation. But when they looked at the “morning” terminator the team discovered this iron signal was gone. They suspect that the cooler temperatures on the night side means the iron condenses, possibly in clouds, and falls as rain during the night.

“The observations show that iron vapor is abundant in the atmosphere of the hot day side of WASP-76b," co-author María Rosa Zapatero Osorio, an astrophysicist at the Centre for Astrobiology in Madrid, Spain, said in a statement.

"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."

The team used the ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations ) instrument on the European Southern Observatory's  Very Large Telescope in Chile to detect and study the chemical variations across the surface of this planet. ESPRESSO was originally designed to hunt for Earth-like planets, but researchers have discovered it's also prime for studying planet's atmospheres, and say this is just the beginning.

“What we have now is a whole new way to trace the climate of the most extreme exoplanets,” said David Ehrenreich, a professor at the University of Geneva in Switzerland, who led the study.

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