WASP-76b, a gas-giant exoplanet located some 640 light-years away in the constellation of Pisces, has a day side where temperatures climb above 2,400 degrees Celsius (4,352 degrees Fahrenheit), high enough to vaporize metals; strong winds carry iron vapor to the cooler night side where it condenses into iron droplets.
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. Image credit: M. Kornmesser / ESO.
WASP-76b is tidally-locked to its star. It takes as long to rotate around its axis as it does to go around the star.
On its day side, the planet receives thousands of times more radiation from its parent star than the Earth does from the Sun. It’s so hot that molecules separate into atoms, and metals like iron evaporate into the atmosphere.
The extreme temperature difference between the day and night sides results in vigorous winds that bring the iron vapor from the ultrahot day side to the cooler night side, where temperatures decrease to around 1,500 degrees Celsius (2,732 degrees Fahrenheit).
Not only does WASP-76b have different day-night temperatures, it also has distinct day-night chemistry.
“One could say that this planet gets rainy in the evening, except it rains iron,” said Professor David Ehrenreich, an astronomer at the University of Geneva.
“This strange phenomenon happens because the iron rain planet only ever shows one face, its day side, to its parent star, its cooler night side remaining in perpetual darkness.”
Professor Ehrenreich and colleagues used the ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) instrument on ESO’s Very Large Telescope (VLT) to identify chemical variations on WASP-76b.
They detected a strong signature of iron vapor at the evening border that separates the planet’s day side from its night side.
“Surprisingly, however, we do not see the iron vapor in the morning. The reason is that it is raining iron on the night side of this extreme exoplanet,” Professor Ehrenreich said.
“The observations show that iron vapor is abundant in the atmosphere of the hot day side of WASP-76b,” said ESPRESSO team leader Dr. María Rosa Zapatero Osorio, an astrophysicist at the Centre for Astrobiology, 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.”
The findings were published in the journal Nature.
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D. Ehrenreich et al. Nightside condensation of iron in an ultrahot giant exoplanet. Nature, published online March 11, 2020; doi: 10.1038/s41586-020-2107-1
