An exoplanet where it rains iron.

Illustration shows a night-side view of the exoplanet WASP-76b. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets. To the left of the image, we see the evening border of the exoplanet, where it transitions from day to night. Credit: ESO/M. Kornmesser

ESO’s Very Large Telescope (VLT) have observed an extreme planet where they suspect it rains iron. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets.

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. Like the Moon on its orbit around the Earth, WASP-76b is ‘tidally locked’: it takes as long to rotate around its axis as it does to go around the star.

On its day side, it 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 vapour from the ultra-hot day side to the cooler night side, where temperatures decrease to around 1500 degrees Celsius.

The observations show that iron vapour is abundant in the atmosphere of the hot day side of WASP-76b.This result was obtained from the very first science observations done with ESPRESSO, in September 2018, by the scientific consortium who built the instrument: a team from Portugal, Italy, Switzerland, Spain and ESO.

ESPRESSO the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations was originally designed to hunt for Earth-like planets around Sun-like stars. However, it has proven to be much more versatile.

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