Hellish planet discovered where it constantly rains molten iron

12 Mar 2020

Illustration of iron rain on WASP-76b. Image: ESO/M Kornmesser

Astronomers have discovered a planet hundreds of light years away where extreme heat results in molten iron raining down on the surface.

Astronomers using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) have discovered what appears to be an exotic planet 640 light years away that might sound hellish to those of us on Earth.

Writing in the journal Nature, the researchers described an ultra-hot giant exoplanet where, on its day side, temperatures can reach above 2,400 degrees Celsius. In such conditions, the planet – dubbed WASP-76b – has temperatures high enough to vaporise metals.

Strong winds carry iron vapour to the cooler night side of the planet, which then condenses to form droplets that fall to the planet’s surface, creating ‘iron rain’. The rain is constant as the planet only ever shows one face – its day side – to its parent star, leaving its shaded side in perpetual darkness.

Meanwhile, its day side is bombarded with thousands of times more radiation from its parent star than the Earth gets from the sun. It’s so hot, researchers said, that molecules separate into atoms, and metals such as iron enter the atmosphere.

These extreme temperatures on either half of the planet result in the strong winds that form the iron rain. On the planet’s ‘cold side’, it reaches temperatures of 1,500 degrees Celsius.

Not the first strange rain discovery

The study also discovered that WASP-76b has distinct day-night chemistry. Using the VLT’s ESPRESSO instrument, the astronomers saw, for the first time, chemical variations on an ultra-hot gas giant.

This led to the detection of iron vapour at the evening border that separates the planet’s day side from its night side. As no iron vapour is detected in the morning, it means the iron rain is on the night side.

“The observations show that iron vapour is abundant in the atmosphere of the hot day side of WASP-76b,” said María Rosa Zapatero Osorio, an astrophysicist at the Centre for Astrobiology in Madrid, Spain, and chair of the ESPRESSO science team.

“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.”

This is not the first exoplanet to be found with peculiar rain. In 2017, the Hubble Space Telescope was used to discover an exoplanet where it ‘snows sunscreen’. Simulations and analysis of Kepler-13Ab’s permanent night side suggested that titanium oxide is carried by powerful winds, where it condenses into crystalline flakes, forms clouds and precipitates as snow.

Colm Gorey was a senior journalist with Silicon Republic