The weather is very strange on WASP-76b. Liquid iron rains down on one side of the exoplanet, every night.
A paper describing the weird finding was published in Nature on Wednesday. Using the European Southern Observatory's Very Large Telescope in Chile, a large international team of astronomers led by the University of Geneva (UNIGE) detected a whiff of iron vapour on one side of the WASP-76b described as the "evening" or "night side".
"Surprisingly, however, we don't see iron vapour on the other side of the planet, in the morning," said Christophe Lovis, a researcher at UNIGE and lead data analyst of the team using the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations instrument known as ESPRESSO. "The conclusion is that the iron has condensed during the night. In other words, it rains iron on the night side of this extreme exoplanet."
WASP-76b's bizarre climate boils down to the way it circles its parent star. Held in a tidally locked orbit, one side of the planet is constantly bathed in sunlight, while the other side is kept hidden in darkness as it rotates on its axis. The "day" side reaches scorching temperatures up to 2,400°C (4,352°F), hot enough to ionise atoms and evaporate metals. The "night" side is colder dropping to 1,500°C (2,732°F).
The researchers reckon that the difference in temperature in both hemispheres drives winds that carry evaporated iron from the hotter day side over to the cooler night side. Here, blobs of iron vapour condensate in the clouds above and raindrops of molten iron come splashing down. Thunderstorms and lightning must be quite the spectacle on this exoplanet.
"On WASP-76b and similarly hot planets, these clouds could be made out of iron droplets, since liquid iron is the most stable high-temperature iron-bearing condensate. Hence, it could literally rain iron on the night side of WASP-76b," the paper concluded.
Initially discovered in 2013 during the Wide Angle Search for Planets programme led by two European-led observatories and six universities, WASP-76b has been described as a "hot Jupiter". It is mostly gassy, with a radius 1.8 times larger than, and a mass 92 per cent of, Jupiter. WASP-76b has a short orbital period, completing a lap around its star once every 1.8 days, and is located about 640 light years away in the constellation of Pisces. ®