Wasp-76b is an exoplanet, a planet that orbits a star outside our Solar System. It lies around 640 light-years away, in the constellation of Pisces. Astronomers have discovered extreme weather conditions on Wasp-76b: temperatures of 2400ºC, winds in excess of 16,000 kilometres per hour (10,0000 mph), and, in one hemisphere, a steady downfall of iron rain. The observations of the distant planet were made by a new instrument at the ESO's Very Large Telescope (VLT) in Chile's Atacama Desert.

Wasp-76b is an ultra-hot gas giant, a planet many times the size of Jupiter, our own Solar System's largest planet. It also orbits its parent star closely—a mere 3% of the distance between the Earth and the Sun—and quickly, in just 1.8 Earth days. These characteristics made this exoplanet relatively easy to find using powerful telescopes. They are able to detect the minute dip in starlight that occurs as the planet moves across the face of its parent star, as viewed from Earth. A clear and regular change in the starlight output from Wasp-76b's parent star could be picked up over a small period of time.

Because it orbits so close to its parent star, Wasp-76b is “tidally locked”, like the Moon’s orbit around Earth. This means that, also just like the Moon, the exoplanet only only ever shows one face to the object it orbits. In this brightly-lit, scorching hemisphere, it stays day all the time. The other hemisphere is, by contrast, plunged into eternal night, and cooler: "only" 1500ºC.

Using a new instrument on the VLT, the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO), astronomers have, for the first time, been able to identify chemical variations on Wasp-76b. They detected a strong signal of iron vapour at the "evening" border that separates the planet’s day side from its night side. “Surprisingly, however, we do not see the iron vapour in the morning,” says study leader David Ehrenreich, a professor at the University of Geneva in Switzerland. Why is this?