2,400 degrees and raining iron – it's just another day on WASP-76b
Astronomers may have discovered an alien world where it rains iron, according to a newly published study. The massive exoplanet is exposed to thousands of times the radiation that the Earth receives from the Sun, rendering the surface hot enough to vaporize metals that later fall as rain on a side of the planet that is bathed in perpetual night.
Every now and then astronomers find an exoplanet that makes you truly glad to have evolved on Earth. In this case, it’s the distant alien world WASP-76b. This nightmarish orb plays host to extreme atmospheric conditions that would be just as much at home in an over the top sci-fi flick as in a scientific research paper.
The exoplanet was observed by the ESPRESSO instrument - which is mounted on the European Southern Observatory’s Very Large Telescope (VLT) - back in September 2018, during the instrument’s first scientific outing. ESPRESSO is a powerful spectrograph capable of breaking down the light captured from distant cosmic objects. Scientists can take this data and analyze it to figure out some of the key characteristics of a distant world.
The instrument was created to hunt down Earth-like planets orbiting stars with similar characteristics to our Sun. However, astronomers soon realized that the instrument/telescope pairing had the flexibility to carry out a range of tasks, including that of probing the atmospheres of extreme, alien worlds such as WASP-76b.
"Extreme" is the perfect word to describe WASP-76b. It is tidally locked to its star, meaning that it takes roughly the same amount of time to turn on its axis as it does to complete one of its yearly cycles. Because of this the planet only presents the same side of its surface to its parent star, which is bathed in constant daylight, while the opposite side is sheathed in perennial night. That is until something dramatic happens to upset the orbital slow dance.
The day side of WASP-76b is estimated to have a temperature exceeding 2,400° C (4,350° F) – hot enough to cause molecules to thermally disassociate into atoms that are borne high into the atmosphere. Hot enough to vaporize metal. The night side of the planet meanwhile exists at a comparatively manageable 1,500° C (2,730° F). This extreme temperature differences could create a powerful wind capable of transferring atmospheric elements from the day side to the night side, and visa versa.
The researchers examined the light signature of the alien world gathered by ESPRESSO, and identified the chemical fingerprint of neutral iron present in the atmosphere. This iron vapor was detected across the day side of the planet and at the evening terminator – the point where day turns to night – but not at the border between the night side and dawn.
This suggests that the iron vapor is transferred from the super-hot day side of the exoplanet by powerful atmospheric winds to the night side. In this cooler environment, the vapor condenses and falls to the surface as iron rain, hence the lack of iron vapor at the dawn terminator.
According to the authors of the paper, their research represents the first time that distinct chemistry between the day and night side of an ultra-hot gas giant planet has been discovered.