Heavy Metal Rains From the Sky of this Exoplanet

When it comes to the discovery of exoplanets, which are worlds that orbit other stars, there is enormous interest in places that might have the right conditions to support life. But every now and then, we need to pay respects to the exoplanets that are aggressively hostile to life, such as WASP-76b, where droplets of metal rain down from the sky.

It “could literally rain iron on the nightside of WASP-76b,” according to a study published on Wednesday in Nature. The type of gnarly metal rain is probably rare in the universe, even on so-called “hot Jupiter” planets, because it requires temperatures high enough to vaporize iron.

But WASP-76b, which was discovered in 2013, is an “ultra-hot Jupiter,” explained study co-author Christophe Lovis, an astronomer at the University of Geneva, in a call.

“Normal hot Jupiters won’t be hot enough for iron to vaporize like this,” he explained. “What’s interesting here is that it’s so hot, you don’t have molecules at all. You have atoms, which are easier to detect and less complex to analyze than what you have when the planet is cooler and you have lots of molecules and complicated chemical compounds, or clouds and hazes.”

WASP-76b orbits a Sun-sized star at a distance of just 3 million miles, about ten times closer than Mercury’s orbit around the Sun, leaving it with a year that lasts just 1.8 Earth days. Because of this tight orbit, WASP-76b is tidally locked, meaning that the same “dayside” is always facing its star, causing temperatures there to exceed 2,400°C (4,350°F).

Since the nightside of the exoplanet is always facing away, it experiences temperatures of about 1,500°C (2,730°F)—still brain-meltingly hot, but not quite so extreme as the opposite hemisphere.

Lovis and his colleagues were able to detect key details about the exoplanet’s atmosphere using a special new instrument called ESPRESSO at the European Southern Observatory's Very Large Telescope.

The team watched the planet transit, or pass in front of its star, on September 2 and October 20, 2018. This stellar backlight revealed that winds blow iron vapor from the dayside to the nightside across a boundary called the evening terminator, but that the vaporized metal did not reemerge on the morning terminator, where the nightside turns back into the dayside.

“We detect iron vapor and then we see it disappear,” Lovis said. “The most likely explanation is condensation in liquid form.”

If you were doomed to be dropped on WASP-76b, would you prefer to be on the dayside and experience unfathomable heat and radiation, or on the nightside where you would be pummeled by metal raindrops? This is one of only many intriguing questions raised by this ultra-hot Jupiter, which is why Lovis and his colleagues want to keep studying its extreme properties.

“We are not finished with this data,” Lovis said. “We can continue to analyze them in more detail to try to find other chemical species, other atoms, in the atmosphere, and we are actually working on that right now.”