Molten iron rains down on exoplanet

Scientists have found signs of rain on a Jupiter-sized exoplanet 391 light years away. 

But it’s not ordinary rain. Rather, they report in the journal Nature, the planet, known as WASP-76b, may be experiencing showers of molten iron.

WASP-76b circles a star hotter than the Sun at a distance of only five million kilometres. That’s so close that it completes an entire orbit once every 1.8 days – and so close that its day side reaches a temperature of 3,000 degrees Celsius – about 150 degrees above the vaporisation temperature of iron.

Meanwhile, says first author David Ehrenreich, an astronomer at the Geneva Observatory in Versoix, Switzerland, its night side is about 1,000 degrees cooler – cool enough for vaporised iron to condense into iron clouds and fall out as iron rain. 

Evidence for this, he says, comes from comparing the amount of iron vapor in the planet’s atmosphere at dusk and dawn. 

To do this, his team began by building an instrument called ESPRESSO (Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) – “the most precise spectrograph built so far” – and mounting it to the European Southern Observatory’s 8.2-meter Very Large Telescope in Chile.

Using these two hyper-sensitive instruments, he says, his team was then able to see that when the planet passed between us and its sun, the atmosphere on its dusk side – where temperatures were still daytime hot – showed absorption bands from vaporized iron…while the dawn side – where the atmosphere was at its nighttime coolest – did not. 

“We cannot see what’s happening on the night side,” Ehrenreich says, “but the conclusion is that the iron is condensing out, and the most likely [way] is in liquid iron droplets. So it must be raining on the night side of the planet.”

“It’s a pretty cool result,” says Rory Barnes, an exoplanet researcher at the University of Washington, Seattle, who was not part of the study team.

Though, he notes, Ehrenreich’s team hasn’t actually proven that the night side of WASP-76b is raining iron. “It’s just inferred,” he says. “There’s always a chance that something else is going on. Maybe the iron gets drawn down into the atmosphere some other way.”

Not that he thinks that’s all that likely. “Their interpretation is probably correct,” he says. “It makes sense.”

Meanwhile, Barnes says, the new find is another example of how different exoplanets can be from anything we’d expect based on the planets in our own Solar System. 

When it comes to exoplanets, he says, “The only thing I’m sure of is there are going to surprises. I’ve been in this business long enough that I continue to expect the unexpected, and they continue to amaze me.”

Ehrenreich adds that the new find is exciting not just because it’s so strange, but because strange planets like WASP-76b are also extremely good laboratories for testing climate models. 

“We can test climate models in conditions that are impossible in [our own] Solar System,” he says.