Weather forecast for this exoplanet: iron rain

If you plan on visiting WASP-76b, a planet orbiting another star roughly 600 light-years away, you'd better pack a hardy umbrella.

In a new study published Wednesday in the journal Nature, scientists found that WASP-76b may produce rain. But this is no ordinary rain: it's iron rain.

The international team of astronomers made the finding using the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument at the Very Large Telescope in Atacama, Chile, which is made up of four individual telescopes.

This type of instrument allows scientists to determine not only the composition of a star — if it's precise enough, it can detect the atmosphere of a distant exoplanet.

And ESPRESSO did this exceptionally well.

"We were not expecting to find this kind of signature [of iron] with this level of detail," said David Ehrenreich, lead author of the paper, who is also an astronomer at the Observatoire astronomique de l'Université de Genève in Versoix, Switzerland.

In fact, the original plan was to do a brief analysis of the instrument's performance and then write a quick paper on the findings. But in doing this, Ehrenreich said he saw "something really intriguing."

"It really blew my mind, actually."

Using ESPRESSO, the researchers measured the atmospheric compositions at two different locations: the region corresponding to morning twilight and the region corresponding to evening twilight.

What they found was on the evening side, where it transitions from day to night, iron vapour is detected. But when gas is circulating from night to day — think of it as the morning — that signature is gone. 

Why it disappeared was puzzling.

Ehrenreich said they concluded that when the wind blows through the night side, it causes the iron vapour to condense, "forming clouds and likely condensing as rain of liquid iron droplets," Ehrenreich said.

'Quite crazy'

While iron rain has been hypothesized on other exoplanets, it's really about how the findings were made.

"The actual observation, the evidence for it, that's what makes this really unique," said Eve Lee, an assistant professor in McGill University's physics department, who studies exoplanets. "Atmospheric analysis of this kind is really difficult and the fact that we have it, it's really new … It's really exciting."

This star system is very different from our own.

At the heart lies a star — WASP-76 — that is similar to our sun in regards to its stellar life stage. But WASP-76 is younger, almost twice as big and hotter than our sun.

As for the planet itself, it's considered a "hot Jupiter," a planet roughly twice the size of our solar system's most massive planet, with a "puffed-up" atmosphere, likely due to the radiation it receives. As opposed to Jupiter, it lies very close to its star — closer than Mercury, the innermost planet in our solar system, is to the sun — completing one orbit every 1.8 days. 

"As a result, the planet is receiving something like 4,000 times the [energy] that the Earth receives from the sun," Ehrenreich said. "That is quite crazy."

WASP-76b is also likely tidally locked with its star. That means the same side always faces the star, just as the moon does with Earth. As such, the day side reaches temperatures around 2,400 C. Temperatures along the evening terminator, or the transition from day to night, is closer to 1,500 C. It's likely this temperature variation is somehow responsible for the condensation of the iron vapour.

As with all scientific findings, Ehrenreich is looking forward to follow-up studies of WASP-76b. And he's looking forward to a time when this kind of technique will be used to determine the composition of Earth-like planets.

For now, he's happy this finding tests astronomers' knowledge about other alien worlds.

"Hot Jupiters are unlike anything in our solar system," Ehrenreich said. "They are like an extreme laboratory to test the most extreme climates that could happen on an exoplanet."