Rick Robinson

Jul 1st 2020

Iron Rain Falling: Sweltering Weather on an Exoplanet


Here on Earth, Iron Rain is the name of a video game, and also, presumably, the name of at least one heavy metal band. But on a planet called WASP-76b, it could be a weather report.

WASP-76b orbits a star some 640 light-years from the sun and is comparable in mass to Jupiter. It orbits its parent star so closely that its “year” is only 43 hours long. And there’s more, notes Sky & Telescope.

According to new observations made using the ultra-precise ESPRESSO spectrograph of the European Southern Observatory in Chile, the atmosphere of WASP-76b has clouds of iron vapor. Along the line of twilight between the planet’s day and night sides, droplets of molten iron fall as rain from upper cloud layers, only to evaporate again into a lower-level fog of iron vapor.

Unearthly Weather

As Axios notes, the iron rain of WASP-76b is not the first exoplanet, or planet orbiting a distant star, to have its weather sussed out by astronomers. Another exoplanet has wind speeds clocked at 5,400 mph, and its atmospheric chemistry likely produces rain droplets of glass. And the entire atmosphere of yet another exoplanet is gradually being blown off into space, forming a tail like a comet.

But — at least for now — WASP-76b probably carries off the prize for the planet with the most rugged weather conditions.

WASP-76b and its weather may be nothing like Earth, but they share one basic feature in common: The main driver of weather is light and heat from the planet’s parent star. WASP-76b gets plenty of both; because of its close orbit, reports EarthSky, atmospheric temperatures on the day side reach about 4,350 degrees F (or 2,400 C).

Like Earth’s moon, WASP-76b has tidally locked rotation: Its day is equal to its year, so one side of the planet is in perpetual daylight, the other in perpetual night. Atmospheric winds, however, transfer heat energy from the day side to the night side, so that even on the night side the temperature remains a toasty 2,730 degrees F (1,500 C).

Between Hot and Hotter

The winds, and the enormous temperature differential between day and night, provide the basis for the iron rain of WASP-76b.

As astronomer David Ehrenreich explains, per Sky & Telescope, the planet “has a twilight zone at a temperature close to the iron condensation temperature.” As winds carry iron vapor from the day side to the night side, temperatures fall enough across the aptly-named twilight zone for the iron to condense into droplets.

Even though the planet always has one side facing its parent star, the dynamics of rotation produce a day-to-night “dusk” wind on one side of the twilight zone, and a night-to-day “dawn” wind on the other side.

The enormous precision of the ESPRESSO spectrograph allowed researchers to identify and distinguish these two wind patterns. And the results showed that iron vapor is present in the atmospheric chemistry of the “dusk” wind, but absent from the “dawn” wind.

The “Iron Cycle”

If iron is disappearing from the atmosphere as the wind carries it to the night side, it has to go somewhere. The only place for it to go is down, condensing into raindrops as the temperature drops.

But because WASP-76b has no solid surface, adds Sky & Telescope, the rain does not end up in iron rivers or seas, instead vaporizing again in deeper, denser, hotter layers of the night side atmosphere. There, invisible to distant observers like human astronomers, it must be carried back to the day side, where rising heat clouds again lift it up to where ESPRESSO can detect it.

Just as the water cycle of terrestrial water evaporation and rainfall plays a crucial role in transferring heat through Earth’s atmosphere — cooling the tropical zones while warming up the poles — the “iron cycle” may play a major role in maintaining the thermal balance of WASP-76b’s atmosphere.

According to astronomer Ehrenreich, reports, “These are likely the most extreme climates we could ever find on a planet.” And certainly WASP-76b is unlikely ever to become a human tourist destination. But observation of weather patterns on even the most unearthly of worlds help us to further understand the processes that shape our own weather and climate here on Earth.