Iron Might Rain on This Hot Jupiter

When astronomers discovered the gas giant WASP-76b in 2016, about 640 light-years from Earth in the constellation Pisces, they already knew it was exotic. WASP-76b circles its star in only 43 hours, making it a “hot Jupiter.”  

Now, a group of astronomers led by David Ehrenreich (University of Geneva, Switzerland) has found that this planet might be even stranger than we thought, hosting clouds and fog made of iron. The research appears in Nature.

WASP-76b is tidally locked so, like the Moon, it always shows the same face toward its star. The planet’s constantly daylit side may reach 2,400°C, and even the permanent nightside only cools to 1,400°C.

The hot Jupiter crosses the face of its star from Earth’s perspective, so astronomers can take a spectrum of the starlight that passes through the planet’s atmosphere during its passes. Ehrenreich and colleagues used the ESPRESSO spectrograph mounted on the European Southern Observatory’s Very Large Telescope in the Chilean Atacama Desert. “ESPRESSO delivers a precision . . . never achieved before by any other spectrograph,” explains coauthor María Rosa Zapatero Osorio (Center of Astrobiology, Spain).

Iron Rain

The high-resolution spectrum reveals lots of iron vapor within the sliver of atmosphere undergoing the transition from day to night. However, this iron vapor signature is missing from the sliver of atmosphere transitioning from night to day. The astronomers think this happens because strong winds push iron vapor to the nightside, where it cools and condenses into clouds.

“This planet has a twilight zone at a temperature close to the iron condensation temperature,” Ehrenreich explains, “so the change in atmospheric composition (with iron vs. without iron) is occurring right where we are able to observe.”

Because the planet is a gas giant, there’s no surface onto which the droplets can fall, says coauthor Nuno Santos (University of Porto, Portugal). But the planet’s gravity likely pulls the clouds downward, enveloping the nightside in iron fog. The global winds then push the clouds and fog onto the dayside, where the vaporization-condensation cycle repeats again.

This isn't the first exoplanet with iron found in its atmosphere — the even-hotter Jupiter KELT-9b also hosts ionized iron and titanium. However, in the case of WASP-76b astronomers are finding neutral iron vapor in clouds rather than in ionized, cloud-free form.

Carole Haswell (Open University), who was not involved with the study, is excited by the new observations. She says,  “These findings show that the weather conditions on exoplanets can be far stranger than we might imagine!”