When I first came back to space science, I attended a meetup at NASA Ames Research Center to learn about the Kepler space telescope and its mission. Finding exoplanets was going to be a big deal in the upcoming years, and I was excited to be a small part of it. Now we’re at over 4000 discovered exoplanets and counting, to the point where exoplanet discovery announcements have to come with some sort of bonus reason as to why they’re important or interesting. Because “woo, look, another exoplanet” just doesn’t make headlines anymore.

So what’s next for the worlds of exoplanet science? One increasingly studied aspect is the atmospheres of those exoplanets. We’re getting to the point where we have instruments sensitive enough to gather data about atmospheric composition. Last week, we reported on finding various carbon isotopes in the atmosphere of a distant world. This isn’t a new science, though. In 2001, Hubble’s spectrographs found less sodium in a planet’s atmosphere than expected. Which led to researchers contemplating clouds as a reason they couldn’t see the sodium.

Huge banks of dark, hot clouds on a planet five times hotter than Earth, though, would make for some interesting weather. It’s unlikely that the droplets making up those clouds would be made of water. So now we have really neat hypotheses about clouds made of liquid sand, iron, rubies, diamonds. Clouds are difficult to study here on Earth, though. They’re simultaneously made of the microscopic and the huge — water droplets coming together to form a wide range of shapes and structures, covering more than two-thirds of the surface of Earth.

And our own solar system has its fair share of weird weather. Venus has sulfuric acid in its clouds. Jupiter has ammonia mushballs. Titan has methane and ethane rain. How do we even begin to understand the clouds on worlds so far away we cannot directly image their surfaces? We go back to the transit of the planet in front of its star, and we use spectrographs to catch the chemical signatures of molecules in the planet’s atmosphere. We take into consideration the temperature of the planet. And we add it all together to get… even weirder forms of rain like the iron rains of WASP-76b.

WASP-76b is a strange planet, to begin with. It’s a gas giant nearly two times the radius of Jupiter. It orbits its star in less than two days because it’s only 0.033AU from that star. It’s tidally locked. The daytime temperature can reach 2,400 Kelvin, but the nightside doesn’t get as hot, and the iron atoms are only detected on the dayside. This difference appears to be evidence for iron vapor to move around the planet via the winds (again, think Jupiter-type storm patterns), condense, and fall as rain on the nightside, where they no longer appear in the atmosphere.

It’s not perfect evidence. It’s not images of iron rain falling. It’s the best we can do at the moment, though, and we’re still waiting for the telescope that shall not be named to do the thing and get launched so we can finally use its infrared capabilities to look at all these exoplanets. Then, finally, we can directly measure what all these weird and interesting clouds are made of.

More Information

The Forecast for Exoplanets is Cloudy but Bright (Eos)