Alien clouds

But when we're looking at some of the exoplanets that we've discovered, they're actually incredibly close to their stars. And because they're very close to their stars, it means that they're hotter; it's like sitting closer to a fire. If we heat up a planet, it's no longer going to be making clouds that are water vapor because it's too hot for water to turn into a liquid or into a solid ice.

The big question is: what are the clouds made of in a hot exoplanet's atmosphere? One of the remarkable things we have discovered is that the clouds forming in these giant hot exoplanet atmospheres are actually made of rock. They're the sand on our beaches, quartz crystals, or molten iron floating around in the atmosphere. My favorite is the material called corundum—it's a ruby or sapphire here on earth—that are forming these aerosols. When we're looking at these other worlds, we're looking at truly alien clouds.

Because it's a big puffy planet, it has a big extended atmosphere. When the planet transits its star, some of that starlight shines through the planet's atmosphere before it reaches our telescope. This means we can measure the things in its atmosphere really precisely. One of the amazing things we measured with the Hubble Space Telescope is a scattering slope: it scattered blue light more than red light. This told us that there must be very small particles in the atmosphere floating around, but we couldn't work out what those particles were made of.

Using JWST, we looked at WASP-17 b's atmosphere and saw absorption features that didn't fit our expectations of sand. Instead, we saw a spike which corresponded to quartz crystals. You see these in any rocks you can pick up on the Earth; it's the shiny bits that you see.

By combining Hubble and JWST data, we can tell you that in WASP-17 b's superheated atmosphere of over 2000°C, you will be bombarded by tiny nanocrystals of quartz flying at thousands of miles per hour. One side is superheated by the star whilst the other faces space, creating a thermal driver that vaporizes the rock on the dayside and condenses it on the night side.

Earth's clouds are made of water, a greenhouse material that absorbs infrared radiation—heat—given out by the Earth. So clouds effectively cool our planet by reflecting sunlight but heat it up by preventing heat from escaping. When we see clouds made of things like quartz, we ask how that changes the way energy is transferred and the probability of thunderstorms versus clear skies. If we don't understand how clouds interact with radiation, we won't understand the environment on any planet, including an Earth twin.

This work is going to help us understand more about our own Earth atmosphere. Fundamental particles like quartz crystals are what is ejected from a volcano or lifted from the Sahara to make orange sunsets. Looking at these materials in other environments allows us to understand the influence they might have on our own planet.

Editor’s note: This article has been faithfully transcribed from the original interview filmed with the author, and carefully edited and proofread. Edit date: 2026