Alien clouds

Clouds on Earth are made of water vapour condensing into droplets of water, or, in very cold parts of the atmosphere, into crystals of ice. On Earth, it's water all the way up.

But many of the exoplanets we've discovered orbit incredibly close to their stars. Because they are so close, they are much hotter and exposed to intense radiation from the star itself. It's like sitting closer to a fire — you can feel the difference in heat.

If we heat up a planet enough, it's no longer going to make clouds out of water vapour because it's too hot for water to become either liquid or solid ice.

So the big question becomes: what are the clouds made of in a hot exoplanet atmosphere?

One of the remarkable things we've discovered, through studying objects across the universe and understanding the chemistry of materials here on Earth, is that the clouds forming in these giant hot exoplanet atmospheres are actually made of rock.

They can be made of quartz crystals, molten iron, or even corundum — the material that forms rubies and sapphires on Earth. These tiny particles suspended in a gas are what we call aerosols, and that's essentially what clouds are.

When we look at these other worlds, we are looking at truly alien clouds and asking: what does that mean, and why does it matter?

WASP-17b is, in my opinion, a beautiful gas giant planet. It has about half the mass of Jupiter but twice the size, which means it has a very low density. If you could find a bathtub big enough, it would float.

It's a huge, puffy planet with a very extended atmosphere. When the planet passes in front of its star from our point of view — what we call a transit — some of the starlight passes through the planet's atmosphere before reaching our telescopes.

Because the atmosphere is so extended, a large amount of starlight travels through it, allowing us to measure its atmospheric properties very carefully and very precisely.

One of the remarkable things we measured with the Hubble Space Telescope early on was that the atmosphere scattered blue light more strongly than red light.

That told us there must be very small particles floating in the atmosphere — particles that scatter blue light efficiently but not red light.

We knew there were tiny particles suspended in the atmosphere, but we couldn't initially determine what those particles were made of.

This matters because if clouds are made of minerals like rock or sand, rather than gases such as water or carbon dioxide, you need to observe them in the mid-infrared wavelengths. That's where those materials interact most strongly with light.

Using JWST, we observed the atmosphere of WASP-17b and detected absorption features that we then tried to reproduce with our atmospheric models.

At first, we expected the clouds to be made of magnesium silicates — essentially a form of sand. But instead, we found a signature corresponding to pure silica, without magnesium.

In other words: quartz crystals.

By combining data from the Hubble Space Telescope and JWST, we were able to show that in the superheated atmosphere of WASP-17b — where temperatures exceed 2,000°C — tiny nanocrystals of quartz are circulating through the atmosphere at enormous speeds.

One side of the planet is intensely heated by its star while the other faces the cold of space. This creates powerful atmospheric circulation: rock vapour forms on the hot dayside and condenses on the cooler side of the planet.

WASP-17b has an entire atmospheric cycle of vaporising and condensing rock, forming crystals of quartz in its atmosphere — and we were able to detect the signal of those crystals in the planet's spectrum.

On Earth, our bright white water clouds scatter sunlight back into space. This reflectivity is what we call albedo.

If a surface has an albedo of one, it reflects all incoming light. If it has an albedo of zero, it absorbs all incoming light.

But clouds also play another role, depending on what they are made of.

Earth's clouds are made of water, which is also a greenhouse material. Water absorbs infrared radiation — heat. So while clouds reflect sunlight and help cool the planet, they also trap heat escaping from Earth's surface, warming the planet at the same time.

Clouds therefore play a major role in controlling a planet's overall environment because they influence both its reflectivity and its heat balance.

When we study clouds on other worlds — clouds made of materials like quartz — we are asking how those clouds change the transfer of energy through the atmosphere.

How do they influence storms? How do they affect atmospheric circulation? How do they shape climate?

These questions become especially important when we eventually begin studying smaller rocky planets similar to Earth.

If we do not understand how clouds interact with radiation, we will not be able to understand what the environment of any planet is truly like.

But by studying giant exoplanets orbiting close to their stars, we can already begin to measure the influence of clouds on atmospheric environments.

This allows us not only to refine the observational techniques we will one day use on Earth-like worlds, but also to test the fundamental physics and chemistry operating in these alien environments.

By studying these giant planets, we are beginning to uncover the basic physical and chemical processes that shape atmospheres throughout the universe.

And ultimately, that knowledge helps us better understand Earth's own atmosphere.

The same quartz particles we observe on distant planets are materials ejected by volcanoes on Earth. Dust particles in our atmosphere influence climate, light scattering, and even the colour of sunsets.

Studying these particles in other worlds allows us to better understand the influence they may also have on our own planet.

One of the most fundamental lessons that has taught us is that there is no Planet B.

Earth is our world for many generations, centuries, and millennia to come. What we do to our planet has a profound influence on its atmosphere.

We therefore need to understand how the changes we make alter the balance of energy entering and leaving our planet.

There is nowhere else for us to go. We need to take care of this one.

And if studying other worlds can help us better understand those influences, then that knowledge becomes incredibly valuable.

In the end, it all comes back to Earth — and to understanding our place in the universe.

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