The list of known exoplanets numbers in the thousands.
The most prolific method of finding these worlds orbiting around other stars is to watch the dimming of the star’s light as the planet passes in front of it, known as a transit.
I study the atmospheres of transiting exoplanets to better understand how their climate is affected by their environments.
Hot Jupiters
I’m most interested in hot gas giants, or hot Jupiters: planets similar in size to Jupiter but orbiting their stars in just a few days, meaning they are baked under the intense radiation.
These ‘hot Jupiters’ are tidally locked with a very hot permanent dayside facing the star, and a cooler nightside facing space.
This dichotomy in temperature drives supersonic winds and changes the atmosphere’s chemistry.
One key driver of how an atmosphere responds to radiation are clouds.
Every Solar System planet with an atmosphere has clouds, and exoplanets are no different.
But at the intense temperatures found on hot Jupiters, what are these clouds made of and how do they change the planet’s atmosphere?
The temperature of hot Jupiters, the available material and the physics of cloud formation all suggest their clouds should be made from magnesium silicates called enstatite and forsterite – better known on Earth as rock or sand.
We can detect the presence of minerals like enstatite but only in the mid-infrared. Until recently, there was no such instrument able to detect it.
Using Webb to understand hot gas giants
Enter the James Webb Space Telescope (JWST).
The JWST is specifically designed to measure infrared light. This means it can pick out the unique fingerprints of whatever is absorbing infrared radiation – what we more commonly think of as greenhouse gases – in an exoplanet’s atmosphere.
The first JWST measurements of exoplanet atmospheres revealed what we were looking for.
The near-IR showed large absorption features of carbon dioxide in hot giant planets and methane in smaller, cooler worlds (still gas giants, but in less extreme conditions).
These are measured in their gas phase, while they were mixed with hydrogen and helium in the planet’s atmosphere, rather than in clouds.
WASP-17b
For one planet that my team and I study, WASP-17b, it was when we measured the mid-IR that we saw the first evidence of what the clouds were actually made from.
The data showed a small but distinct spike, where infrared radiation was being absorbed at 8.6 microns.
The spike was too narrow to be the enstatite or forsterite we expected.
Instead, our models showed that the clouds of WASP-17b are made of quartz – silica that is uncontaminated by magnesium.
Our modelling showed that the silica is in its crystalline from, which is likely forming lower down in the atmosphere before being transported to higher altitudes where we could see it.
With the help of data from Hubble in the UV-optical we found that the cloud particles were sub-micron in size, each a fraction of the width of a human hair.
Silica is one part silicon to two parts oxygen, so these clouds are locking away a vast amount of the planet’s oxygen, which means it’s unavailable to form water, carbon compounds or other minerals key to planet formation.
Once we know the amount of material within the planet’s clouds, we can begin to make links back to the planet’s formative conditions, to better understand how hot Jupiter exoplanets form, and what this means for the likelihood of planets like our own existing out there.
This article appeared in the August 2024 issue of BBC Sky at Night Magazine
