Cloudbusting with JWST: characterising aerosols, aurorae and chemistry in substellar atmospheresto the water cloud regime

JWST will revolutionise the study of alien worlds via its sensitivity to the wavelengths where most of their heat escapes into space - wavelengths of light that cannot be observed easily from the ground. One of the most important groups of planets that JWST will target are giant planets, similar to Jupiter, that orbit other stars. These worlds are important for understanding how planetary systems form and evolve because they represent most of the mass in their planetary systems, in the same way that Jupiter dominates our own Solar system. And, like Jupiter, giant exoplanets are thought to play an important role in delivering water to rocky, possibly habitable, planets in their systems. Giant planets are also the easiest planets to observe outside the Solar System, so they also provide a testing ground for enhancing our ability to study all kinds of exoplanets, even potentially habitable ones.

One of the most challenging problems in studying exoplanets is how to deal with clouds. Clouds block our view, and also change the composition and other properties of alien atmospheres in complicated and difficult to measure ways. Because of this, clouds are often seen as an obstacle to exoplanet science. However, thanks to JWST and the analysis tools that we have developed, clouds will soon be able provide new insights into the composition, chemistry and dynamics of alien worlds.

Alongside clouds, powerful aurorae have been discovered on free floating Jupiter-like worlds known as brown dwarfs, and we expect to find similar aurorae on giant exoplanets in the future. Interestingly, all the brown dwarfs that show aurorae also show variations in their brightness as they rotate. Such variability has been seen in many other brown dwarfs and even some giant exoplanets, and it is generally thought to be due to weather creating bright and dark patches due to differing cloud cover. It is also possible that some of the variability is driven by aurorae, or even that weather, and associated large scale atmospheric flows, may interact with the aurorae in ways yet to be discovered.

With this funding, we will learn how to decode JWST spectra of distant worlds to reveal their cloud properties, composition, and thermal structure, and understand how their aurorae interact with their atmospheres. We will also use JWST to study some of the coolest brown dwarfs known, called Y dwarfs. Y dwarfs are cool enough to have water ice clouds in their atmospheres. These will be the first studies of water clouds beyond the Solar system. Thanks to prior knowledge of the compositions of some of these brown dwarfs, we will be able to calibrate our understanding of clouds, and then use this knowledge to study giant planets. We will also be able to test different theories of cloud formation and chemistry, as well as study the weather on these alien worlds. This work will develop techniques that we will need in order study all kinds of cloudy exoplanets. It will also pave the way for robust estimates of the composition of giant exoplanets - an essential piece of evidence for revealing their formation histories and that of their home planetary systems.