To answer these and other questions about the atmosphere of exoplanets, in an effort to shed light on our own solar system, we need to measure and understand their atmospheres. Over the last decade we have been using the Hubble Space Telescope to perform large-scale surveys of exoplanet atmospheres from giant Jupiter sized worlds extremely close to their stars dubbed hot Jupiters, to Neptune sized worlds, and even rocky terrestrial worlds around small cool stars. To measure the atmosphere, we observe the transit of the exoplanet as it passes in front of its star, this causes a small dimming effect on the measured starlight which corresponds to the relative size of the planet. If the planet has an atmosphere surrounding it, some of the star's light will be filtered through the exoplanet atmosphere before reaching the telescope. Each atom and molecule have its own unique spectral fingerprint. If an atom or molecule is present in the exoplanet's atmosphere it will imprint its signature on the spectrum of the starlight which can be measured to determine the atmospheric composition.
We have discovered a huge diversity in the atmospheres of exoplanets from clear strong atomic and molecular signatures including water vapour, to scattering and muted or obscured features indicative of clouds high in the planet's atmosphere. Interestingly, due to the temperature of these worlds the clouds are not made of water, but instead things commonly found as rocks or minerals here on Earth such as enstatite (sand), or corundum (the basis of rubies and sapphires). In the coming years the James Webb Space Telescope (NASA/ESA/CSA) will allow us to look even deeper into these exoplanet atmospheres and identify different molecules and measure the composition of these clouds directly, to better understand their composition, structure, and environment. As newer exoplanets amenable to exoplanet characterization are discovered with missions such as TESS and PLATO, we will use our expertise with HST data analysis to develop and apply novel observing and analysis methods to JWST to better place our solar system in a planet-filled universe.
Links to planet formation modelling
When a planet forms, it is thought that its location and formation process in the protoplanetary disk can leave imprints in the composition of the atmosphere. By measuring the composition and abundances of different molecules in exoplanet atmospheres across a whole range of worlds around different stars, we can build up a more complete view of formation markers and place links back to simulations of our solar system.
Links to Brown Dwarf atmospheres
Another exciting avenue of study for atmospheric physics and chemistry are brown dwarfs - objects greater than 13 Jupiter masses but not massive enough to fuse hydrogen at their cores to become stars. Brown dwarf atmospheres are measured in emission where we are generally looking at lone objects not irradiated by a host star. However, their atmospheric chemistry can be very similar to that of exoplanets and they offer a direct look at the composition of exotic clouds in their atmosphere. We use JWST measurements to characterize the clouds in brown dwarf atmospheres and make comparisons to the physical formation mechanisms in irradiated exoplanet atmosphere.