Exoplanets, planets that orbit stars other than our sun, are likely more numerous than the stars themselves. In the last 30 years over 4,000 exoplanets have been discovered. These worlds come in all sizes, orbital periods, and around all different types of stars. These changing environments will have a direct impact on the nature of these exoplanets. The most common size of world is one we do not have in our solar system, these planets have radii greater than 1.8 Earth radii, yet, smaller than that of Neptune at 4 Earth radii. In our solar system the Earth a rocky terrestrial world and Neptune a gas giant with a hydrogen dominated atmosphere, represent distinctly different types of planets. However, these abundant exoplanets have been discovered at sizes spanning this whole range. Leading us to question: At what size or mass does a planet stop having a hydrogen dominated atmosphere? How does the environment that planet lives in change what we expect to see? Are their atmospheres more like Neptune or like Earth? Is there a smooth transition from one type of planet to the other is there a more dominant unknown type of planet composition and structure we have not measured yet?
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.