Playing detective for distant worlds: exoplanet spectral retrieval

The innovative transit spectroscopy technique has provided us with a window into the atmospheres of exoplanets for the last 20 years. When a planet in a close orbit around its parent star passes in front of the star, the starlight is filtered through the planet’s atmosphere and emerges with the fingerprints of different absorbing gases. The Hubble Space Telescope was for many years the go-to observatory, providing the first insights into the diversity of hot, tidally locked gas giants; however, the restricted infrared spectral coverage, relatively small primary mirror and low thermal stability of the telescope have limited our ability to fully understand the chemistry of these objects. JWST, operating since summer last year, has extended our reach into the mid-infrared and has provided exquisitely precise spectra that are already reshaping our understanding of exoplanet atmospheres.

As in the solar system, spectral retrieval techniques are powerful tools for disentangling the spectral effects of different atmospheric properties. Whilst for solar system planets our detailed measurements from orbiting satellites mean that algorithms such as optimal estimation – which requires only a small number of individual simulations – are adequate, the lack of prior knowledge and wide potential parameter space for exoplanets means we resort to much more computationally expensive approaches such as nested sampling to explore possible solutions. Retrieval modelling for exoplanets is therefore a careful balance between detail and practicality. With the arrival of JWST, modelling approximations that were previously suitable are no longer adequate to describe our observations.  

In this seminar, I will discuss how atmospheric retrieval approaches for exoplanets have evolved through the Hubble era to current approaches for JWST. I will touch on how our understanding of exoplanet clouds has changed over time, and the latest challenge of adapting our models to encompass the 3D nature of exoplanets – even though we only see them as a point source.