Searching for Exoplanet Atmospheres
How can we use thermal observations to characterize rocky exoplanets? I originally started studying exoplanet thermal phase curves during my PhD (Koll & Abbot 2015; Koll & Abbot 2016), largely because they offer a way to probe the atmospheric dynamics of exoplanets (see below). Along the way, however, we discovered that thermal observations of hot rocky exoplanets can be used to search exoplanets for atmospheres more broadly.
During my postdoc, my collaborators and I proposed that secondary eclipses with JWST could help us quickly scan exoplanets and identify those with thick atmospheres (Koll et al 2019; Mansfield et al 2019; Malik et al 2019). This method has since then become one of the most effective ways to search rocky exoplanets for atmospheres (e.g., Crossfield et al 2022; Mansfield et al 2024; Xue et al 2024). As of late 2024, 500 hours of JWST time will be dedicated to searching nearby exoplanets for atmospheres using this method (news).
Atmospheres and Climates of Tidally-locked Exoplanets
Most exoplanets on short-period orbits are believed to be tidally locked, but how does this affect their atmospheres? Our Solar System has no planets that are tidally locked to the Sun, but tidal models have long suggested that many exoplanets should be tidally locked to their host stars. My PhD introduced some tools from dimensional analysis to analyze the large-scale atmospheric dynamics of such planets (Koll & Abbot 2015). Largely inspired by old tropical cyclone papers, I then showed that tidally-locked rocky planets closely resemble planetary heat engines (Koll & Abbot 2016), in which radiative heating and cooling allows these atmospheres to sustain dissipation via friction. Interestingly, hot Jupiters also behave like heat engines (Koll & Komacek 2018), which allows us to directly compare theory against observations.
One nice application of the heat engine approach is that it provides a quick way to predict heat redistribution on rocky exoplanets (Koll 2022). We had a great opportunity to test an early version of this approach in 2019, when we measured the first thermal phase curve of any small rocky exoplanet (Kreidberg, Koll et al 2019). The data combined with heat redistribution theory allowed us to rule out most atmospheric scenarios, so this planet must likely have no atmospheres whatsoever.
Spectral Foundations of Earth’s Climate
What do other planets tell us about our own planet’s climate? It turns out, quite a lot! During my postdoc we started investigating how the runaway greenhouse limit influences a planet’s radiation balance, and we ended explaining a fundamental property of Earth’s climate (Koll & Cronin 2019).