Caltech/IPAC Lunch Seminar

Almost everything we know about exoplanet atmospheres has come from reconciling telescope spectra with atmospheric chemistry and climate models. Until recently, these models were applied mainly to uninhabitable worlds such as hot Jupiters and brown dwarfs. Now, in the era of the James Webb Space Telescope (JWST), claims of habitable—or even inhabited—worlds depend directly on such simulations. I present Photochem, an open-source and general 1-D photochemistry–climate model designed to interpret exoplanet spectra. Photochem reproduces the observed compositions and climates of Venus, Earth, Mars, Jupiter, and Titan, and is thus a validated tool for understanding exoplanets. First, we use Photochem to interpret recent JWST observations of K2-18 b, a habitable-zone sub-Neptune exoplanet. These observations revealed CH4 and CO2 in a H2-rich atmosphere, which some researchers have interpreted as a sign that the planet has a habitable liquid water ocean. Overall, our simulations find that a gas-rich mini-Neptune with no habitable surface is a better explanation of the data, because photochemistry prohibits simultaneous CO2 and CH4 in a habitable and lifeless scenario and challenges with maintaining a habitable climate. Second, we use Photochem to develop a novel method of evaluating telescope spectra for signs of exoplanet life. Specifically, we invert the model to extract the rate a planet's surface and atmosphere exchange gases (i.e., surface gas fluxes), a quantity critical for determining the presence of life. We apply the method to synthetic JWST observations of TRAPPIST-1e assuming it is an inhabited world similar to the Archean Earth and assess the feasibility of inferring a biological methane flux on such a world. Together, these results demonstrate how physics-constrained, open-source modeling is necessary to do astrobiology with JWST.