Reading between the rainbows: comparative exoplanet characterisation through molecule agnostic spectral clustering

Rocky exoplanets are faint and difficult to observe due to their small size and low brightness compared to their host star. Despite this, the James Webb Space Telescope (JWST) has recently allowed us new methods and opportunities to study them, and accurately characterising exoplanets could offer insights not only into the planetary demographics of rocky worlds in the universe, but also how our own Earth compares. Previous work has simulated the observational spectra of planets with various models of atmospheric composition and described the conditions under which specific molecules would be observable. We seek a different approach by locating features that do not depend on specific molecules and correlating them with each other. Thusly we can investigate the formation of clusters with our prior knowledge of the simulated atmospheres and draw even further conclusions from both new and existing data.

This study uses 42 synthetic transit spectra acquired from the Virtual Planetary Laboratory Spectral Explorer in a range matching that of JWST’s MIRI instrument to show that it is possible to characterise exoplanet atmospheres from this level of observation. Spectra were denoised, normalised and split into bands for which the enclosed areas were calculated as a measure of signal strength. Using the HDBSCAN clustering algorithm, separation patterns and clusters were found across these bands, which correlated to various a priori planet characteristics, such as relative gas concentration. We also clearly separated reducing vs. oxic atmospheres by combining the areas under three different 0.7µm bands between 7 and 12µm. This initial set was sufficient to allow very strong constraints (mixing ratios accurate to within two orders of magnitude) in the identification of an unknown sample. Through this, we have demonstrated that it is possible to differentiate and analyse exoplanet atmosphere type from agnostic interrogation of their transit spectra.