Isotopes as a Tool for Exploring Exoplanet Atmospheres

Abstract

Isotopes are a powerful tool to understand the formation and evolution of celestial bodies. Isotope ratios have been used extensively in planetary science to help better understand the history of the solar system. In this thesis, I evaluate how we can characterize exoplanet atmospheres using isotopologues. I also extend this to the detectability of novel biosignature gases by way of carbon isotope ratios.

I evaluated the detectability of ¹³CO₂ in the atmospheres of temperate terrestrial and sub-Neptune sized planets and assess its suitability as a biosignature gas. While the search for signs of life has focused on terrestrial planets around M dwarf stars, an unambiguous detection of life—including through isotopologues—on a rocky world will likely remain out of reach in the JWST era. Due to their size and ability to retain large scale height, H₂-dominated atmospheres, sub-Neptunes are relatively easier to study using transmission spectroscopy compared with terrestrial planets. I simulated observations of CO₂ isotopologues in the H₂-dominated atmospheres of our nearest (< 40 pc), temperate (equilibrium temperature of 250-350 K) sub-Neptunes with M dwarf host stars. I find Earth-like fractionation of ¹³CO₂ to be distinguishable only if the atmosphere is H₂-dominated with a few percentage points of CO₂. I find that carbon isotopes via CO₂ are only observable in the atmospheres of sub-Neptune planets for the most idealized targets. I extended this work to fully consider the requirements and considerable challenges of using isotopologues as biosignature gases in exoplanet atmospheres. I conclude that isotopologue measurements should be used to evaluate formation mechanisms of planets and exoplanetary systems rather than potential biosignature gases.

Carbon isotopic composition in planetary atmospheres can inform both the formation location and heating processes within the protoplanetary disk. Through simulations, I assessed the detectability of isotopologues in the atmospheres of exoplanets with JWST and determined the best dozen targets to measure atmospheric carbon isotope ratios using transmission spectroscopy. The similarity of carbon isotope ratios in our solar system bodies is at odds with simulations, which predict that carbon isotope ratios should vary both radially and axially within a protoplanetary disk due to temperature and UV radiation variations. Measuring the ¹²C/¹³C ratio in planets and their host stars will help to assess if (1) the homogenization of ¹²C/¹³C is common in planetary system formation and (2) evaluate which mechanism(s) could be responsible.

Finally, TRAPPIST-1 e is one of our best, potentially habitable, terrestrial exoplanets. In preparation for the JWST GTO Exoplanet Transit Spectroscopy team’s observations of TRAPPIST-1 e (PI: N. Lewis), I evaluate which atmospheric scenarios we will be able to constrain with our limited data. The four transits of our GTO Program will permit initial reconnaissance of TRAPPIST-1 e’s atmosphere, though a close consideration is required for which atmospheric scenarios we will be able to support or reject with our expected data. We will be able to evaluate specific atmospheric compositions such as a low mean molecular weight, primordial H₂-dominated atmosphere, and the presence of strong absorbers, such as CO₂ and CH₄. I find that in the best-case scenario, we may see hints of CO₂ and CH₄ within our data, though this is unlikely due to the strong chance that the atmosphere has a small scale height and that the atmospheric signal will be entangled with stellar noise from the M dwarf host star. However, we will be able to put additional constraints on the mean molecular weight of the atmosphere and use our observations to inform on the best path forward for making conclusive detections of the two most detectable molecular species—CO₂ and CH₄—in the atmosphere of TRAPPIST-1 e.