Exo-Earths from the Inside Out: Characterizing Earth-like Exoplanets with Future Space Telescopes

Abstract

Future space-based telescopes will seek extraterrestrial life on Earth-like planets via transmission and direct imaging spectroscopy. NASA has prioritized the development of the Habitable Worlds Observatory (HWO), a flagship ultraviolet/optical/infrared direct imaging telescope to study Earth-like planets around Sun-like (FGK) stars, and has recommended the development of the Origins Space Telescope (Origins), a probe-class mid-infrared telescope capable of transmission spectroscopy to study Earth-like planets around M-dwarf stars. To assess habitability and biosignatures with observations from these future missions, we require (1) coupled interior-atmosphere models to predict observables and discriminants to rule out abiotic planets, and (2) robust retrieval tools to analyze observed spectra and infer surface and atmospheric properties with their uncertainties.In the first part of this thesis work, we develop a novel "whole-planet" model to simulate the abiotic Earth through time by enhancing a coupled stellar-core-mantle-lithosphere thermal evolution model to include the carbonate-silicate cycle, the deep water cycle, a global surface water cycle, and climate with radiative transfer calculations. The upgraded model predicts surface temperature, surface albedo, CO2 and H2O abundances in the atmosphere and oceans, and ocean pH, as well as mantle and core properties described in previous work. We validate our model on measurements of key properties of the pre-industrial Earth, including but not limited to surface temperature, surface albedo, atmospheric and oceanic abundances of CO2 and H2O, ocean pH, upper mantle temperature, mantle viscosity, and the inner radius of the core. Next, we extend our abiotic Earth model to explore the time-evolving boundaries of the habitable zone (HZ) around representative F-, G-, and K-type stars. We produce HZ phase diagrams by upgrading our whole-planet model with a self-consistent treatment of fractional ice coverage, exploring two end-member weathering prescriptions that bracket the poorly constrained efficiency of abiotic silicate weathering. We find that the width, stability, and temporal evolution of the HZ depends sensitively on both stellar type and weathering assumptions. In both models, the outer HZ boundary contracts over the star's main sequence lifetime as volcanic degassing declines. Our nominal model predicts limit cycle regimes in the outer HZ for all three stellar types, while our suppressed weathering model predicts either stable habitability or permanent snowball states. The K-dwarf case is notably distinct: the runaway greenhouse boundary remains fixed throughout the evolution owing to the star's slow luminosity evolution, and outer HZ planets show a pronounced tendency toward permanent glaciation in both models. This is a consequence of degassing declining faster than stellar luminosity increases. Our results are broadly consistent with previous work but predict substantially narrower HZ widths. In contrast to previous work, for G- and K-stars we find a narrow range of planets that remain habitable for the duration of the star's main sequence lifetime. In the latter portion of the thesis, we use a novel retrieval tool to assess accuracy in characterizing near-surface habitability and biosignatures via simulated transmission and direct imaging spectra, based on the Origins and LUVOIR (HWO precursor) mission concepts. In transmission, we assess our ability to discriminate between an Earth-like and a false-positive O3 TRAPPIST-1 e, and in reflected light, we assess the robustness of retrieval results to un-modeled cloud extinction. We find that Origins-like transmission observations may detect the CO2/CH4 pair on M-dwarf planets and differentiate between biological and false-positive O3 using H2O and abundant CO. In contrast, direct-imaging observations with an HWO-like observatory are better suited to constraining O2 and O3, and may be sensitive to wavelength-dependent water cloud features, but will struggle to detect modern-Earth-like abundances of methane. For direct imaging, we weakly detect a stratospheric ozone bulge by fitting the near-UV wings of the Hartley band. Finally, we assess the spectral resolving power required to detect and characterize biosignature gases across Earth-through-time atmospheres using HWO. Combining analytical detectability calculations spanning resolutions R=20-5000 with atmospheric retrievals, we find that the nominal visible resolution R_Vis=140 is sufficient for detecting O2 in Phanerozoic-like atmospheres, while indirect inference via O3 (detectable at R_UV~7) may provide the most efficient path for characterizing low-O2 Proterozoic atmospheres. In the near-IR, we find that R_NIR>=40 is necessary to avoid degeneracies between CO2 and CO that could produce false positive detections of abundant CO, and the nominal R_NIR=70 is sufficient for characterizing all Earth-through-time cases. This thesis work contributes to HWO mission development by anticipating potential challenges in interpreting habitability and biosignatures due to abiotic planets, assessing the observational advantages of different stellar types for exoplanet surveys, and providing actionable guidance for finalizing spectrometer requirements while maintaining technological feasibility for the search for life on exoplanets.