The Formation and Evolution of Habitable Worlds

Abstract

The next generation of ground- and space-based telescopes will probe the atmospheres, and possibly the surfaces, of Earth-sized planets in the habitable zone (HZ). However, observing time will be both expensive and limited making it essential to target only the most promising planet candidates in the hunt for habitable and inhabited worlds. In addition, the measurements of these exoplanets may be noisy and the spectral signals of interest weak, necessitating a robust framework of theoretical and numerical models to guide collection and interpretation of the data, of which the studies presented here are a part. Conventionally, for a planet or moon to be habitable it must be sufficiently small to not retain abundant nebular gas after formation, but large enough to ensure subsequently outgassed or delivered surface volatiles, such as H2O, are not rapidly lost to space. In Chapters 2 and 3 we consider these upper and lower bounds by modeling the atmospheric escape of planets and moons around Sun-like stars. We show that rocky planets are unlikely to exist with radii larger than ~1.8 Earth radii and the smallest habitable planets and habitable zone icy moons may be similar to, or greater in size than Ganymede. However, size alone does not guarantee exoplanet habitability. Crucially, atmospheric pressure and composition, particularly atmospheric CO2, must conspire such that surface H2O does not rapidly evaporate and escape to space or freeze. The Earth has sustained a wide range of atmospheric CO2 levels over its history, from a few hundred ppmv today to possibly >70% at 2.7 Ga, which we show from micrometeorite oxidation in Chapter 4. In Chapter 5, from a coupled climate and carbonate-silicate weathering model, we show that similarly broad ranges for atmospheric CO2 are expected on Earth-like planets in the HZ. Despite this predicted spread in atmospheric CO2 in the HZ, we show a relationship between CO2 and incident flux should exist if Earth-like, carbonate-silicate weathering is ubiquitous and so the HZ hypothesis can be tested observationally. In Chapters 6 and 7 we focus on inhabited exoplanets and model an Earth-like biosphere around other stars, i.e. one dominated by oxygenic photosynthetic organisms. In Chapter 6 we model the optimal pigment absorption profile of photosynthetic organisms and find they may preferentially absorb blue photons around F type stars, red photons around G, K, and early M type stars, and infrared photons around late M type stars. Then in Chapter 7 we show that around the latest M type stars, the lack of high-energy photons available for photosynthesis may limit total biosphere size and detectability. The chapters in this work provide theoretical and numerical constraints on the properties of habitable and potentially inhabited worlds. We model the plausible size limits on habitable worlds in Chapters 2 and 3, consider CO2 abundances in habitable atmospheres in Chapters 4 and 5, and predict total biosphere size and absorption properties in Chapters 6 and 7. These findings may aid future detection and characterization of habitable worlds.