The Habitability of Proxima Centauri b: II: Environmental States and Observational Discriminants (1608.08620v1)

Abstract: Proxima Centauri b provides an unprecedented opportunity to understand the evolution and nature of terrestrial planets orbiting M dwarfs. Although Proxima Cen b orbits within its star's habitable zone, multiple plausible evolutionary paths could have generated different environments that may or may not be habitable. Here we use 1D coupled climate-photochemical models to generate self-consistent atmospheres for evolutionary scenarios predicted in our companion paper (Barnes et al., 2016). These include high-O2, high-CO2, and more Earth-like atmospheres, with either oxidizing or reducing compositions. We show that these modeled environments can be habitable or uninhabitable at Proxima Cen b's position in the habitable zone. We use radiative transfer models to generate synthetic spectra and thermal phase curves for these simulated environments, and instrument models to explore our ability to discriminate between possible planetary states. These results are applicable not only to Proxima Cen b, but to other terrestrial planets orbiting M dwarfs. Thermal phase curves may provide the first constraint on the existence of an atmosphere, and JWST observations longward of 7 microns could characterize atmospheric heat transport and molecular composition. Detection of ocean glint is unlikely with JWST, but may be within the reach of larger aperture telescopes. Direct imaging spectra may detect O4, which is diagnostic of massive water loss and O2 retention, rather than a photosynthesis. Similarly, strong CO2 and CO bands at wavelengths shortward of 2.5 {\mu}m would indicate a CO2-dominated atmosphere. If the planet is habitable and volatile-rich, direct imaging will be the best means of detecting habitability. Earth-like planets with microbial biospheres may be identified by the presence of CH4 and either photosynthetically produced O2 or a hydrocarbon haze layer.

Overview of Environmental States and Observational Discriminants for Proxima Centauri b

The paper, "The Habitability of Proxima Centauri b: II: Environmental States and Observational Discriminants," provides a comprehensive analysis of the prospective atmospheric and environmental states of Proxima Centauri b, a terrestrial exoplanet in the habitable zone of its host M dwarf star. Utilizing a range of one-dimensional (1D) coupled climate-photochemical models, the researchers investigate a spectrum of potential evolutionary scenarios that impact the planet's ability to support life. These include atmospheres rich in oxygen (O₂), carbon dioxide (CO₂), and combined O₂/CO₂ environments, evaluating both oxidizing and reducing compositions. Models generate self-consistent atmospheric profiles, yielding synthetic spectra and thermal phase curves to predict observable atmospheric characteristics.

Summary of Key Findings

1. Oxygen-Dominated Atmospheres: Scenarios where Proxima Centauri b retains, or has lost, a significant fraction of its water through photolysis are explored. Oxygen-rich atmospheres (partially due to massive water loss) depict strong absorption features in synthetic spectra, notably at wavelengths associated with O₂ and O₃.
2. Evolved CO₂/O₂ Atmospheres: The paper discusses scenarios where the atmosphere evolves into coexistence of CO₂ and O₂ through volcanic outgassing and subsequent desiccation. This results in Venus-like conditions with thick CO₂ atmospheres and surface temperatures significantly above those conducive to life.
3. Potentially Habitable States: Under scenarios where Proxima Centauri b maintained sufficient water to avoid desiccation or underwent a post-formation inward migration following stable volatile conditions, the processes might leave behind a planet reminiscent of early Earth or a modern Earth-like biosphere. Enhanced greenhouse gases to compensate for reduced stellar insolation from the host M dwarf are presented.
4. Spectral Observations: The paper highlights key observational discriminants that can distinguish between habitable and uninhabitable scenarios. The James Webb Space Telescope (JWST) and large ground-based telescopes are considered for their capability to detect significant atmospheric biomarkers, including O₂, CH₄, and CO₂ using direct imaging and thermal emission phase curves.

Implications and Future Directions

This work posits significant implications for understanding the dynamics of terrestrial exoplanets in close proximity to M dwarf stars. While the location within the habitable zone suggests potential for life, evolutionary processes driven by stellar and planetary interactions introduce substantial variability in habitability outcomes. The comprehensive modeling efforts put forth pave the way for spectroscopic assessments using next-generation telescopes, like JWST, which are needed to characterize atmospheric compositions and surface conditions definitively.

These models necessitate further refinement through incorporation of higher-dimensional atmospheric dynamics and continued observational campaigns to characterize stellar activity, particularly the ultraviolet spectrum, which influences atmospheric chemistry significantly. Analyzing the potential effects of frequent stellar flares via observational constraints will enhance the predictive accuracy of volatile inventories and atmospheric states.

In conclusion, this analysis of Proxima Centauri b underscores the importance of detailed, scenario-based climate and photochemistry modeling for assessing the habitability of terrestrial exoplanets. These insights provide a framework for future observational efforts directed at our nearest stellar neighbor, forming a crucial step in expanding our understanding of potential life-bearing conditions beyond Earth.