The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants

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 1-D coupled climate-photochemical models to generate self-consistent atmospheres for several evolutionary scenarios, including high-O2, high-CO2, and more Earth-like atmospheres, with both oxic and anoxic 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 use 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. We find that James Webb Space Telescope (JWST) observations longward of 10 μm 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 absorption, which is diagnostic of massive water loss and O2 retention, rather than a photosynthetic biosphere. Similarly, strong CO2 and CO bands at wavelengths shortward of 2.5 μm 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—which has a longer atmospheric lifetime under Proxima Centauri's incident UV—and either photosynthetically produced O2 or a hydrocarbon haze layer. Key Words: Planetary habitability and biosignatures—Planetary atmospheres—Exoplanets—Spectroscopic biosignatures—Planetary science—Proxima Centauri b. Astrobiology 18, 133–189.

THE SPACE WEATHER OF PROXIMA CENTAURI b

“…Turbet et al (2016) studied two likely rotation modes (tidally locked and 3:2 resonances) reaching the conclusion that, for both synchronous and non-synchronous rotation, there are scenarios which would allow liquid water to be present. Other studies have aimed at understanding the likely climate, photosynthetic properties and observational discriminants of the atmosphere of Proxima b (Goldblatt 2016;Lopez et al 2014;Meadows et al 2016;Kreidberg & Loeb 2016). We have shown here that space weather conditions of Proxima b are extreme and very different to those experienced on Earth.…”

“…Apart from its approximate mass and orbital parameters, rather little is currently known about Proxima b itself, although a handful of studies have already examined its likely irradiation history and possible climate and evolution in relation to potential habitability Turbet et al 2016;Barnes et al 2016;Meadows et al 2016). …”

Gravity-Darkened Seasons: Insolation Around Rapid Rotators

“…By constraining the planet's formation and migration history, high-energy irradiance, incoming stellar particle winds, and tidal interactions, along with the host star's evolution history, one can estimate the planet's atmospheric loss rate, its water budget, and its overall climate regime (Ribas et al 2016;Turbet et al 2016). Barnes et al (2016b) and Meadows et al (2016) demonstrate that a planet's geologic behavior can be constrained by modeling its orbit evolution and tidal history, as well as heavy element abundances in the planets core. Such works provide possible next steps toward characterizing the nature of exoplanets in early-type systems.…”

The Pale Green Dot: A Method to Characterize Proxima Centauri b Using Exo-Aurorae

“…Consequently, in advance of larger diameter ground-and space-based telescopes, and improvements in visible AO systems, we must initially consider observations that do not rely on transits or current coronagraphy to search for and characterize the atmosphere of Proxima Cen b. Phase curves may offer one of the first means to study the atmosphere of Proxima Cen b Kreidberg & Loeb 2016;Meadows et al 2016) by potentially showing the reduction in day-night thermal emission contrast associated with an atmosphere. Phase curves have proven to be a successful means to characterize the atmospheres of planets larger and hotter than Proxima Cen b Knutson et al 2007; Knutson et al 2008;Crossfield et al 2010;Brogi et al 2012;Zellem et al 2014;Stevenson et al 2014), including ones that do not transit (Selsis et al 2011;Faigler & Mazeh 2011;Maurin et al 2012;Brogi et al 2014).…”

“…Since the Proxima system is only 1.3pc away, it is perhaps the best-case scenario for the detection of the faint auroral signal from a terrestrial exoplanet. Even though the planet-star contrast ratio in reflected visible light is poor ( 10 −7 ; see Turbet et al 2016;Kreidberg & Loeb 2016;Meadows et al 2016), if Proxima Cen b exhibits auroral emission, this will brighten the planet and potentially boost the planet-star contrast by one or more orders of magnitude at the wavelengths of the auroral emission features. The short wavelength of the oxygen green line also improves the contrast of the planet relative to the star due to the star's cool temperature and TiO absorption, which strongly suppresses the brightness of the star in the visible.…”