The habitability of planets orbiting M-dwarf stars

Abstract

The prospects for the habitability of M-dwarf planets have long been debated, due to key differences between the unique stellar and planetary environments around these low-mass stars, as compared to hotter, more luminous Sunlike stars. Over the past decade, significant progress has been made by both space-and ground-based observatories to measure the likelihood of small planets to orbit in the habitable zones of M-dwarf stars. We now know that most M dwarfs are hosts to closely-packed planetary systems characterized by a paucity of Jupiter-mass planets and the presence of multiple rocky planets, with roughly a third of these rocky M-dwarf planets orbiting within the habitable zone, where they have the potential to support liquid water on their surfaces. Theoretical studies have also quantified the effect on climate and habitability of the interaction between the spectral energy distribution of M-dwarf stars and the atmospheres and surfaces of their planets. These and other recent results fill in knowledge gaps that existed at the time of the previous overview papers published nearly a decade ago by Tarter et al. (2007) and Scalo et al. (2007). In this review we provide a comprehensive picture of the current knowledge of M-dwarf planet occurrence and habitability based on work done in this area over the past decade, and summarize future directions planned in this quickly evolving field.

Is Proxima Centauri b Habitable? A Study of Atmospheric Loss

“…In contrast, for lower values of the pressure, the magnetized case can potentially sustain an atmosphere over Gyr timescales. It has been widely established that the HZ of M-dwarfs evolves over time (Shields et al 2016). If the stellar wind pressure was higher at earlier epochs, this would have led to increased escape rates.…”

“…Hence, there have been numerous HZ studies of Mdwarf exoplanets (Tarter et al 2007;Scalo et al 2007;Shields et al 2016). This area received a major impetus recently with the potential discovery of a super-Earth (a planet slightly larger than the Earth) orbiting Proxima Centauri (Anglada-Escudé et al 2016), which was christened Proxima Centauri b (PCb).…”

Stellar and Planetary Parameters for K2's Late-type Dwarf Systems from C1 to C5

“…There are still significant discrepancies between theoretical models and observations of M dwarf spectra (e.g., Hoeijmakers et al 2015), and we are still uncertain as to why the occurrence rate of small, short-period planets is higher for M dwarfs and the occurrence rate of gas giants (on both close and wide orbits) is lower for M dwarfs when compared to solar-like stars, as shown in studies of the Kepler field (Dressing & Charbonneau 2013;Gaidos et al 2014;Morton & Swift 2014;Dressing & Charbonneau 2015;Muirhead et al 2015) and other surveys (Shields et al 2016). There are a few exceptions to the low occurrence rate of gas giants around M dwarfs; there has been at least one confirmed gas giant orbiting an M dwarf (Johnson et al 2012).…”

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

“…M dwarfs are also prime targets for current and upcoming exoplanet searches and characterization efforts (see Scalo et al 2007;Shields et al 2016, for comprehensive overviews), due to their ubiquity in the solar neighborhood (Henry et al 2006), high occurrence rates of small exoplanets (Dressing & Charbonneau 2015), and the larger transit and radial velocity signals their planets provide. The important UV region of an M dwarf's SED cannot yet be predicted by models, although semi-empirical modeling efforts are under way for individual stars (see Fontenla et al 2016, and references therein).…”