The Habitability of Planets Orbiting M-dwarf Stars (1610.05765v3)

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 Sun-like 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.

• The paper presents a comprehensive review of habitable conditions on M-dwarf planets using observational data from missions like Kepler.
• It examines how stellar flares and atmospheric interactions influence surface climates and the retention of potential biosignatures.
• The study highlights tidal locking effects on planetary environments and outlines future exploration directions with missions such as TESS and JWST.

The Habitability of Planets Orbiting M-dwarf Stars

This paper presents a comprehensive review of the habitability prospects for planets orbiting M-dwarf stars. The field of exoplanetary studies has seen a significant shift, focusing on Earth-sized planets orbiting these low-mass stars due to their prevalence and proximity, which present advantageous conditions for detectability and characterization.

Observational Overview and Insights

Recent advancements, particularly from NASA's Kepler mission, have dramatically increased our understanding of the demographics and characteristics of planets orbiting M-dwarfs. Most notably, small rocky planets are frequently found within the habitable zones of these stars, with approximately a third of these planets potentially able to support liquid water. Contrary to systems orbiting Sun-like stars, M-dwarf systems often lack Jupiter-sized planets but are rich in smaller, terrestrial-type planets.

Stellar and Planetary Environments

M-dwarfs, which constitute a significant portion of the galaxy's stellar population, offer prolonged stellar lifetimes due to their slow nuclear fuel consumption. This longevity theoretically supports prolonged periods of planetary and biological evolution. However, the proximity of habitable zones to M-dwarfs raises concerns due to intense stellar activity, such as flares that emit significant XUV radiation, which could strip atmospheres of their planets over extended timeframes, especially during the prolonged pre-main sequence phase.

Atmospheric and Surface Interactions

The paper discusses the role of atmospheric compositions and how spectral energy from M-dwarfs interacts with these planetary atmospheres. For example, planets orbiting M-dwarfs have atmospheres that absorb more strongly in the near-infrared, potentially leading to warmer climates in comparison to planets orbiting stars with spectra peaking at shorter wavelengths. Consequently, these planets might possess more stable climates, resisting global glaciation events compared to their counterparts orbiting Sun-like stars, through diminished ice-albedo feedback due to reduced reflectivity of ice at longer wavelengths emitted from M-dwarfs.

Tidal Interaction and Rotation

Tidal forces resulting from the close-orbiting nature of planets in the habitable zones of M-dwarfs often lead to synchronous rotation states, where a planet's rotational period equals its orbital period. This synchronous rotation raises concerns about atmospheric vulnerability and potential runaway greenhouse conditions on the day side, although current models suggest the presence of sufficient atmospheres could modulate temperatures and prevent atmospheric collapse.

Future Directions and Implications

The paper indicates that future observational missions, such as TESS, CHEOPS, PLATO, and subsequent analyses with instruments like the James Webb Space Telescope, will be critical in further characterizing these worlds. It emphasizes that M-dwarfs might indeed be primary contenders in the search for life due to the abundance and proximity of their planets, despite the challenging environments.

In conclusion, the research landscape regarding the habitability of M-dwarf planets is rapidly evolving. While concerns prioritize strong stellar activity and atmospheric retention capabilities, advancements in observation and modeling offer encouraging avenues suggesting these worlds should not be discounted in the quest to find habitable environments beyond Earth. The interplay between stellar radiation, planetary atmospheres, and surface conditions will guide the prioritization of targets for future explorations aimed at detecting biosignatures of life.