M Stars as Targets for Terrestrial Exoplanet Searches And Biosignature Detection

Abstract

The changing view of planets orbiting low mass stars, M stars, as potentially hospitable worlds for life and its remote detection was motivated by several factors, including the demonstration of viable atmospheres and oceans on tidally locked planets, normal incidence of dust disks, including debris disks, detection of planets with masses in the 5-20 M() range, and predictions of unusually strong spectral biosignatures. We present a critical discussion of M star properties that are relevant for the long- and short-term thermal, dynamical, geological, and environmental stability of conventional liquid water habitable zone (HZ) M star planets, and the advantages and disadvantages of M stars as targets in searches for terrestrial HZ planets using various detection techniques. Biological viability seems supported by unmatched very long-term stability conferred by tidal locking, small HZ size, an apparent short-fall of gas giant planet perturbers, immunity to large astrosphere compressions, and several other factors, assuming incidence and evolutionary rate of life benefit from lack of variability. Tectonic regulation of climate and dynamo generation of a protective magnetic field, especially for a planet in synchronous rotation, are important unresolved questions that must await improved geodynamic models, though they both probably impose constraints on the planet mass. M star HZ terrestrial planets must survive a number of early trials in order to enjoy their many Gyr of stability. Their formation may be jeopardized by an insufficient initial disk supply of solids, resulting in the formation of objects too small and/or dry for habitability. The small empirical gas giant fraction for M stars reduces the risk of formation suppression or orbit disruption from either migrating or nonmigrating giant planets, but effects of perturbations from lower mass planets in these systems are uncertain. During the first approximately 1 Gyr, atmospheric retention is at peril because of intense and frequent stellar flares and sporadic energetic particle events, and impact erosion, both enhanced, the former dramatically, for M star HZ semimajor axes. Loss of atmosphere by interactions with energetic particles is likely unless the planetary magnetic moment is sufficiently large. For the smallest stellar masses a period of high planetary surface temperature, while the parent star approaches the main sequence, must be endured. The formation and retention of a thick atmosphere and a strong magnetic field as buffers for a sufficiently massive planet emerge as prerequisites for an M star planet to enter a long period of stability with its habitability intact. However, the star will then be subjected to short-term fluctuations with consequences including frequent unpredictable variation in atmospheric chemistry and surficial radiation field. After a review of evidence concerning disks and planets associated with M stars, we evaluate M stars as targets for future HZ planet search programs. Strong advantages of M stars for most approaches to HZ...

The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up

“…Some previous workers have considered possible positive roles for UV radiation for early life. Scalo et al (2007) hypothesize that highly variable M-dwarf UV emission could drive variations in mutation rates on orbiting planets, which might enhance the rate of evolution. Buccino et al (2007) argue based on the "Principle of Mediocrity" 11 that habitable planets should receive stellar irradiance similar to Archaean Earth in order to power potential prebiotic chemistry, and use it to suggest that life cannot arise on planets orbiting inactive, low-UV Mdwarfs because in order to receive Earthlike UV instellation, planets will need to orbit within the inner edge of the habitable zone.…”

“…Extensive work has been done on M-dwarf planet habitability, i.e., whether life as we know it could endure on these worlds; see, e.g., Tarter et al (2007), Scalo et al (2007), and Shields et al (2016) for reviews of work on this topic. However, far fewer investigations have been conducted as to the favorability of M-dwarf planets for abiogenesis (the origin of life), i.e., whether life as we know it could emerge on these worlds.…”

On the Role of Dissolved Gases in the Atmosphere Retention of Low-mass Low-density Planets

“…However, it is important to model the F XUV flux before 100 Myr as mass loss during nascent stage of the planet are especially important. We do so by considering a saturated XUV flux from the star for the first 100 Myr fixed at the value of F XUV obtained from the above equation at t = 0.1 Gyr (Scalo et al 2007). The efficiency of conversion of XUV flux energy to usable work, ò, contains all of the atmospheric and atomic physical processes that occur in the radiative region.…”

Magnetism, dynamo action and the solar-stellar connection

“…From an astronomical perspective, these stars are interesting partly because they are so common: in their spatial distribution we can discern the influence of dynamical heating in the Galactic disk (e.g., West et al 2006); in their signatures in the integrated light of other galaxies, some authors have suggested evidence of variations in the initial mass function (van Dokkum and Conroy 2010). These stars have also become major targets in the search for "Earth-like" exoplanets (see, e.g., Tarter et al 2007;Scalo et al 2007;Berta et al 2012). With this has come an appreciation of the magnetic activity in such stars: because, for example, the "habitable zone" in these stars is likely to be comparatively close in (e.g., Pierrehumbert 2010;Haswell 2010), it is conceivable that magnetic activity in these stars would exert an especially great influence on the environment of any orbiting planets (e.g., Walkowicz et al 2008).…”