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...

Planetary targets in the search for extrasolar oxygenic photosynthesis

“…They make up approximately 75% of all stars and they have more stable environments over longer time periods than solar-type stars (Tarter et al 2007). They are, therefore, important targets in the search for extrasolar biosignatures (Scalo et al 2007). M stars, however, present three unusual problems for photosynthesis.…”

The CARMENES search for exoplanets around M dwarfs

“…Spectroscopy of M dwarfs has become a very active research field because potentially habitable planetary companions cause Doppler variations that are more easily detectable around stars of lower mass (Martín et al 2005;Scalo et al 2007;Tarter et al 2007). These stars also constitute the vast majority of potential planet hosts in our immediate vicinity, and a detailed characterization of their planets is believed to be easier than in the Article number, page 1 of 63 arXiv:1711.06576v3 [astro-ph.SR] 9 Feb 2018 A&A proofs: manuscript no.…”

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

“…C1 show the relationships graphically. Next, we examine the mass-luminosity relationships detailed in RH00, Scalo et al (2007) (S07) and B98. For the former, we again fit their data with a third-order polynomial.…”