A reappraisal of the habitability of planets around M dwarf stars (original) (raw)
Recently, an exoplanetary system containing seven Earth-sized terrestrial planets, three of which orbit the central M-dwarf star in its habitable zone, was discovered. Such systems may be common, as this type of star makes up almost 80% of all stars. Based on a critical literature review, this paper defines the criteria for exoplanet habitability and adopts the habitable zone criterion as the most basic one for use in absence of data and the novel Statistical- likelihood Exo-Planetary Habitability Index as the presently most suited for assessing M-dwarf exoplanets. It further describes the feasible methods employed for the detection of this type of exoplanets (the transit and radial velocity method), provides an account of the environmental condition constraints imposed by their close orbit around an M- dwarf star, and evaluates these conditions against those deemed suitable for the development and survival of life. The details of the known and suspected population of exoplanets in the habitable zone of M-dwarf stars are then compared to the adopted habitability criteria, and it is concluded that the frequency with which any such planets can indeed be said to be habitable cannot currently be estimated due to the small sample size. The prospects of further characterising this subset of exoplanets in the future, identifying them as habitable and possibly detecting any bio-signatures are subsequently discussed. In order to accelerate the quest for habitable and inhabited exoplanets, aggressive, focused searches and targeted studies directed at planets in the habitable zone of M-dwarf stars are proposed.
M dwarfs: planet formation and long term evolution
Astronomische Nachrichten, 2005
The first part of this paper discusses how planet formation proceeds in the disks orbiting M dwarf stars. These environments are different from those associated with solar-type stars in several ways: The planet forming clock (set by orbits) runs slower, the disks are more prone to evaporation, the supply of raw material is lower, the snowline is closer in, and planetary systems are more easily disrupted. Because of these considerations, red dwarfs are less likely to harbor giant planets, but can readily produce smaller planets. The second part of this paper describes stellar evolution calculations for M dwarfs, which live far longer than the current age of the universe. These diminutive stellar objects remain convective over most of their lives, continue to burn hydrogen for trillions of years, and do not experience red giant phases in their old age. Instead, red dwarfs turn into blue dwarfs and finally white dwarfs. This work also shows (in part) why larger stars become red giants.
Habitability of planets around red dwarf stars
Origins of life and evolution of the biosphere : the journal of the International Society for the Study of the Origin of Life, 1999
Recent models indicate that relatively moderate climates could exist on Earth-sized planets in synchronous rotation around red dwarf stars. Investigation of the global water cycle, availability of photosynthetically active radiation in red dwarf sunlight, and the biological implications of stellar flares, which can be frequent for red dwarfs, suggests that higher plant habitability of red dwarf planets may be possible.
Terrestrial planets and water delivery around low-mass stars
Astronomy & Astrophysics, 2016
Context. Theoretical and observational studies suggest that protoplanetary disks with a wide range of masses could be found around low-mass stars. Aims. We analyze planetary formation processes in systems without gas giants around M3-and M0-type stars of 0.29 M and 0.5 M , respectively. In particular, we assume disks with masses of 5% and 10% of the mass of the star. Our study focuses on the formation of terrestrial-like planets and water delivery in the habitable zone (HZ). Methods. First, we use a semi-analytical model to describe the evolution of embryos and planetesimals during the gaseous phase. Then, a N-body code is used to analyze the last giant impact phase after the gas dissipation. Results. For M3-type stars, five planets with different properties are formed in the HZ. These planets have masses of 0.072 M ⊕ , ∼0.13 M ⊕ (two of them), and 1.03 M ⊕ , and have water contents of 5.9%, 16.7%, 28.6%, and 60.6% by mass, respectively. Then, the fifth planet formed in the HZ is a dry world with 0.138 M ⊕. For M0-type stars, four planets are produced in the HZ with masses of 0.28 M ⊕ , 0.51 M ⊕ , 0.72 M ⊕ , and 1.42 M ⊕ , and they have water contents of 26.7%, 45.8%, 68%, and 50.5% by mass, respectively. Conclusions. M3-and M0-type stars represent targets of interest for the search of exoplanets in the HZ. In fact, the Mars-mass planets formed around M3-type stars could maintain habitable conditions in their early histories. Thus, the search for candidates around young M3-type stars could lead to the detection of planets analogous to early Mars. Moreover, Earth-mass planets should also be discovered around M3-type stars and, sub-and super-Earths should be detected around M0-type stars. Such planets are very interesting since they could maintain habitable conditions for very long.
International Journal of Astrobiology, 2016
We review the latest findings on extra-solar planets and their potential of having environmental conditions that could support Earth-like life. Focusing on planets orbiting red dwarf (RD) stars, the most abundant stellar type in the Milky Way, we show that including RDs as potential life supporting host stars could increase the probability of finding biotic planets by a factor of up to a thousand, and reduce the estimate of the distance to our nearest biotic neighbour by up to 10. We argue that binary and multiple star systems need to be taken into account when discussing habitability and the abundance of biotic exoplanets, in particular RDs in such systems. Early considerations indicated that conditions on RD planets would be inimical to life, as their habitable zones would be so close to the host star as to make planets tidally locked. This was thought to cause an erratic climate and expose life forms to flares of ionizing radiation. Recent calculations show that these negative fa...
Because of their large numbers, low mass stars may be the most abundant planet hosts in our Galaxy. Furthermore, terrestrial planets in the habitable zones (HZs) around M-dwarfs can potentially be characterized in the near future and hence may be the first such planets to be studied. Recently Dressing & Charbonneau(2013) used Kepler data and calculated the frequency of terrestrial planets in the HZ of cool stars to be 0.15^{+0.13}_{-0.06} per star for Earth-size planets (0.5-1.4 R_{Earth}). However, this estimate was derived using the Kasting et al.(1993) HZ limits, which were not valid for stars with effective temperatures lower than 3700 K. Here we update their result using new HZ limits from Kopparapu et al.(2013) for stars with effective temperatures between 2600 K and 7200 K, which includes the cool M stars in the Kepler target list. The new habitable zone boundaries increase the number of planet candidates in the habitable zone. Assuming Earth-size planets as 0.5 - 1.4 R_{Earth}, when we reanalyze their results, we obtain a terrestrial planet frequency of 0.48^{+0.12}_{-0.24} and 0.53^{+0.08}_{-0.17} planets per M-dwarf star for conservative and optimistic limits of the HZ boundaries, respectively. Assuming Earth-size planets as 0.5 - 2 R_{Earth}, the frequency increases to 0.51^{+0.10}_{-0.20} per star for the conservative estimate and to 0.61^{+0.07}_{-0.15} per star for the optimistic estimate. Within uncertainties, our optimistic estimates are in agreement with a similar optimistic estimate from the radial velocity survey of M-dwarfs (0.41^{+0.54}_{-0.13}, Bonfils et al.(2011)). So, the potential for finding Earth-like planets around M stars may be higher than previously reported.
The bio-habitable zone and atmospheric properties for planets of red dwarfs
International Journal of Astrobiology, 2019
The Kepler data show that habitable small planets orbiting Red Dwarf stars (RDs) are abundant, and hence might be promising targets to look at for biomarkers and life. Planets orbiting within the Habitable Zone of RDs are close enough to be tidally locked. Some recent works have cast doubt on the ability of planets orbiting RDs to support life.
M Stars as Targets for Terrestrial Exoplanet Searches And Biosignature Detection
Astrobiology, 2007
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 ᮍ 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 shortfall 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 hab-85 itability. 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 ϳ1 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 detection are offset by their faintness, leading to severe constraints due to accessible sample size, stellar crowding (transits), or angular size of the HZ (direct imaging). Gravitational lensing is unlikely to detect HZ M star planets because the HZ size decreases with mass faster than the Einstein ring size to which the method is sensitive. M star Earth-twin planets are predicted to exhibit surprisingly strong bands of nitrous oxide, methyl chloride, and methane, and work on signatures for other climate categories is summarized. The rest of the paper is devoted to an examination of evidence and implications of the unusual radiation and particle environments for atmospheric chemistry and surface radiation doses, and is summarized in the Synopsis. We conclude that attempts at remote sensing of biosignatures and nonbiological markers from M star planets are important, not as tests of any quantitative theories or rational arguments, but instead because they offer an inspection of the residues from a Gyr-long biochemistry experiment in the presence of extreme environmental fluctuations. A detection or repeated nondetections could provide a unique opportunity to partially answer a fundamental and recurrent question about the relation between stability and complexity, one that is not addressed by remote detection from a planet orbiting a solar-like star, and can only be studied on Earth using restricted microbial systems in serial evolution experiments or in artificial life simulations. This proposal requires a planet that has retained its atmosphere and a water supply. The discussion given here suggests that observations of M star exoplanets can decide this latter question with only slight modifications to plans already in place for direct imaging terrestrial exoplanet missions. Key Words: M star planets-Habitable planets-Life and stellar activity-Spectral biosignatures-Terrestrial planet formation-Exoplanet properties. Astrobiology 7(1), 85-166. SCALO ET AL. 86 1.8. HZs for M star planets 97 A. Specificity of water 97 B. Biomolecular recognition and water habitability 98 C. Varieties of habitability 99 i. Liquid water thermal HZ 99 ii. Cosmic ray environment 101 iii. Geologically sustainable habitability 103 iv. Dynamical habitability 104 a. Known exoplanet systems 104 b. Formation and stability: nonmigrating giants 105 c. Terrestrial planet formation in the presence of migrating giants 107 and large eccentricity 1.9. Catalogues of M star target properties 111 2. M star habitable planet and planet search programs 113 2.1. Trends of protostellar disk mass with stellar mass 114 2.2. Implications of existing exoplanet results for M star planets 118 i. Frequency of giant planets in M star systems 120 2.3. The new face of M star planets: five low mass planets 121 2.4. Future detection of HZ terrestrial planets 122 i. Radial velocity detection of M star HZ planets 123 ii. Astrometric detection of M star HZ planets 123 iii. Transit detection of M star terrestrial exoplanets 124 iv. Direct imaging of M star terrestrial HZ planets 126 a. Spectroscopic signatures from direct imaging 127 b. Photometric biomarkings from direct imaging 129 v. Additional possibilities 129 3. Flares, CMEs, X-rays, and EUV and UV activity of M stars 130 3.1. Flares 130 3.2. Age-activity relation and X-ray luminosity 133 3.3. Expected CMEs 137 3.4. Star spot-type phenomena 137 3.5. UV spectra 138 4. Activity effects on atmospheres of Earth-like exoplanets within M star HZs 139 4.1. UV influence on photochemistry and surface biota of Earth-like exoplanets 139 within M star HZs 4.2. Effect of energetic particles on M star planet biomarkers 141 4.3. Thermospheric heating of M star HZ planets due to X-ray and EUV fluxes 143 4.4. Life in a fluctuating environment 145 5. Conclusion 148 6. Synopsis 149 7. Acknowledgments 152 8. Abbreviations 153 9. References 153 M STAR RELEVANCE IN TERRESTRIAL PLANET SEARCH 87
Biosignatures from Earth-Like Planets Around M Dwarfs
Astrobiology, 2005
Coupled one-dimensional photochemical-climate calculations have been performed for hypothetical Earth-like planets around M dwarfs. Visible, near-infrared and thermal-infrared synthetic spectra of these planets were generated to determine which biosignature gases might be observed by a future, space-based telescope. Our star sample included two observed active M dwarfs, AD Leo and GJ 643, and three quiescent model stars. The spectral distribution of these stars in the ultraviolet generates a different photochemistry on these planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially longer lifetimes and higher mixing ratios than on Earth, making them potentially observable by space-based telescopes. On the active M-star planets, an ozone layer similar to Earth's was developed that resulted in a spectroscopic signature comparable to the terrestrial one. The simultaneous detection of O2 (or O3) and a reduced gas in a planet's atmosphere has been suggested as strong evidence for life. Planets circling M stars may be good locations to search for such evidence.
Astronomy & Astrophysics, 2020
Context. Planets orbiting low-mass stars such as M dwarfs are now considered a cornerstone in the search for planets with the potential to harbour life. GJ 273 is a planetary system orbiting an M dwarf only 3.75 pc away, which is composed of two confirmed planets, GJ 273b and GJ 273c, and two promising candidates, GJ 273d and GJ 273e. Planet GJ 273b resides in the habitable zone. Currently, due to a lack of observed planetary transits, only the minimum masses of the planets are known: Mb sin ib = 2.89 M⊕, Mc sin ic = 1.18 M⊕, Md sin id = 10.80 M⊕, and Me sin ie = 9.30 M⊕. Despite its interesting character, the GJ 273 planetary system has been poorly studied thus far. Aims. We aim to precisely determine the physical parameters of the individual planets, in particular, to break the mass–inclination degeneracy to accurately determine the mass of the planets. Moreover, we present a thorough characterisation of planet GJ 273b in terms of its potential habitability. Methods. First, we exp...