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M stars as targets for terrestrial exoplanet searches and biosignature detection

Scalo, John and Kaltenegger, Lisa and Segura, Antigona and Fridlund, Malcolm and Ribas, Ignasi and Kulikov, Yu. N. and Grenfell, John Lee and Rauer, Heike and Odert, Petra and Leitzinger, Martin and Selsis, Frank and Khodachenko, Maxim L. and Eiora, Carlos and Kasting, Jim and Lammer, Helmut (2007) M stars as targets for terrestrial exoplanet searches and biosignature detection. Astrobiology, 7 (1), pp. 85-166. Mary Ann Liebert Inc.. doi: 10.1089/ast.2006.0125. ISSN 1531-1074.

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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 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 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 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 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 alreadyin place for direct imaging terrestrial exoplanet missions.

Item URL in elib:https://elib.dlr.de/43771/
Document Type:Article
Title:M stars as targets for terrestrial exoplanet searches and biosignature detection
AuthorsInstitution or Email of AuthorsAuthor's ORCID iDORCID Put Code
Scalo, JohnDepartment of Astronomy,University of Texas, AustinUNSPECIFIEDUNSPECIFIED
Kaltenegger, LisaHarvard-Smithsonian Center for Astrophysics, CambridgeUNSPECIFIEDUNSPECIFIED
Segura, AntigonaCalifornia Institute of Technology / Jet Propulsion Laboratory,PasadenaUNSPECIFIEDUNSPECIFIED
Fridlund, MalcolmEuropean Space Agency (ESA),ESTEC, NoordwijkUNSPECIFIEDUNSPECIFIED
Ribas, IgnasiInstitut d'Estudis Espacials de Catalunya and Instituto de Ciencias del Espacio, BarceloneUNSPECIFIEDUNSPECIFIED
Kulikov, Yu. N.Polar Geophysical Institute (PGI), Russian Academy of Sciences, MurmanskUNSPECIFIEDUNSPECIFIED
Odert, PetraInstitute for Physics, University of Graz, GrazUNSPECIFIEDUNSPECIFIED
Leitzinger, MartinInstitute for Physics, University of Graz, GrazUNSPECIFIEDUNSPECIFIED
Selsis, FrankCentre de Recherche en Astrophysique de Lyon, and Ecole Normale Superieure, LyonUNSPECIFIEDUNSPECIFIED
Khodachenko, Maxim L.Space Research Institute (IWF), Austrian Academy of Sciences, GrazUNSPECIFIEDUNSPECIFIED
Eiora, CarlosDpto. Fisica Teorica, C-XI, Facultad de Ciencas, Universidad Autonoma de Madrid, MadridUNSPECIFIEDUNSPECIFIED
Kasting, JimDepartment of Geosciences, Pennsylvania State University, State College, PennsylvaniaUNSPECIFIEDUNSPECIFIED
Lammer, HelmutSpace Research Institute (IWF), Austrian Academy of Sciences, GrazUNSPECIFIEDUNSPECIFIED
Journal or Publication Title:Astrobiology
Open Access:No
Gold Open Access:No
In ISI Web of Science:Yes
Page Range:pp. 85-166
Publisher:Mary Ann Liebert Inc.
Keywords:M stars, terrestrial exoplanets, biosignature, astrobiology
HGF - Research field:Aeronautics, Space and Transport (old)
HGF - Program:Space (old)
HGF - Program Themes:W EW - Erforschung des Weltraums
DLR - Research area:Space
DLR - Program:W EW - Erforschung des Weltraums
DLR - Research theme (Project):W - Vorhaben Extrasolare Planeten (old)
Location: Berlin-Adlershof
Institutes and Institutions:Institute of Planetary Research > Extrasolar Planets and Atmospheres
Deposited By: Hedelt, Pascal
Deposited On:09 Jul 2007
Last Modified:06 Sep 2019 15:23

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