title: A reappraisal of the habitability of planets around M dwarf stars.
abstract: Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.
authors.affiliation: SETI Institute, Mountain View, California 94043, USA. tarter@seti.org
authors.forename: Jill C
authors.initials: JC
authors.lastname: Tarter
authors.forename: Peter R
authors.initials: PR
authors.lastname: Backus
authors.forename: Rocco L
authors.initials: RL
authors.lastname: Mancinelli
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Aurnou
authors.forename: Dana E
authors.initials: DE
authors.lastname: Backman
authors.forename: Gibor S
authors.initials: GS
authors.lastname: Basri
authors.forename: Alan P
authors.initials: AP
authors.lastname: Boss
authors.forename: Andrew
authors.initials: A
authors.lastname: Clarke
authors.forename: Drake
authors.initials: D
authors.lastname: Deming
authors.forename: Laurance R
authors.initials: LR
authors.lastname: Doyle
authors.forename: Eric D
authors.initials: ED
authors.lastname: Feigelson
authors.forename: Friedmann
authors.initials: F
authors.lastname: Freund
authors.forename: David H
authors.initials: DH
authors.lastname: Grinspoon
authors.forename: Robert M
authors.initials: RM
authors.lastname: Haberle
authors.forename: Steven A
authors.initials: SA
authors.lastname: Hauck
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Heath
authors.forename: Todd J
authors.initials: TJ
authors.lastname: Henry
authors.forename: Jeffery L
authors.initials: JL
authors.lastname: Hollingsworth
authors.forename: Manoj M
authors.initials: MM
authors.lastname: Joshi
authors.forename: Steven
authors.initials: S
authors.lastname: Kilston
authors.forename: Michael C
authors.initials: MC
authors.lastname: Liu
authors.forename: Eric
authors.initials: E
authors.lastname: Meikle
authors.forename: I Neill
authors.initials: IN
authors.lastname: Reid
authors.forename: Lynn J
authors.initials: LJ
authors.lastname: Rothschild
authors.forename: John
authors.initials: J
authors.lastname: Scalo
authors.forename: Antigona
authors.initials: A
authors.lastname: Segura
authors.forename: Carol M
authors.initials: CM
authors.lastname: Tang
authors.forename: James M
authors.initials: JM
authors.lastname: Tiedje
authors.forename: Margaret C
authors.initials: MC
authors.lastname: Turnbull
authors.forename: Lucianne M
authors.initials: LM
authors.lastname: Walkowicz
authors.forename: Arthur L
authors.initials: AL
authors.lastname: Weber
authors.forename: Richard E
authors.initials: RE
authors.lastname: Young
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D017753:Ecosystem; D018559:Exobiology; D005118:Extraterrestrial Environment; D000069497:Origin of Life; D016083:Planets