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.

High-dispersion yellow-red spectra of eight representative N-type carbon stars have been matched by Minneart model synthesis. CN, C 2 , and Fe line strengths imply high abundances of free carbon, C-O, such that N-star surface carbon must originate from 3-a Heshell burning, rather than from CNO equilibrium processes. CN/C 2 ratios ranging from 4 to 200 imply values of log(C -0)/H from -2.7 to -4.5; these lead to C/O ratios from 2.9 to 1.033, if the oxygen abundances are solar. Nitrogen abundances for these stars are also essentially solar, with a mean value log (N/H) = -4.10 ± 0.18, also ruling out substantial contributions from CNO processing.Values of the 12 C/ 13 C ratio range from 3.5 to > 25. Zr/Ti ratios are a factor of 2 to 10 greater than solar, similar to the S stars. Ba is enriched only in some high Zr stars. Rareearth abundances are subject to severe error. Lithium abundances range from the meteoritic to below solar. The range of logg is -0.3 to 1.7. Excitation temperatures, both atomic and and molecular, are between 2500° K and 3150° K. Microturbulent velocities are in most cases near 6 km sec -1 . Relations to other red-giant stars are considered.

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Recent advances in deformable mirror technology for correcting wavefront errors and in pupil shapes and masks for coronagraphic suppression of diffracted starlight enable a powerful approach to detecting extrasolar planets in reflected (scattered) starlight at visible wavelengths. We discuss the planet-finding performance of Hubble-like telescopes using these technical advances. A telescope of aperture of at least 4 meters could accomplish the goals of the Terrestrial Planet Finder (TPF) mission. The '4mTPF' detects an Earth around a Sun at five parsecs in about one hour of integration time. It finds molecular oxygen, ozone, water vapor, the 'red edge' of chlorophyll-containing land-plant leaves, and the total atmospheric column density-all in forty hours or less. The 4mTPF has a strong science program of discovery and characterization of extrasolar planets and planetary systems, including other worlds like Earth. With other astronomical instruments sharing the focal plane, the 4mTPF could also continue and expand the general program of astronomical research of the Hubble Space Telescope.