CIRCUMSTELLAR HABITABLE ZONES - AN OVERVIEW

We review aspects of circumstellar habitable zones based on results re ported at the First International Conference on Circumstellar Habitabl e Zones (held in 1994 at NASA Ames Research Center). Recent advances i n atmospheric radiative transfer modeling have shown that circumstella r habitable zones are wider than previously thought. New consideration s may allow a much shorter time scale both for the origin as well as t he evolution of biological forms. The most abundant M dwarf stars, con trary to previous views, appear to be able to support the necessary co nditions for a habitable zone around them. New planet formation models indicate that at least one planet should form within the circumstella r habitable zone of single dwarf stars regardless of mass. Biogenic ma terials also appear to be widespread and delivery to terrestrial plane ts via cometary impacts may be a viable mechanism. Finally, biology wi ll modify a planet and provide positive feedback, in general, to incre ase the habitability of a planet. Overall, new astronomical, planetary , and biological considerations each seem to indicate that habitable z ones around other stars may be both more widespread and more stable th an previous research had indicated. (C) 1998 Elsevier Science Ltd. All rights reserved.