HABITABLE ZONES ABOUT MAIN-SEQUENCE STARS

Calculations show that a main sequence star which is less massive than the Sun has a continuously habitable zone about it which is not only closer in than the corresponding zone about the Sun, but is also relatively narrower. Let L(t) represent the luminosity after t billion years of a main sequence star of mass M, and let rinner and router represent the boundaries of the continuously habitable zone about such a star-that is, the zone in which an Earthlike planet will undergo neither a runaway greenhouse effect in the early stages of its history nor runaway glaciation after it develops an oxidizing atmosphere. Then our computer results indicate that router rinner is roughly proportional to [ L(3.5) L(1.0)] 1 2. This ratio is smaller for stars less massive than the Sun (because they evolve more slowly), and the width of the continuously habitable zone about a main sequence star is therefore a strong function of the initial stellar mass. Our calculations show that rinner = router for M∼0.83M{circled ring operator} (i.e., K1 stars), and it therefore appears that there is no continuously habitable zone about most K stars, nor any about M stars. © 1979.