CIRCUMSTELLAR HABIT ABLE ZONES:

ASTRONOMICAL CON SID ERA TIO NS

Daniel P. Whitmire University of Southwestern Louisiana

Lafayette, LA 70504-4210

Ray T. Reynolds Theoretical Studies

NASA Ames Research Center Moffett Field, CA 94035

Abstract

From an astronomical perspective, stellar luminosity and its evolu­tion in time are the dominant factors in determining the location, size and evolution of circumstellar habitable zones (HZ). Other stellar properties that can affect HZs are stellar temperature, mass, mass loss, spot coverage and multiplicity. Whether planets are likely to form within a star's HZ and under what conditions their orbits would be dynamically stable for significant periods of time are separate important questions. In this paper we review these topics and suggest areas for future work in light of the recent proposal (Goldin 1994) that NASA adopt as a new unifying focus the search for a habitable planet around a nearby star.

INTRODUCTION

Following Kasting, Whitmire and Reynolds (1993; here­after KWR) and others, we define the habitable zone as the radial shell around a star within which an Earthlike planet could support surface liquid water. For our purposes an Earthlike planet is one similar in mass and composition to Earth and having comparable surface inventories of CO2, H20, and Nz. The upper mass limit is

1.1, 0.53

1.4, 0.36

1.76, 0.32