The habitable zone (HZ) around a star is defined as the region in which an Earth-like planet could support liquid water. The continuously habitable zone (CHZ) represents the overlap of the HZs at two different instants in time. HZs move outward with time because main sequence stars get brighter as they age. The inner edge of the HZ is set by loss of water by way of photodissociation followed by escape of hydrogen to space. A conservative (i.e., pessimistic) estimate for the solar flux at which this phenomenon occurs is 1.1 S₀, where S₀ is the present solar flux at Earth's orbit, 1370 W/m² (1). The outer edge of the HZ is set by CO₂ condensation, which shuts off the stabilizing feedback provided by the carbonate-silicate cycle. Within the HZ, atmospheric CO2 concentrations should increase with orbital distance as a consequence of this cycle. A conservative estimate for the solar flux at the outer edge of the HZ is 0.53 S₀ (1). In terms of distance, the HZ for our own Solar System extends from at least 0.95 AU to 1.37 AU, and the 4.6-Gyr CHZ extends from at least 0.95 AU to 1.15 AU. Corresponding fluxes and distances for other types of stars are tabulated in ref. (1).

The number of planets expected to lie within the HZ or CHZ around a given star depends on how planets are spaced and on how rapidly the star evolves. If planets are spaced geometrically, as they are in our own Solar System (i.e., according to Bode's Law), then roughly equal numbers of instantaneously habitable planets are expected around all types of stars. Early-type stars (type O, B, and A) will have fewer planets that remain continuously habitable because they evolve in luminosity much more rapidly than does the Sun. Late-type stars (late K and M) may have few continuously habitable planets because their HZs lie within the tidal locking radius of the star. Any potentially habitable planets are likely to become locked in synchronous rotation, allowing their atmospheres to condense out on their dark sides. Stars not too different from the Sun (early F to mid K) have about a 50% chance of harboring a long-lived habitable planet if Bode's Law spacing is obeyed (1).

A glaring deficiency in the climate model on which these predictions are based is that it fails to account for the apparently warm climate of early Mars (2,3). Either greenhouse gases other than CO₂ and H₂O were important, or some other aspect of the Martian climate system has been poorly represented. New ideas about how to solve this problem will be discussed if circumstances permit. In any case, the existence of a warm climate on early Mars implies that HZ around a star is probably wider than has been calculated to date.

The exciting development in this field is that it now appears feasible to look for Earth-sized planets around other stars using a space-based, infrared interferometer and to examine their atmospheres spectroscopically (4). The practical way to search for life on extrasolar planets is to look for the 9.6-µm band of O₃ (5). O₃ is a sensitive indicator of atmospheric O₂, and O₂ is, under most circumstances, a strong indicator of photosynthetic life. Exceptions to this rule include planets on either side of the HZ, as such planets can conceivably accumulate large amounts of O₂ abiotically (7). A planet with O₃ in its atmosphere and liquid water on its surface, however, is very likely to be inhabited. The fact that we are on the verge of being able to identify such planets suggests that building such an instrument should be a top NASA priority.

1. J. F. Kasting, D. P. Whitmire, and R. T. Reynolds, Icarus 101:108-128 (1993).

2. J. F. Kasting, Icarus 94:1-13 (1991).

3. S. W. Squyres and J. F. Kasting, Science 265:744-749 (1994).

4. J. R. P. Angel and N. J. Woolf, Scientific American, April, 60-66 (1996).

5. A. Leger, M. Pirre, and F. J. Marceau, Astron. Astrophys. 277:309-313 (1993).

6. J. F. Kasting, H. D. Holland, and J. P. Pinto, J. Geophys. Res. 90:10,497-10,510 (1985).

7. J. F. Kasting, in Proceedings of NASA Ames Bluedot Workshop, D. Des Marais, ed., June, 1996.

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