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 HZ's at two different instants in time. HZ's 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 So, where So is the present solar flux at EarthÕs orbit, 1370 W m^-2 (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 So (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 reason that the HZ is important is two-fold: First, it indicates where inhabited planets might be found. And, second, it provides useful information about abiotic production of O₂. The only net abiotic source for O₂ is photodissociation of H₂O, followed by escape of hydrogen to space. The rate at which this occurs is governed by the mixing ratio of H₂O in the stratosphere, according to the principle of diffusion-limited flux (2,3). For Earth, the stratospheric H₂O mixing ratio is small (~4 ppmv), and the corresponding O₂ production rate is only ~5 x 107 O₂ molecules cm^-2s^-1. This is about 400 times smaller than the rate of O₂ production by photosynthesis followed by organic carbon burial (4) and about 100 times smaller than the rate of O₂ consumption by reaction with reduced volcanic gases (5). Thus, Earth's atmosphere would be virtually anoxic in the absence of life (3,6). A planet near or inside the inner edge of the HZ, however, could have a much higher stratospheric H₂O mixing ratio and a correspondingly larger abiotic O₂ source (1). Venus, for example, could have accumulated tens or even hundreds of bars of O₂ during the time that it lost its water (7). Thus, the identification of O₂ in a Venus-like planet's atmosphere would not necessarily indicate that life was present.

A second type of planet that could conceivably build up a high abiotic O₂ abundance would be one that was slightly larger than Mars, located beyond the outer edge of the HZ. The planet would need to be larger than Mars because Mars loses oxygen by dissociative recombination of O₂⁺ (8). So, Mars loses water to space rather than just hydrogen. Raising Mars' mass from 0.1 MÅ to 0.17 MÅ would be enough to shut off the loss of oxygen and allow O₂ to accumulate. But the planet could not be as large as Earth, because an Earth-sized planet would presumably emit reduced volcanic gases which would consume atmospheric O₂ as rapidly as it was produced (3,6). The planet would also need to be cold, like Mars, because a planet with liquid water on its surface would lose oxygen by rainout of oxidized gases, e.g. H₂SO₄, H₂O₂, followed by reaction of these species with reduced minerals in rocks (9, 10). Further discussion of this issue can be found in refs. 11 and 12.

The practical way to search for life on extrasolar planets is to look for the 9.6-µm band of O₃ (13), which could be detected in nearby planetary systems with a space-based, infrared interferometer (14). O₃ is a sensitive indicator of atmospheric O₂. The relationship between O₂ and O₃ concentrations in Earth's atmosphere has been studied by numerous investigators (e.g., 15- 17) and is reasonably well understood. Further work needs to be done to estimate the strength of the 9.6-µm band as a function of atmospheric O₂ level. Observations could presumably tell us where an extrasolar planet was located with respect to its primary's HZ, so we could determine whether the planet was Venus-like. Observations of other planets in the same system and their mutual interactions might allow us to derive planetary masses, so we could determine whether the planet in question might be Mars-like. If we could exclude both these possibilities, we could conclude that the planet was Earth-like and that the presence of substantial O₃ in its atmosphere was a strong indication of life.

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

2. D. M. Hunten, J. Atmos. Sci. 30, 1481-1494 (1973).

3. J. C. G. Walker, Evolution of the Atmosphere (Macmillan, New York, 1977).

4. H. D. Holland, The Chemistry of the Atmosphere and Oceans (Wiley, New York, 1978).

5. H. D. Holland, The Chemical Evolution of the Atmosphere and Oceans (Princeton University Press, Princeton, 1984).

6. J. F. Kasting, Science 259, 920-926 (1993).

7. J. F. Kasting and J. B. Pollack, Icarus 53, 479-508 (1983).

8. M. B. McElroy, Science 175, 443-445 (1972).

9. J. F. Kasting, Origins of Life 20, 199-231 (1990).

10. Kasting, J. F., in The Chemistry of Life's Origins, J. M. Greenberg, C. X. Mendoza-Gomez and V. Pirronello, eds. (Kluwer Academic Publishers, Dordrecht, 1993), pp. 149-176.

11. J. Rosenqvist and E. Chassefiere, Planet. Space Sci. 43, 3-10 (1995).

12. Kasting, J. F., Planet. Space Sci. 43, 11-13 (1995).

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

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

15. J. S. Levine, P. B. Hays and J. C. G. Walker, Icarus 39, 295-309 (1979).

16. J. F. Kasting and T. M. Donahue, J. Geophys. Res. 85, 3255-3263 (1980).

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