Mathematics – Logic
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...193.5003k&link_type=abstract
American Astronomical Society, 193rd AAS Meeting, #50.03; Bulletin of the American Astronomical Society, Vol. 30, p.1328
Mathematics
Logic
2
Scientific paper
The recent discovery of extrasolar giant planets has led to new interest in the possibility that life might exist on Earth-like planets around other stars. Although the existence of such planets has not yet been demonstrated, there are good reasons to believe that they are relatively commonplace. Planetary formation is thought to be a natural companion to star formation, at least in the case of single stars. The habitable zone (HZ) - the region where liquid water can exist on a planet's surface - is also thought to be relatively wide. Calculations by Kasting et al. (Icarus 101, 108, 1993) placed a lower limit of 0.4 AU for the width of the current HZ around the Sun. This estimate presumed that the outer edge of the HZ was set by the formation of CO2 clouds. Recent calculations by Forget and Pierrehumbert (Science 278, 1273, 1997) suggest that this assumption is too conservative because CO2 clouds should tend to warm a planet's surface. Hence, the HZ is probably wider than previously thought, and the probability of finding a planet within it in a given planetary system is therefore quite high, perhaps approaching unity. These ideas about the existence of habitable planets are, for the moment, just speculation. Within the next 20 years, however, it may be able to test them by observing extrasolar planets with NASA's proposed Terrestrial Planet Finder (TPF) mission. In theory, TPF will not only find Earth-sized planets, but it will also perform thermal-IR spectroscopy on their atmospheres. A strong 9.6micron ozone band would, in most cases, be a good indicator for life (because O3 is produced photochemically from O2, and O2 is produced mainly by photosynthesis). Some possible exceptions to this criterion will be discussed. An early-Earth type planet would not be expected to have O2 and O3, but it might have high concentrations of biologically-produced CH4. CH4 has a strong absorption feature near 7.7 microns that should be observable by TPF. The possibility of using this methane band as a remote signature of life will be discussed.
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