Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p21c..01k&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P21C-01
Other
0325 Evolution Of The Atmosphere, 0343 Planetary Atmospheres (5405, 5407, 5409, 5704, 5705, 5707), 5705 Atmospheres: Evolution, 5749 Origin And Evolution
Scientific paper
The habitable zone, or HZ, is defined as the region around a star in which liquid water can exist on the surface of a planet. Liquid water can exist in subsurface environments on planets or moons outside of the HZ (e.g., Jupiter's moon, Europa), but this possibility is of little significance, as such environments cannot be observed or otherwise investigated. The continuously habitable zone, or CHZ, is the region that remains habitable over some finite period of time. The CHZ for any extended time interval is narrower than the HZ because all main sequence stars grow brighter as they age, so that their HZs move outward with time. The conservative estimate for the inner edge of the HZ in our own Solar System has not changed in the last 13 years. It is at 0.95 AU, which is the distance at which Earth's water would be lost, assuming zero cloud feedback on climate. If clouds tend to cool the climate as the solar flux increases, as seems likely, the inner edge of the HZ could be well inside that distance. Venus' orbital distance, 0.72 AU, provides an empirical, 'optimistic' limit for the inner edge of the HZ. The outer edge of the HZ could lie as far out as 2.4 AU if CO2 ice clouds warm a planet's surface, as predicted by some authors (Forget and Pierrehumbert, Science, 1997). However, the warming by CO2 clouds is problematical, as it depends critically on poorly-known factors such as cloud optical depth, cloud height, and fractional cloud cover. Indeed, it is a challenge to figure out how Mars, at 1.52 AU, managed to stay warm enough to allow liquid water to flow on its surface early in its history when the Sun was less bright. Greenhouse warming by CH4 may be at least part of the answer. If so, the outer edge of the HZ depends on a variety of factors that are not easy to predict. The exciting news on the horizon is that it may eventually be possible to observe planets within stellar HZs by way of NASA's Terrestrial Planet Finder (TPF) mission. If it works, TPF will provide either visible or thermal infrared spectra that will allow us to characterize planetary atmospheres and, perhaps, to look for evidence of life. Once this happens, the study of habitable zones and habitable planets will assume much more practical significance.
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