Biology
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p21c1675d&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P21C-1675
Biology
[0406] Biogeosciences / Astrobiology And Extraterrestrial Materials, [5210] Planetary Sciences: Astrobiology / Planetary Atmospheres, Clouds, And Hazes, [5210] Planetary Sciences: Astrobiology / Planetary Atmospheres, Clouds, And Hazes, [5225] Planetary Sciences: Astrobiology / Early Environment Of Earth
Scientific paper
The habitable zone is the region around a star for which liquid water might be stable at the surface of a planet. This is roughly defined as orbital distances greater than those for which runaway greenhouses are triggered or water loss via H escape becomes rapid, yet less than those for which snowball Earth conditions are unavoidable. Both of these limits are inherently tied to surface temperature of the planet. Exoplanet observers have often defined habitable zones based on estimates of the equilibrium temperature for a planet, using that as a proxy for surface. However, the calculation of equilibrium temperature requires knowledge of the planet's albedo, which is usually not known. Furthermore, translating a planet's equilibrium temperature into a surface temperature requires estimations of greenhouse and anti-greenhouse effects that are also unknown. Venus presents both of these problems: it has a much-higher albedo than the value commonly assumed for Earth-like exoplanets, and yet its surface temperature is hundreds of degrees higher than its equilibrium temperature. Without knowledge of the albedo of a planet or the magnitude of the greenhouse effect, equilibrium temperature is an unknown quantity that provides unreliable estimates of the surface temperature of a planet. For these reasons, atmospheric modelers have incorporated the effects of albedo and of greenhouse effects into definitions of the habitable zone. Historically, these definitions have been based on the luminosity of the host star and the semi-major axis of the planet's orbit. This has served the community well, as planets are treated and analyzed on a case-by-base basis. However, the presence of two criteria for habitability (semi-major axis and stellar luminosity) presents an impediment to plotting planets in 2-dimensional diagrams that also include geophysical parameters such as planetary radius, mass, or density. While such plots were not previously warranted for ~Earth-sized planets because very few were known, the large number of ~Earth-sized planets currently being discovered by exoplanet surveys such as NASA's Kepler mission increase the need for a single metric that represents the possibility for liquid water to be stable at the surface of a planet. In this presentation, we propose the use of installation - the amount of energy reaching the top of a planet's atmosphere - as a metric for habitability that can be calculated strictly from measured properties and that also allows for display of "surface water stability" on the same chart as other geophysical parameters. The habitable zone presented here is primarily derived from information on planets in our solar system, including knowledge of the history of those planets. We compare this new definition of the habitable zone to traditional ones, and apply it to the February 2011 release of data from the Kepler data set and the database of confirmed extrasolar planets.
Domagal-Goldman Shawn D.
Virtual Planetary Laboratory
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