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Life Zones and Suitable Stars for E.T.

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    1088
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    This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

    For reasons explained in the habitable planets and bio-markers sections below, our search for inhabited exoplanets is focusing on those that have water-based life existing on the surface of the exoplanet. The habitable zone, or life zone, is the distance from the star where the temperature on the surface is between the freezing point (0° C) and boiling point (100° C) of water. If you consider a planet with the same reflectivity (clouds and surface material) as the Earth, reradiates the solar energy it absorbed as efficiently as the Earth does, and rotates as quickly as the Earth does, then the habitable zone for the Sun (a G2 main sequence star) is between approximately 0.63 and 1.15 A.U. Calculations that include the effects of the greenhouse effect and whether or not there is a runaway process and ultraviolet dissociation of water like what happened on Venus shift the Sun's habitable zone outward so that the Earth is nearer the inside edge of the habitable zone. Climate research is still at the beginning stages of development, so the habitable zone boundaries are a bit uncertain. Note that the discussion in this section ignores the effect of internal heating that could create liquid water places at much greater distances from the star (e.g., tidal heating of a jovian planet's moon).

    The habitable zone of a hotter main sequence star will be farther out and wider because of the hotter star's greater luminosity. Using the same line of reasoning, the habitable zone of a cooler main sequence star will be closer to the star and narrower. You can use the inverse square law of light brightness to determine the extent of the habitable zones for different luminosity stars. The boundary distance is

    star boundary = Sun boundary × Sqrt[(star luminosity)/(Sun luminosity].

    For example, if the Sun's habitable zone boundaries are 0.9 and 1.5 A.U, the inner and outer bounds of the habitable zone for a star like Vega (an A0-type main sequence star with (Vega luminosity/Sun luminosity = 53) are 6.6 to 10.9 A.U., respectively. For a cool star like Kapteyn's Star (a M0 main sequence star with Kapteyn's star luminosity/Sun luminosity = 0.004), the habitable zone stretches from only 0.056 to 0.095 A.U.

    One of the first exoplanets discovered that orbits in its star's habitable zone is "Gliese 581c". Gliese 581c orbits in the habitable zone of an M3-type star, Gliese 581 about 20.4 light years from us. Gliese 581 has a luminosity = 0.013 solar luminosities (even though it is a cooler spectral type than Kapteyn's star). That would put the habitable zone of Gliese 581 between 0.1 AU and 0.17 AU. What is even more intriguing is that Gliese 581c has a mass of just five times the Earth (though that is a minimum derived mass), so it should have a solid surface that liquid water could collect upon as well as enough gravity to hold onto an atmosphere. This planet and the another slightly more massive planet (at a minimum of 8 Earth masses) orbiting Gliese 581 will certainly be studied a lot over the coming years! A major problem with the planet's habitability is its very close distance to the star as described in the next section. Since its discovery, many other exoplanets have been discovered in the habitable zones of their planetary systems, especially by the Kepler mission as described in exoplanet section elsewhere on this website.

    The much-heralded conclusion that 1 in 5 sun-like stars have an Earth-sized planet (1 to 2 Earth diameters in size) orbiting in their habitable zone is for stars of spectral type G and K and uses an optimistic estimate of the habitable zone boundaries where the inner boundary is the planet receiving up to four times the flux of energy from its star than the Earth receives from the Sun and the outer boundary is the planet receiving as little as 0.25 times (one-fourth) the flux of energy from its star than the Earth receives from the Sun. For a star like the Sun, that corresponds to 0.5 to 2 A.U. Both Venus and Mars would fit within that definition of the habitable zone. More conservative (pessimistic) estimates of the habitable zone that narrow the region drop the fraction of sun-like stars with an Earth-sized planet in the habitable zone down to 6 to 9%. Even with this smaller fraction, though, the potential Earth-analogues in our galaxy alone would number in the hundreds of millions for just the sun-like stars. As explained in the next section, it is possible that suitable stars with habitable planets orbiting them could be much more than just the sun-like stars.

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