The Search for Other Earths

star field

3 The Habitable Zone

The main interest in the search for extra-solar planets is in those at about the same size and gravity as earth and located in a zone around a star where the conditions are suitable for life. At present, few planets of this size have been detected, although the Kepler space telescope is expected to find increasing numbers. The standard definition of the habitable zone around a star is the range of distances where liquid water can be found, this in turn depends on the temperature of the star and on the atmospheric characteristics of the planet. A planet like Venus may appear to be at the right distance for liquid water, but its cloud cover maintains the surface temperature at an average of 464 deg C.

These considerations, and the variability of temperature across a planet, or the effects of changing planetary albedo, make the definition of the life zone somewhat indeterminate: it might be 0.5 AU to 1.1 AU, or perhaps out to 2 AU. Then there is the complication of moons about planets. Jupiter at 5.2 AU from the sun may not have surface water, but its moon Europe has, because it is heated by tidal effects of Jupiter rather than be radiation from the sun. Or, like the earth, a planet may have an internal source of heat and liquid water could exist at some depth below the surface, even at a distance from the sun that would freeze the entire surface. Life could emerge under such circumstances, whether it could evolve into an advanced civilization is another matter.

If we are looking for a place where an advanced civilization may have evolved, then the planet should be continuously in its star’s habitable zone. But the star’s output of energy and its surface temperature varies during its life time, so the habitable zone shifts in or out. High mass stars vary in this respect over shorter time periods, and have an average radiance that is much brighter than the sun and thus may be poor candidates for habitable planetary systems that exist over great lengths of time. Low mass stars are also poor candidates because  their low mass means they are much dimmer. The life zone is therefore much closer to the star, in a region where the spin period of the planet becomes locked to its orbital period (like Mercury) so that one hemisphere faces the sun all the time while the other is in darkness. There is a further requirement that the star’s gravitational forces should not be strong enough to drive a violent plate tectonics that is constantly resurfacing the planet with new lava flows. In addition to the temperature extremes that this produces, low mass stars emit intense flares of radiation that are inimical to complex life.

 In general, stars about the mass of the sun appear to be the best place to look for habitable planets. Then deciding whether a planet is in the habitable zone comes down to individual cases: where is the star in its evolutionary progress, what is the atmosphere like on the planet, is it spinning freely or does it rotate once a year, showing the same face always to its star? One can also worry whether the star is in a habitable part of the galaxy, but the distances in that habitable zone are very much larger than the search distances involved at present.

If an earth-sized planet in a habitable zone is discovered, high resolution spectroscopy can detect elements in the planet’s atmosphere. In addition,  monitoring the light change as the planet passes behind its star can yield information on the amount of light radiated back into space by the planet and so its temperature and perhaps the presence of cloud formations. This technique was used successfully  by the Spitzer Space Telescope in 2005.

Jianpo Guo and his colleagues at the Yunnan Observatory point out that some planets outside the habitable zone of a host star during the main sequence phase may enter the habitable zone during later phases. They calculate the after the main sequence phase of the Sun, the orbits of Mars and Jupiter orbit will enter the habitable zone during the subgiant branch phase and the red giant branch phase, respectively. And the orbit of Saturn will enter the habitable zone during the He-burning phase for about 137 million years. Titan, the rocky moon of Saturn, may be suitable for biological evolution and become another Earth during that time. For low-mass stars, there are similar habitable zones during the He-burning phase as in our Solar system, because there are similar core masses and luminosities for these stars during that phase.


NASA Astrobiology Roadmap

Habitable zones of host stars during the post-MS phase, by  Guo, Jianpo; Zhang, Fenghui; Han, Zhanwen.  Astrophysics and Space Science, Volume 327, Issue 2, pp.233-238, April, 2010.

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