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habitable zone (HZ)
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The single most crucial factor to the evolution of terrestrial life has been the ready availability of liquid water. Not only does water serve as the solvent for biochemical reactions but it also furnishes the hydrogen upon which living matter depends. Water will remain liquid under a pressure of 1 bar (terrestrial sea-level pressure) between 0°C and 100°C. Even on Earth, however, geographical, seasonal, and diurnal variations can cause the temperature to fall well below freezing (as low as -70°C in Antarctica), yet life can survive under these conditions. At the other end of the scale, although most terrestrial organisms cannot endure temperatures consistently above 45°C, hyperthermophiles have been found thriving around hydrothermal vents at 113°C.
It is reasonable to assume that if the Earth had been marginally closer or further from the Sun, life of some kind would still have evolved on it. But the limits of the Sun's HZ are not easy to fix. The discovery of primitive life, or fossils, on Mars would extend the HZ of the Sun out to at least 225 million km (140 million miles). On the other hand, the absence of Martian biology would not rule out the possibility that if the Earth, with its greater mass able to retain a denser atmosphere, had orbited at the distance of Mars it would have been capable of nurturing life. Indeed, Mars itself enjoyed warmer, wetter conditions in the past under which primitive organisms may have evolved. Much further from the Sun are the gas giants and their large moons, at first sight inhospitable to life. Yet the effect of tidal heating on these worlds, resulting possibly, in the case of Europa and Callisto, in the existence of large amounts of sub-surface liquid water, makes them biologically interesting.1 Were life to be discovered on one of Jupiter's moons, the outer margin of the Sun's HZ would be pushed out to almost 800 million km (500 million miles). Its inner margin is somewhat easier to define. Venus is approximately terrestrial in size, mass, and composition, but its proximity to the Sun has prevented its development from following an Earthlike course. On our own planet, temperatures were low enough for most of the water vapor released in large quantities from primordial volcanoes or colliding comets to condense and form oceans of water (see oceans, origin). These oceans subsequently mopped up the bulk of the carbon dioxide also produced by volcanic activity. On Venus, however, it was too hot from the outset for the water vapor to condense. As a result, the massive amounts of carbon dioxide pumped into the primitive Venusian atmosphere remained there and served to bring about a runaway greenhouse effect. The already uncomfortably high surface temperature soared to around 460°C, at which point any kind of complex biochemistry along terrestrial lines was ruled out. Based on present knowledge, therefore, and until more is known about conditions on worlds such as Europa (and possibly Titan), we can reasonably put the extent of the Sun's HZ for an Earth-sized planet at between 120 million and 240 million km (75 million and 150 million miles) – that is, 0.8 to 1.6 times the radius of the Earth's orbit.
Regarding other stars, the size and location of the HZ depends on the nature of the star in question.2, 3 In general, hot, luminous stars – those of spectral types "earlier" than that of the Sun (G3-G9, F, A, B, and O) – all have wide HZs, the inner margins of which are located relatively far out. For example, to enjoy terrestrial temperatures around Sirius (26 times more luminous than the Sun), an Earth-sized planet would have to orbit at about the distance of Jupiter from the Sun. By contrast, if the Sun were replaced by Epsilon Indi, which has only about one-tenth the Sun's luminosity, the Earth would need to be in a Mercurian orbit to receive its present level of warmth. The situation becomes even more extreme in the case of a red dwarf, such as Barnard's Star (about 2,000 times less luminous than the Sun), the HZ of which would extend only between about 750,000 and 2 million km (0.02 to 0.06 AU). Even if planets exist so close to their parent star (as now seems likely following the discovery of epistellar jovians) the development of life might be made problematic through a gravitational lock.
By contrast, the HZ of a highly luminous star would in principle be very wide, its inner margin beginning perhaps several hundred million km out and stretching to a distance of a billion km or more. However, this promising scenario is spoiled by the fact that massive, bright stars are much more short-lived than their smaller, dimmer cousins. In the case of the giant O stars and B stars, these very massive objects race through their life-cycles in only a few tens of millions of years – too quickly to allow even primitive life-forms to emerge. Given the rate of evolution of life on Earth, it is possible that microorganisms might have time to develop on worlds around A stars (see stars, lifetimes). But in the search for extraterrestrial intelligence, the HZs around F stars and later must be considered the most likely places to look (see Sunlike stars; target stars). Among extrasolar planets discovered to date, only one, the companion of 70 Virginis, appears to move within its star's HZ. It is, however, an extremely massive planet with an orbit of highly eccentricity, both of which factors argue against it supporting a terrestrial-type ecosystem.
Of course, it might be that life can develop along very varied lines. If it could be based on ammonia as a solvent, instead of water, this would allow it to thrive at low temperatures. Then, again, it may be that life is possible in the atmospheres of Jupiter-like planets. Such novel biological forms would extend the HZs of stars beyond those considered.4 The idea of galactic habitable zones, and of preferred regions within the Galaxy to search for intelligent life, has also been discussed.5
Notes
If all of the complicating factors discussed above are ignored and the habitable zone is defined simply as the distance from a star where the effective temperature is in the range 0° to 100°C then it is straightforward to calculate the radii of the HZ's inner and outer bounds. The relevant formula is:
In the case of Vega, L(star)/L(sun) = 53, which gives a range for HZ of 5.1 to 10.9 AU. In the case of Kapteyn's Star, L(star)/L(sun) = 0.004 and the corresponding HZ range is 0.044 to 0.095 AU.
References
- Reynolds, R. T., McKay, C. P., and Kasting, J. F. "Europa, Tidally Heated Oceans, and Habitable Zones Around Giant Planets," Advances in Space Research, 7 (5), 125 (1987).
- Hart, M. "Habitable Zones Around Main Sequence Stars," Icarus, 37, 351 (1979).
- Kasting, J. F., Whitmire, D. P., and Reynolds, R. T. "Habitable Zones Around Main Sequence Stars," Icarus, 101, 108 (1993).
- Doyle, L. R., ed. Circumstellar Habitable Zones, Proceedings of the First International Conference on Habitable Zones, San Francisco, Menlo Park, Calif.: Travis House (1996).
- Balás, Béla A. "SETI and the Galactic Belt of Intelligent Life," a paper presented at Bioastronomy 99: A New Era in Bioastronomy, Kohala Coast, Hawaii, August 2-7, 1999.
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• ASTROBIOLOGY
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