Salem Press

Solar System
Habitable Zones

Category: Life in the Solar System

Habitable zones are the places beyond Earth where there is the best chance of finding life. As such, they are a major focus of scientific consideration and investigation that inspire exploratory efforts.

Overview
At root, the idea of a habitable zone around a star is simple. A planet must not be too close to a star to be too hot for life. On the other hand, it must not be too far from the star to be too cold for life. The spherical shell around a star where a planet will be "just right" for life is the habitable zone of that star. Another name is the circumstellar habitable zone; this name distinguishes it from the galactic habitable zone, the region of a galaxy most favorable to life.

Unfortunately, it is not a simple matter to calculate the limits of this shell for a given star. However, using Earth and the Sun as the basic measure and noting that energy concentration of light from a star decreases as the square of the distance, scientists can predict the rough average radius of the habitable zone around a star. The result is simply the square root of the ratio of the stellar brightness to that of the Sun. The radius will then be expressed in astronomic units (AU), where 1 AU is the average distance of Earth from the Sun. The brightness of the star must be rated at a standard distance, known as its bolometric luminosity.

To complicate matters, a planet might not stay in the habitable zone. To be habitable, a planet should have an orbit that remains in the habitable zone for billions of years. Only planets with relatively circular orbits can do that. Also, the habitable zone moves away from a star as that star ages and grows hotter. Stars are not all the same. Some burn their fuel quickly and do not last for billions of years. The habitable zone of a bright, hot star will not be the same as that of a dim, cool one. Thus, the habitable zone is determined by the star, the planet, and the life-form under consideration.

The type of life involved is, of course, a critical consideration. We one cannot discuss habitable zones without first establishing the expectations we have of life around a star. We can identify three very basic requirements for life. First, living things have bodies. Second, a life-form uses a flow of nutrients and energy to sustain its body and bodily processes. Third, life reproduces itself. Reproduction requires bodies (and, most likely, molecules) able to retain the complex and detailed information required for constructing more living forms. Life may be more than this, but it will surely never be less.

With these criteria we find, by an argument too extended to give here, that life in a habitable zone will be water- and carbon-chemistry-based. The habitable zone, then, can be calculated based on the requirement that a planet in the zone will long be able to hold water in liquid form. The actual calculation is complicated by many factors; chief among them is the fact that water vapor is a major greenhouse gas. Hence, one cannot simply find the incident energy from the star at various distances because the presence of water retains the heat supplied by the star and thereby expands the habitable zone. An early estimate by Michael Hart had the habitable zone of our Sun between 99 and 105 percent of the Earth's current distance. This was too conservative; a more likely estimate is 95 to 137 percent.

The habitable zone is unique to the star and is determined by the stellar mass and, to a lesser extent, the age of the star. In terms of spectral classes, stars such as Earth's Sun (in class G) and some K- and F-class stars can have habitable zones. The range also corresponds to stellar surface temperature range of a bit less than 4,500 Kelvins (K) to a bit above 7,000 Kelvins. Our Sun, at 5,777 Kelvins, is in the middle of this range. Stars with a mass 20 percent or more greater than that of the Sun (that is, 1.2 M_S) will not have habitable zones, because they emit deadly amounts of ultraviolet radiation (UV) along with their visible and infrared radiation (IR). UV destroys water molecules and, at high intensity, will eventually strip a planet of the water critical for life. About 1 percent of stars are so large that they consume their fuel and die long before life can form. Indeed, all stars larger than 1.5 M_S would turn into red giant stars and swallow up any life-bearing planets around them before intelligent life could appear. Stars with more than ten times the mass of our Sun are so intensely bright that planets cannot form around them, because light creates pressure on anything it strikes. This radiation pressure is usually too small to matter, but for these very large stars it is great enough that all the material around the star that might eventually form planets is pushed away from the star and is disbursed too quickly for planets to form.

On the other hand, stars with less than about 0.80 M_S do not produce enough high-energy UV light to support life on any planet; their UV output is insufficient for important atmospheric effects such as ozone creation. Any planet close enough to a star of less than about 0.65 M_S to receive sufficient heat will be so gripped by the stellar gravity that it will show the star one face, as the Moon does our Earth. If this is not the case, an effect called spin-orbit coupling will almost certainly force the rotation rate of the planet to be almost as slow as the planetary year, thus frying the planet on one slowly changing side while freezing it on the other. Mercury is such a case. It revolves around the Sun in 88 days but rotates once every 58.7 days, exactly two-thirds of the orbital period. Both these effects are due to the fact that no planet is perfectly spherical. In either case, the planetary face toward the star will be too hot for water, the side away from the star too cold.

Another factor in habitability is variability of stellar output. Our Sun has an eleven-year sunspot intensity cycle that causes a variation in solar luminosity of about 0.1 percent. However, 18 Scorpii, an almost identical star in the constellation Scorpius with a mass of 1.03 M_S and a temperature of 5,789 K, has a much greater variability that would over a 9 to perhaps 13 year cycle. If great enough, this would make its habitable zone move in and out rapidly thereby its benefits for any planet in a basically fixed orbit.

Knowledge Gained
The idea of a circumstellar habitable zone has stimulated wide-ranging research resulting in a significant extension of our knowledge of planetary systems generally, as well as of our own solar system in particular. A circumstellar habitable zone imposes quite severe limitations on where best to look for life in the universe. Responses to these limitations are likewise limited. One either accepts the limitations, at least tentatively, and looks for suitable planets around only suitable stars, or one must in some way challenge the limitations.

Tentative acceptance of the limitations takes us in the direction of what has become a successful search for exoplanets, planets orbiting stars other than our Sun. The list is large and growing. The primary technique used in this search detects stellar motion due to the stellar reaction to the orbital motion of the planet. Since large planets create more stellar reaction that is more easily detected, this technique is biased toward discovering large planets. It is no surprise then that most of the known exoplanets are large. It is a bit of a surprise that they tend to be relatively close to their star and, hence, are sometimes called "hot Jupiters." If this trend continues, it may require revisions in the theories of how planetary systems form. Hot Jupiters are not expected to harbor life even if they are in the habitable zone.

Another puzzling result of these searches is that exoplanets seem to prefer highly elliptical orbits compared with those in our solar system. Such orbits are risky in that they may take the planet out of the habitable zone annually. On a more positive note, the work on exoplanets has confirmed that, as expected, planets tend strongly to be found around stars with high metal content. (In this context, "metal" means any element other than hydrogen or helium.)

Challenges to the idea of the circumstellar habitable zone have either been attempts to show there are niches of habitability outside the habitable zone or efforts to extend the habitable zone in size or to more types of stars, especially to red dwarfs. This later direction seems promising in light of the discovery of planets around the red dwarf Gliese 581. One of them, Gliese 581 c, is said to be the smallest planet yet discovered in the habitable zone of another star. That, of course, assumes that a red dwarf has a habitable zone.

Looking for niches of habitability in our solar system—and, hence, potentially elsewhere—offers the possibility of confirmation by direct examination in the not too distant future and is accordingly fairly popular. Thus, attention has become focused on Mars and some large satellites of Jupiter and Saturn.

Mars has received the most attention, as attested by missions such as the Mars Exploration Rover (which began operating on the Martian surface in early 2004). The Cassini-Huygens nission to Saturn (which entered into orbit around Saturn in mid-2004) included flybys of Titan, revealing its liquid methane oceans and dense atmosphere, while the Galileo mission to Jupiter in the late 1990's gathered a great deal of data on two of Jupiter's satellites, volcanic Io and Europa, whose deep ice sheet appears to have water in liquid form beneath it.

Context
The dream of "other races of men" on other worlds has been the currency of cosmological speculation at least since the ancient Greek atomists. Men on the Moon were described by the ancient Pythagoreans and in the seventeenth century by Johannes Kepler, and even the eighteenth century philosopher Immanuel Kant gave opinions on the inhabitants of Mars, Venus, and Jupiter. Modern science has tried to inform and thereby reduce this speculation. The concept of a habitable zone is a product of this effort, although it imposes limitations that would, no doubt, have disappointed earlier enthusiasts.

Enthusiasm for finding life elsewhere in the universe is by no means dead. The high profile of the Search for Extraterrestrial Intelligence (SETI) and the advent of the new academic discipline of astrobiology are proof of that. Both of these developments are inextricably connected with the concept of habitable zones and are all but inconceivable without it. The prospect of habitable zones has also stimulated thinking and research in other areas. One such development is the idea of a galactic habitable zone.

The concept of habitable zones also connects to larger cosmological issues, such as questions of the "fine tuning" of the universe that makes life possible somewhere in the universe and the related issue of the anthropic principle, the notion that the universe must contain conditions that allow for the existence of an observing intelligent life-form

John A. Cramer

Further Reading
Aczel, Amir D. Probability 1. New York: Harcourt Brace & Company, 1998. Aczel argues that the large number of stars outweighs the limitations of habitable zones to the point where intelligent life must occur throughout the universe.

Cohen, Jack, and Ian Stewart. Evolving the Alien: The Science of Extraterrestrial Life. London: Ebury Press, 2002. Cohen and Stewart dispute that alien life will be similar enough to terrestrial forms to frame a meaningful idea of a habitable zone. They also argue the case for various niches of habitability.

Cramer, John A. How Alien Would Aliens Be? Lincoln, Nebr.: Writers Club Press, 2001. The first half of the book shows how physical constraints limit where intelligent life might be found in the universe. Hence, it surveys many of the limitations that lead to the idea of a habitable zone and the possibility of habitable niches.

Dole, Stephen H. Habitable Planets for Man, 2d ed. New York: Elsevier, 1970. Something of a classic on habitable planets, this is one of the earliest discussions of habitable places for human colonization. It gives a good if somewhat dated account of what makes a place habitable for intelligent life, a more restrictive notion than a habitable zone for any life.

Gonzalex, Guillermo, and Jay W. Richards. The Priviledged Planet. Washington, D.C.: Regnery, 2004. Gonzales and Richards consider the idea that planetary habitability may be connected with the planet's suitability as a platform for observing the universe.

Grinspoon, David. Lonely Planets: The Natural Philosophy of Alien Life. New York: HarperCollins, 2004. This is a wide-ranging and readable book covering habitable zones and many related topics.

Ward, Peter, and Donald Brownlee. Rare Earth: Why Complex Life Is Uncommon in the Universe. New York: Springer, 2000. Ward and Brownlee make the case that the limitations on habitable zones are severe to the point of making planets like Earth quite rare.

See Also
Earth's Origin; Extraterrestrial Life in the Solar System; Life's Origins; Mars: Possible Life; Search for Extraterrestrial Intelligence (SETI).