The concept of habitable zones is perhaps most interesting as applied to stars other than the sun. The brightness of the star determines the location of its habitable zone, but brightness in turn depends on the star's size, type, and age.
For stars more massive than our sun, the outward migration of their HZs through time is much faster and of much shorter duration. More massive stars have shorter lifetimes. The sun should be fairly stable for nearly 10 billion years after its birth, but a star 50% more massive than the sun enters its red giant stage after only 2 billion years. When a star becomes a red giant, its brightness increases by a factor of a thousand, and the HZ retreats greatly beyond its original bounds. We have already noted that a 1.5-solar-mass star would not be around long enough for animals to evolve at the leisurely pace enjoyed by terrestrial life. More massive stars have habitable zones farther from the star—or they may have no habitable zones at all. More massive stars are hotter and radiate substantially more ultraviolet light than the sun. Ultraviolet (UV) light breaks the bonds of most biological molecules, and life must be shielded from it to survive. UV also can be disastrous for the atmospheres of Earth-like planets. It is strongly absorbed at the top of such atmospheres and is a potent high-altitude heat source than can lead to escape of the atmosphere. The sun, with its effective surface temperature of 5780 kelvin emits less than 10% of its energy in the ultraviolet range, whereas hotter stars like Sirius radiate most of their energy in the UV. Atmospheric loss may prevent terrestrial planets with oceans and atmospheres from forming around more massive stars. This atmospheric problem with planets orbiting more massive stars is in addition to limitations imposed by their shorter stellar lifetimes.
It is often said that the sun is a typical star, but this is entirely untrue. The mere fact that 95% of all stars are less massive than the sun makes our planetary system quite rare. Less massive stars are important because they are much more common than more massive ones. For stars less massive than the sun, the habitable zones are located farther inward.
The most common stars in our galaxy are classified as M stars; they have only 10% of the mass of the sun. Such stars are far less luminous than our sun, and any planets orbiting them would have to be very close to stay warm enough to allow the existence of liquid water on the surface. However, there is danger in orbiting too close to any celestial body. As planets get closer to a star (or moons to a planet), the gravitational tidal effects from the star induce synchronous rotation, wherein the planet spins on its axis only once each time it orbits the star. Thus the same side of the planet always faces the star. (Such tidal locking keeps one side of the Moon facing Earth at all times.) This synchronous rotation leads to extreme cold on the dark side of a planet and freezes out the atmosphere. It is possible that with a very thick atmosphere, and with little day/night variation, a planet might escape this fate, but unless their atmospheres are exceedingly rich in CO2, planets close to low-mass stars are not likely to be habitable because of atmospheric freeze-out.
We can thus look at various stars in our Milky Way galaxy and ask whether they are appropriate places for life or, indeed, have habitable zones at all. For example, could there be habitable planets orbiting binary stars or multiple star systems, places where two or more stars are locked in a complex orbital dance? Can planets with stable orbits and relatively constant regimes of temperature be found in such settings? Can planets even form in such settings? These questions are highly relevant to understanding the frequency of life beyond Earth, because approximately two-thirds of solar-type stars in the solar neighborhood are members of binary or multiple star systems. Astrobiologist Alan Hale, who has written on the problems of habit-ability in binary or multiple star systems, notes, "The effects of nearby stellar companions on the habitability of planetary environments must be considered in estimating the number of potential life-bearing planets within the Galaxy."
Two scenarios can be considered: the case where the stellar components (the stars of the binary or multiple star system) are quite close together, and the planets orbit both or all of the stars, and the case where the stellar companions are far apart, and the planets orbit a single star. But can planets even form in such stellar systems? Some recent work suggests that planets may not be able to form there unless the stars are at least 50 times the distance from Earth to the sun or 50 AU, although this has not been proved. Alan Hale suggests that stable orbits will be achieved in multiple star systems only where the companion stars are at less than 20 million miles apart or farther than a billion miles apart. And, of course, if planets do form in such systems, two or more bodies will affect their orbits.
The most pressing question is whether planets, once formed in a multiple star system, can achieve stable orbits. The rise of life (at least on Earth) seems to require long periods of constant conditions, which require stable or bits. Highly elliptical orbits wherein a planet moves in and out of the CHZ might allow microbial life to form and even flourish but probably would be lethal to animal life. In such systems planets might form, but their orbits would be perturbed by the various gravitational forces of more than a single star, which would eventually either eject the planets or cause them to fall into one of the stars.
A second problem with multiple star systems as habitats for life is insolation (the stellar energy a planet receives). S.H. Dole, in his groundbreaking 1970 book Habitable Planets for Man, estimated that the average amount of energy received by a planet could vary by as much as 10% without affecting its habitability. (This too is debatable: Our sun undergoes far less variation in output than 10%, yet even these small fluctuations produce major swings in climate that drastically affect the evolution of life forms.) Where planets orbit in the same plane as the companion stars, insolation will also be affected by eclipses of one star by another.
Finally, the residents of any planet in a multiple star system will have to deal with the stellar evolution of two or more suns. Our sun is getting brighter through time. This gradual brightening causes the habitable zones to migrate ever outward. With two or more suns undergoing the same process, we might expect habitable zones to migrate even faster through time. Although this might not adversely affect microbial life, it could inhibit animal life. All in all, it appears that multiple star systems might be regions that could support life, but perhaps not animal life. They are certainly less favorable habitats for animal life than solitary stars.
Other types of stars might be even less suitable. Variable stars (those that exhibit rapidly changing insolation) are surely poor candidates for producing planets habitable by animals (though here again, microbial life might gain and maintain a foothold, assuming that planets form). Unusual stellar entities such as neutron stars and white dwarf stars are probably uninhabitable by any form of life.
What of regions where star frequency (the number of stars in a volume of space) is very high? Such regions include open star clusters and globular star clusters. Open clusters are unlikely to be hospitable to animal life because they are too young. Most are composed of relatively new stars, where life— at least advanced life such as higher plants and animals—would not yet have had a chance to develop. Many open clusters are dispersed by the time they have orbited their galaxy several times. Others are more long-lasting, but they too have problems. Because neighboring stars are so close, planetary orbits can be perturbed, causing planets to be ejected, to enter highly elliptical orbits, or even to fall into their suns.
In globular clusters the density of stars is extremely high: Some globular clusters can have as many as 100,000 stars packed into a space some tens to hundreds of light-years across. The nearest star to our own, Proxima Cen-tauri, is 4.2 light-years away. There are a total of 23 known stars within 13 light-years of the sun. In a globular cluster, the same distance might hold 1000 stars or more. For example, the M15 globular cluster has 30,000 stars packed into a space only 28 light-years across. There would be no night on any planets in such clusters. There might be habitable stellar systems in such regions, but the very number of stars would make them more dangerous and less congenial to the maintenance of animal life than more widely separated stars; there is too much radiation and particles, too many chances for gravitational changes to affect the orbits of planets in any such mass. Being in a high concentration of stars increases the risk of a nearby star going nova (exploding) or belching hard radiation into nearby space. A second great disadvantage of globular clusters is that they are composed of old (and thus heavy-element-poor) stars, all of about the same age. The low abundance of "heavy elements" such as carbon, silicon, and iron makes it unlikely that any Earth-size terrestrial planets would form. These heavy elements are required not only to provide habitats for life but also to build life as we know it.
Even if some of the stars did manage to have Earth-like planets, the stars would be so old that 1-solar-mass stars would have evolved to the point where their HZs had retreated outward beyond the inner planets. Globular clusters thus may be devoid of all life. This conclusion illustrates real progress in our understanding of the limits of life in the cosmos. In 1974 a group of as tronomers led by Frank Drake directed a radio signal toward the globular cluster M13. It was hoped that other radio-astronomers living around one of the 300,000 stars in the cluster might receive the message. Today, only a few decades later, we realize that there is no chance anyone will be there to take the call when the radio message arrives at M13, some 24,000 years from now. If the experiment were to be repeated, the beam would be directed toward stars more likely to have planets and life.
About other stellar regions we can only speculate. Stars are continuing to form: Is there some aspect of their formation that is beneficial—or deleterious—to habitability? Would a planet in a region with newly forming stars be able to sustain life? What about stellar systems in the middle of nebulae? Are these regions neutral to life, or does the presence of great quantities of interstellar gas have some effect on life's presence or existence? Our sun probably formed in a low-density star cluster that dispersed soon thereafter and thus avoided disruption of the orbits of Jupiter, Saturn, Uranus, and Neptune.
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