Are Narrow

Give me more love or more disdain; the torrid or the frozen zone.

Thomas Carew, Mediocrity in Love Rejected

Even if terrestrial planets form readily around stars, another condition must be met before life as we know it can survive for the billions of years needed for a technological civilization to develop. A terrestrial planet has to be in a system's habitable zone (HZ) before life can evolve.191

The key to life is water. in essence, the habitable zone around a star is the region in which a planet like Earth could support liquid water. The location of the inner edge of the HZ is set by the point at which a planet loses water due to the high temperatures close to a star. The outer edge of the HZ is set by the point at which water freezes.192 Many scientists believe that the HZ for our Solar System extends from 0.95 AU to 1.37 au. Venus, with a mean distance of 0.723 au from the Sun, lies inside the inner edge of the habitable zone; Mars, with a mean distance of 1.524 au from the Sun, lies outside the outer edge of the habitable zone. only Earth lies in the right place. Nevertheless, the habitable zone is rather wide: if the HZ was the full story, one would expect most other systems to have planets in the zone. It is, of course, not the full story.

Michael Hart argued that the habitable zone around a star varies with time. Main-sequence stars become brighter and hotter as they grow older, so the HZ moves outward as a star ages. What is important, according to Hart, is the continuously habitable zone (chz).

Typically, the CHZ is defined as the region in which an Earth-like planet can support liquid water for 1 billion years — the timescale evolution presumably requires to develop complex life. In the case of the Solar System, the CHZ has existed for 4.5 billion years, and Earth has been fortunate enough to be precisely in the middle of the zone. Clearly, though, the CHZ must be narrower than the HZ. In 1979, Hart published the results of computer models that seemed to show that the CHZ is extremely narrow.193 It is widest around GO main-sequence stars (the Sun is a G2 star) and shrinks to zero at cool K1 stars and hot F7 stars. In all cases, though, the CHZ was narrower than 0.1 AU. For the Solar System, for example, he calculated an inner edge of the CHZ at 0.95 AU and an outer edge at 1.01 AU. With such a narrow CHZ, one would expect Earth-like planets — those that can support life over billions of years — to be much rarer than is commonly supposed.

While Hart's finding did not prove ETCs could not exist, it clearly had a bearing on the Fermi paradox. If the number of potential life-bearing planets is much smaller than most estimates suppose, then the number of potential ETCs out there must also be smaller. Depending upon the values of the other factors in the Drake equation, the total number of communicating civilizations might be reduced to one: us.

Recent calculations, however, employ more sophisticated models of the Earth's early atmosphere; they also take account of the recycling of CO2 by plate tectonics, a phenomenon not known to Hart. The results are encouraging for those who would believe in the existence of ETCs (or at least in the existence of planetary homes for ETCs). Models developed by James Kasting and co-workers suggest that the 4.6-billion-year CHZ for our Solar System extends from 0.95 AU to 1.15 AU — larger than the range calculated by Hart.194 Other scientists believe the Solar System's CHZ may be even wider. The CHZ around other stars, too, is wider than first thought.

So: how likely is it that a given planetary system will have a planet that lies within the CHZ? The answer depends both upon the type of star and the distribution of the planets in the system. If planets are distributed as they are in our Solar System — in other words, if the distances of planets from the central star follow the Titius-Bode law — then roughly the same number of planets will exist in the instantaneously habitable zones of all stellar types. However, planets around hot stars of type O, B and A will not long remain in a habitable zone, as the stars themselves evolve in lumi nosity too quickly. Planets around cold stars of type K and M are unlikely to be continuously habitable: the HZ in these systems lie close to the central star, and the planet will thus become tidally locked. (When a planet is tidally locked, one side of the planet always faces the heat of the star, while the other side always faces the cold of open space. This situation is presumably inimical to life.) Around stars not too different from the Sun, however, a planetary system, if it obeys the Titius-Bode law, has roughly a 50:50 chance of containing a planet in the CHZ.

If our current models of planetary formation, stellar evolution and long-term planetary atmospheric evolution are correct (and it must be admitted that there are places where scientists are surely uncertain on the details), then the conclusion seems to be that there are potentially millions of continuously habitable planets in the Galaxy. One caveat, though. We saw in an earlier section that only certain types of star have sufficient metallicity to possess terrestrial planets; and only certain parts of the Galaxy are sufficiently protected from the violence of the central regions. We may need to define a galactic habitable zone (GHZ) — which is an annulus containing perhaps only 20% of the stars in the Galaxy. For complex life to evolve, a CHZ must be within the GHZ — and this reduces the possibilities.195

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