Earths

Earths are defined here as rocky terrestrial planets which stably orbit their suns for long periods of time at a distance which allows a proper temperature/radiation input so as to keep the solvent-of-life, water, in its liquid state.

The frequently of occurence of these objects has been the point of a quite intense debate, which is not totally resolved. The core material initiating the debate was provided by Michael Hart, who felt that certain facts and models indicated that our Earth was a very lucky, exceptional place, perhaps even unique s1Hart 1978, 1979. The majority of the "pessimistic" commentators, however, seem merely to repeat Hart's conclusions, or, at best, build slightly off his basic model. The motivations of this school of thought seem to range from a need to explain the "absence" of ETI visiting our solar system (a position which not only assumes the absence of evidence in the UFO phenomenon, but also ignores the obvious fact that we have not explored most likely locations in our system for evidence of present and past ETI), to apparently emotional concerns about humanity's place and future role in the universe. The most vocal of this school are enthousiasts for either human interstellar migration via advanced spaceships or for the "anthropic principle" as seen as a "proof" that the universe has been designed particularly to evolve human intelligence as some sort of climatic pinnacle s2Bond & Martin 1980 s3Martin & Bond 1983 s4Tipler 1980, 1981. If we scrape away the irrelevancies, the argument, as regards "earths," is still based on essentially one thing: Michael Hart's conceptualization of what he called the "Continously Habitable Zone" (CHZ) for life-bearing planets.

To critique this issue we should begin with the standard version of what planetary theorists think would go on in the formation of a system around a sun. When a sunlike star condenses by gravity out of a heavy molecular cloud (a hydrogen/helium cloud littered with substantial amounts of heavier elements), other grains and lumps and centers of attraction also form. Such meteoritic or cometary lumps aggregate and condense into the cores of planets surrounded by the hydrogen-rich gas of the cloud. The cloud condenses, spins, flattens until there is a disk-like system with the proto-sun at the center and the proto-planets revolving in a flattened plane about it. An early super-bright phase of star-formation then blows the primaeval light gas of the original cloud from the rocky cores of the planets-to-be which are nearest the star. The cores continue to condense and heat-up as heavy elements engage in radiactive decay. Solids melt and metals sink to the center, while a lighter crust forms and floats. The crust fractures and gases escape to reform the atmosphere (more hydrogen and helium, but, more importantly, carbon dioxide, water vapor, nitrogen, and a few other components). The solar wind has now abated, and this new atmosphere becomes the true primordial atmosphere of our earth-like planets s5Torbett & al. 1982 s6Lewis & Prinn 1984.

Now the planet cools. This is the critical phase. Will the planet cool enough to rain out its vaporous oceans-to-be? If the planet is too near the sun, it will not. Instead, insufficient liquid water will be present to dissolve the carbon dioxide. CO₂' will pack the atmosphere as continued venting of gases occurs in the crust. This "greenhouse gas," CO₂', will trap more and more heat until the atmosphere and surface temperatures are at a level unsuited for even elementary life. Such was the fate of Venus. Thus, some promising planets will be too near their star.

They can also be too far. On such a planet the rains will be complete and the CO₂' will be dissolved. The processes leading to life may well begin. But as the primordial heat of the planet, insufficiently augmented by the incoming radiation of its star, continues to drop, liquid water freezes and glaciation begins. Such an early potential life-generating planet will die. This was probably the fate of Mars s7Pollack & al. 1987. Even better placed life-generators may reach a later crisis caused by atmosphere changes due to the biogenic release of massive quantities of oxygen. Such changes also result in less heat retention and potential irreversible glaciation. This last risk may be substantially modulated by the atmosphere controlling activities of the most primitive life forms in the oceans (the so-called GAIA force), however s8Lovelock 1980 s9Margulis 1982.

Therefore, there is a life zone surrounding each sun-like star, a strip within which a planet must luckily form if it is to be a liquid-water earth. What are the odds that such a stroke of luck will occur? Hart and the school of minority opinion say that the chances are so slight that it is almost impossible to get a planet slotted into this narrow channel. Hart's models indicate that the galaxy is filled with Venuses and Mars lookalikes, and the Earth, the fabulous fluke, could be unique.

This position is now largely discarted or severly modified even by the pessimists. The reason are several:

1. The original atmospheric models have turned out to be overly simplistic and even directly inaccurate in some of what they did include s10Schneider & Thompson 1980.
2. The original models totally ignored the effect of life forms (microorganisms) in stabilizing atmospheres;
3. More complex, and probably more accurate, modelling of early atmospheres predicts the probability of much wider liquid water zones, particularly on the "cold side" of the strip s11Kasting & al. 1988.
4. Our own Earth's history shows adaptation to widely differing solar energy inputs while maintaining remarkable temperature stability at the surface, a stability impossible if the pessimists' models were anywhere nearly correct s12Schneider & Thompson 1980.

Newer models of atmospheres and temperatures point to life zones six or seven times wider than the Hart estimate. In our own solar system with the Earth at the reference distance of 1.0 astronomical unit, Hart's model pointed to a life zone between 0.95 and 1.01 AU. The new estimates increase the local life zone to between 0.86 and 1.25 (or greater) AU. Venus, for reference, is too hot at 0.72 AU. Mars is a bit too cold at 1.52 AU. With this wider zone what are the odds of an earthlike planet forming there? We do have some guides with which to estimate this answer.

When we look at the spacing of the planets in our own system, we are struck with an intuition of a patterned array. The great rocks seem to lie in lanes of movement at "respectful" distances from one another, gradually widening the gaps as we look further from the Sun. The Bode-Titius equation hints at a regularizing mathematical physics which rules their positions, as if primaeval forces of gravitational resonnance, collisions, available mass, or whatever, determined the design. As our theories of system formation become better at approximating the realities we see in our own planets, we are able to alter the initial parameters (star size, cloud metallicity, angular momentum) and watch as our computers form alternative planetary arrays in moments. The arrays stay essentially the same: small rocky terrestrials in close to the star, a transitional zone, big Jovian gas balls further out, all gradually widening their gaps to their next further neighbor. Our own system should not be widely deviant from the others of the galaxy.

If the arrangement of our terrestrial planets was precisely the rule for our galaxy, it would be an easy task to lay down a grid containing the "too hot," "habitable zone," and "too cold" regions, and overlay the spacing of our four terrestrials on it. We could then slide the planets up and down and make a quick estimate of how often one would happen to fall in the zone. For our system, a planet falls in the life zone over 90% of the time (about 92.4% actually). If our system was average in this sense, then the vast majority of extra-solar systems would have a terrestrial planet in the zone. Our own spacing would allow a few systems (about 8.5%) to have two earths in the zone. The fact that the two numbers add up to something very close to 100 is not mysterious; it simply follows from the fact that our life zone's width (0.39 AU) is about equal to our average planetary spacing in the terrestrial zone (0.38 AU). This is perhaps just a coincidence, and maybe not even that true, given our future refinements of life zone width estimates. But it may also be just another intuitive reason to believe that earths are a natural product of the cosmos.

Such reasoning and the perusal of many computer-generated arrays has led researchers to estimate varying numbers for the amount of earth-like worlds. Planets do form and almost always one falls in the ecozone, but other concerns (axis inclination, mass, orbital eccentricity, and period of rotation) moderate many of the guesses. Depending particularly on what the model used says about planetary mass, estimates made upon widened (non-Hart) life zones would place earthlike planets with all the proper characteristics in the zones between one-third and two-thirds of the time for stars very much like the sun. Because most of the suitable stars will be smaller, perhaps calling for generally smaller planes as well, the odds may drop. Stephen Dole drops them by a factor of ten (to 1 earth in every 200 stars in the disk); Martyn Fogg drops them by a factor of fifty (1 in 1,000 stars); and the "Hart school enthusiasts" of Bond and Martin drop them by a factor of five hundred (1 in 6,000 to 12,000 stars). Bond and Martin, and even Fogg, used modified Hart models and their estimates would seem too low. Dole seems more legitimate and perhaps his guess is best for the moment (see Fogg 1986ab, for comparisons). If there are more determinant factors ensuring proper mass contents for terrestrial planets near the life zones (and other orbital characteristics), then the following more optimistic estimate by Sebastian von Hoerner of the National Radio Astronomy Observatory could well be true:

Some astronomical estimates show that probably about 2 percent of all stars have a planet fulfilling all known conditions needed to develop life similar to ours. If we are average, then on half of these planets intelligence has developed earlier and farther, while the other half are barren or underdeveloped s13quoted in Ridpath 1975.