How do we commence looking for other habitable planets in the local Galaxy ? Objects of planetary size at the distances of the closest stars can not be detected optically, even under the most favorable conditions with the use of the Palomar Observatory's 200-inch telescope, the world's largest "eye" on space.
The present approach will be as follows: wherever possible, use will be made of available factual material and known relationships; where gaps exist in our knowledge, reasonable inferences will be based on models constructed from observations of conditions in the solar system. Some dependence on theory will be necessary where no direct data exist; all known physical and chemical laws must be obeyed elsewhere in the universe.
In the past, many serious writers have speculated about the existence of life on other planets—Percival Lowell (1908), H. Spencer Jones (1949), Harlow Shapley (1959), and Hubertus Strughold (1955), to name a few. But, generally speaking, the subject has been covered in only a qualitative manner. It is the present objective to try to establish reasonable quantitative limits from which the prevalence of habitable planets can be estimated. In setting such an objective, it is necessary to define more precisely what is meant by a habitable planet. Several different categories of habitable planets can be imagined: planets that could support some unknown form of life; planets that could support microscopic forms of carbon-based life, which is life as we know it; and planets that could support various extreme forms of terrestrial life. However, in the present context, a habitable planet is defined as one suitable for human life. This sets aside any speculations about organisms living in seas of liquid ammonia or breathing gaseous sulfur or existing under any other such alien conditions. Further, the use of the term "habitable planet" is meant to imply a planet with surface conditions naturally suitable for human beings, that is, one that does not require extensive feats of engineering to remodel its atmosphere or its surface so that people in large numbers can live there.
Under the present plan we will first circumscribe the environmental conditions required by human beings. Then we will attempt to delineate the astronomical circumstances that produce these requisite environmental conditions. Finally, we will make an estimate of the probabilities that these astronomical concatenations will be found elsewhere in the Galaxy and where they might be found in the immediate neighborhood of the Sun.
It will be realized that many of the questions raised here can not be answered definitively at the present time because of deficiencies in our knowledge of the universe. Therefore many of the conclusions made here must be tentative—based on premises and assumptions that, although believed to be reasonable within our present state of knowledge, may eventually be proved incorrect.
In the spirit of this study, then, conclusions based on clearly labeled assumptions will sometimes be drawn. One of the methods of approach used most frequently will be based on the conviction (based on numerous bits of evidence) that our solar system is not an extraordinarily rare assemblage of bodies, but is rather a typical planetary system, and that its members can be treated as a good, although not numerically large, sample of the types of bodies that exist in proximity to other stars. Every effort will be made to supply a reasonable answer to the following question: In full recognition of the incompleteness of our knowledge, what can now be said regarding the prevalence of habitable planets in our Galaxy ?
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