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Assuming the above volume to be typical, the contents of any other reascnable volume can be found by varying the number of stars proportionately with the volume, or with the radius cubed, S, = 22 x ( r _)5, where Sg is number of eligible stars and r is the radius 16 of the volume in light years. (This formula should only be used for radii creater thah 16 lisht years. For smaller samples we call for a recount. For example, only one known eligible star other than the Sun lies within eizht light years), Having an estimate of the number of useable stars, it is now necessary to make a zuess as to the nhmber of habitable planets. We have only one observed sample, the Solar System, and the pruess must be made with low confidence, since intelligent lifé may not be randomly distributed at all. The Sun has nine planets, arranged in a fairly regular progression of orbits (see reference 1, Appendix I) trat lends credence to theories that many stars have planets. Of the nine planets, (one, the “arth) is completely suitable for life. Two more (in adjacent orbits) are near misses: Mars has extremely rigorous livine conditions and Venus has an unsuitable atmosphere. Viewed very broadly indeed, this could mean that each star would have a series of planets so spaced that one, or possibly two, would have correct temperatures, correct moisture content and atmosphsre to support civilized life. let us assume that there is, on the average, one habitable planet per elirible Star. There is no line of reasoning or evidence which can indicate whether life will, actually develop on a phanet where the conditions re suitable. Here again, the Earth may be unique rather than a random sample. This writer can only inject some personal intuition into the discussion with the view that life is not unique on Earth, or even the random result of a low probability, but is practically insggltable in the right conditions. This is to say, the number of inhabited planets is equaz to those that are suitable! One more item needs to be considered. Knowing nothinz at all about other races, we must assume that Man is avera:6 as to technical advancement, environmental difficulties, etc. That is, one half of the other planets are baBhind us md have no space travel and the other half. are shead and have various levels of space travel. We can thus imagine that in our satmmle volume threre are 11 races of beings who have berun space exdlorations. The formula on page 3 above now becomes Be dae x Gr) 16 where R is the mumber of races exploring space in a spherical volure of radius mr. =S. 16 lignt years. Arguments like those applied to. Martians on pate 2 need not apply to races from other star systems. Instead of being a first port of call, Farth would possibly be reached only after many centuries of de- velopment and exploration with space ships, so that a visiting race would be expected to be far in advance of Man. 31 Declassification Authority: NND 57565