What interests many people is the possibility of finding an Earth-like planet, and many science fiction stories have been woven around the possibility of there existing a planet somewhere in the universe which is an exact replica of the Earth. There are too many variable quantities for this to be a possibility worth considering. What is possible, if planetary systems are common as they seem to be, is the existence of planets where the conditions are similar to conditions on the Earth and to which our form of life could rapidly adapt. If life had gained a foothold on such a planet, it is possible that life closely paralleling our own planet could have developed.

What sort of conditions is necessary for life as we know it to develop? First of all of course a suitable planetary body is essential. Given this, then two vital conditions must be satisfied. The temperature must neither be too hot nor too cold, since intense heat breaks down organic molecules and severe cold prevents activity from going on. Too much short-wave radiation also upsets living organisms. The other prerequisite is a suitable atmosphere sufficiently dense to give protection from radiation and meteorites and containing oxygen and water vapour in reasonable quantities.

658. This passage suggests that there

659. The hypothesis about the possibility of planets parallel to the Earth gets its strength from the fact that

660. The statement that "If life had gained a foothold on such a planet" means that

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