Terrestrial life is delicately balanced and due to accidental factors. Therefore, the probability of planets that can be colonized without elaborate aids is low. Agreed.
Necessary:
an oxygen atmosphere
the right radiation and temperature ranges
enough gravity to keep the air
not enough gravity to upset body fluid adjustments
microscopic life to support humanly nourishing vegetation and domestic animals
nitrogen-fixing bacteria
saprophytes
earthworms
enough duplicated vitamins
Extraterrestrial life is probably either inedible or poisonous. Domes and tank food support only a few. Natives, if present, resist invasion.
1 comment:
Kaor, Paul!
I hope we do find some extra-Solar planets which meets enough of the parameters you listed to be habitable for mankind (and without intelligent races of their own). Altai, in "A Message in Secret," is probably about as cold as human beings would find tolerable. And the gravity of Imhotep, in THE GAME OF EMPIRE, seems to have been as high as Anderson thought humans could tolerate, long term (30 percent greater gravity than on Terra).
Sean
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