The Limiting Sizes of the Habitable Planets

The astrobiological problem of the occurrence of life in the universe is sdiscussed from the standpoint of the size and nature of planets upon swhich living organisms might arise. The conclusion is tentatively drawn sthat the most likely radius of a habitable planet lies between s10⁸ cm and 2 x 10⁹ cm. Conditions of temperature sand density also bear upon the occurrence of life; thus the moon and sMercury, although both fall within the range of favorable radii, are snevertheless believed uninhabited by indigeneous life.