The Sizes of Habitable Planets

Abstract

In order to obtain some idea of the sizes of habitable planets, it is necessary first to examine the conditions under which life may emerge. Authorities generally agree that coacervates in the aqueous solution represent the earliest stage of evolution of organic substances into living organisms. 1 Therefore a habitable planet must have a solid crust and be able to hold water gravi-tationally. Indeed, the existence of living organisms without the support of some substances in the liquid form is quite inconceivable. Since stars and planets are formed out of the interstellar medium, it is likely that in the early stages of evolution a planet (or more properly a proto-planet) will have a high percentage of hydrogen, especially in its outer envelope. 2 Life may first appear under reducing conditions, 1 but an atmosphere composed mainly of hydrogen does not favor the development of a high form of life. Thus, the hydrogen atmosphere must first be at least partially dissipated. 3 The present atmosphere of the earth is actually a secondary one, formed as a result of chemical processes that took place subsequent to the formation of the planet. 4 We therefore impose the condition that for a planet to be habitable, the hydrogen atmosphere must be lost completely or partially in a time short compared with that of stellar evolution. For the stars most likely to support life in their vicinity, the evolution time is of the order of 10 9 to 10 11 years. 5 With this range of values we can derive an upper limit for the total mass of a habitable planet because a very massive planet (Jupiter, for example) will retain its hydrogen atmosphere too long. 3 On the other hand, for life of an advanced form to arise, the secondary atmosphere must not be lost before living organisms have evolved. The timescale of biological evolution on the earth is of the order of 3 X 10 9 years. This condition gives us a lower limit to the mass of a habitable planet. We would like to emphasize that in discussing the dissipation 489