Liquid Water on a Planet over Cosmic Periods

Abstract

From the equation that determines the average temperature of a planet, it is readily seen that even small changes in the distance, albedo and greenhouse effect of the planet, if not properly compensated, can elliminate liquid water from the surface of the planet either through runaway glaciation or through a runaway greenhouse effect. Since active life, at least as we know it, requires the presence of liquid water, and since its evolution to higher intelligence seems to require billions of years, it follows that the ability of planets to maintain liquid water over cosmic periods is probably the critical factor that determines the abundance of advanced civilizations in the Cosmos.