Our civilization on Earth resides around a yellow dwarf star, but orange and red dwarf stars are much more numerous in the galaxy. Does this suggest that we should orient the search for extraterrestrial intelligence (SETI) toward these smaller and redder star systems? Or are certain star types better abodes for intelligent life than others?

Myself and Ravi Kopparapu address these questions in a chapter titled, “The Drake equation as a function of spectral type and time,” which appears in the book Habitability of the Universe Before Earth. Our approach invokes the Drake equation, which is a probabilistic device for estimating the number of communicative civilizations in the galaxy. The conventional terms of the Drake equation are the rate of star formation, the fraction of stars with planets, the number of habitable planets per system, the probability of life, the probability of intelligence, the probability of communicative technology, and the average lifetime of such civilizations. The product of all these terms yields the number of civilizations in the galaxy that SETI could conceivably discover.

Our book chapter analyzes calculations of the liquid water habitable zone for plants orbiting yellow, orange, and red dwarf stars. These habitable zone calculations provide constraint on the terms of the Drake equation. In particular we consider the dependence of each term of the Drake equation on both the stellar type (color) of the star, as well as the change in each parameter over time since the galaxy formed. We suggest that the habitability of red dwarf systems may peak in the far future, while the present time is optimal for habitability around yellow and orange dwarf stars.