Theoretical work on the origin of solar systems suggests that planets are a frequent, if not invariable, accompaniment of stars. If there are billions of planets, if the origin of life occurs readily under general cosmic conditions, and if there are many worlds much older than the Earth for evolution to work upon, why shouldn’t the galaxy be brimming over with life?
At the time Sagan delivered his Christmas Lectures in 1977, the only known planets were the ones in our own solar system. There was no evidence to suggest planets existed outside our solar system, or that there was a star other than our own sun producing planets. This was to change just over a decade later when the first evidence of planets orbiting a star was detected by radio astronomers in 1991.
In the last of his six Christmas Lectures, Carl Sagan explores the concept of solar systems outside our own, and asks if this were the case, how similar they might be to ours. With more than 450 extra-solar planets discovered in the past twenty years, Sagan’s final lecture serves as a reminder of how far we’ve come in our understanding of what exists beyond Earth.