Venus today is far too hot to sustain surface liquid water or any biology requiring it, but did Venusian life exist in the past? Nearly all remains of past life would have been destroyed by now, but it is plausable–and perhaps likely–that Venus once boasted Earth-like oceans. This possibility exists because the sun may not have been as bright in the past.

Models of stellar evolution predict that a star brightens by about 30% over it’s main sequence lifetime due to core contraction as the star fuses hydrogen into helium. The inference of warmer-than-present temperatures for the early Earth despite the reduction in solar luminosity is known as the faint young sun paradox (one of my research interests). Earth seems to have sustained both liquid water and life for nearly 4 billion years, continually maintaining habitable temperatures.

So what happened to Venus? Eventually the surface received enough energy to evaporate the oceans and enter a positive feedback known as a runaway greenhouse. Today, we see a cloudy world with sulfuric acid rain and a surface hot enough to melt lead. But 4 billion years ago life could have arisen in the Venusian oceans and perhaps even lasted long enough to develop intelligence.

I can imagine a Venusian scientist carefully working out models of stellar evolution–only to find that their parent star was slowly and steadily brightening! We look back in time and find a faint young sun paradox, but the Venusians would have a contemporary warming sun catastrophe! The greatest climate disaster in the history of our solar system may well have been when Venus entered its runaway greenhouse state to become uninhabitable.