The existence of extra-terrestrial life is one of the most compelling debates in science. When we ask "Are we alone?", we are wondering about how life itself begins, and what our place and function is in this universe.
A new paper, published in Astrobiology, uses the most recent discoveries of exoplanets to estimate the likelihood that other technologically advanced civilizations have ever existed. The authors’ pessimistic limit is that humanity is unique if the odds of an advanced civilization existing on a planet is less than one chance in about 10 billion trillion.
“One in 10 billion trillion is incredibly small. To me, this implies that other intelligent, technology producing species very likely have evolved before us,” said Adam Frank, lead author of this study, in a statement.
“Think of it this way. Before our result, you’d be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet were, say, one in a trillion. But even that guess, one chance in a trillion, implies that what has happened here on Earth with humanity has in fact happened about 10 billion other times over cosmic history [owing to the estimated number of stars and planets in the universe]!”
This probability was constructed from the famous Drake Equation, a probabilistic argument to how likely or not intelligent life is. The Drake equation uses seven probabilistic parameters to obtain an estimate of the number of civilizations in the Milky Way. The purpose of it was never to provide a precise number, but to stimulate the debate about alien life.
The Drake equation on top, and the version employed by Frank & Sullivan on the bottom
In the paper, Frank and his colleague Woodruff Sullivan simplified the equation significantly. The number of advanced civilizations is equal to the number of habitable planets in a given volume of the universe multiplied by the likelihood of a technological species arising on one of these planets.
The likelihood is obviously a matter of guesswork, but by expecting humans to be alone, they were able to provide their pessimistic limit.
“From a fundamental perspective, the question is ‘has it ever happened anywhere before?’” said Frank. “Our result is the first time anyone has been able to set any empirical answer for that question and it is astonishingly likely that we are not the only time and place that an advance civilization has evolved.”