Are humans alone in the universe, or is there intelligent life out there somewhere? Scientists have taken another look and found that recent discoveries of exoplanets combined with a broader approach to the question makes it possible to assign a new empirically valid probability to whether any other advanced civilizations have ever existed.
"The question of whether advanced civilizations exist elsewhere in the universe has always been vexed with three large uncertainties in the Drake equation," said Adam Frank of the University of Rochester and co-author of the new study. "We've known for a long time approximately how many stars exist. We didn't know how many of those stars had planets that could potentially harbor life, how often life might evolve and lead to intelligent beings, and how long any civilizations might last before becoming extinct."
NASA's Kepler satellite and other planetary searches have shown that roughly one-fifth of stars have planets that are in the "habitable" zone. This zone is where temperatures could potentially support life as we know it. But more interesting is the question of how long civilizations might survive on possibly habitable planets.
"The fact that humans have had rudimentary technology for roughly ten thousand years doesn't really tell us if other societies would last that long or perhaps much longer," Frank said.
In this latest study, the researchers decided to focus on the question: Are we the only technological species that has ever arisen?
The scientists calculated the odds against other advanced civilizations arising in the entire history of the observable universe. While this doesn't tell the researchers how many may be out there, it does tell them how likely it is that a technological species will evolve on a given habitable planet.
So, what did they find? It turns out that human civilization is likely to be unique in the cosmos only if the odds of a civilization developing on a habitable planet are less than about one in 10 billion trillion, or one part in 10 to the 22nd power.
"One in 10 billion trilling is incredibly small," Frank said. "To me, this implies that other intelligent, technology producing species very likely have evolved before us. 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 happened here on Earth with humanity has in fact happened about 10 billion other times over cosmic history!"
The findings were published in the April 22 issue of the journal Astrobiology.