One of the most vexing mysteries of the universe is whether or not we're alone. There are potentially hundreds of billions of planets in the Milky Way alone. How likely is it that life is out there?
A new equation calculates the probability of other technological civilisations evolving, and has found that it's wildly unlikely we're the only time advanced society has appeared.
Adam Frank from the University of Rochester and Woodruff Sullivan from the University of Washington based their new equation on the Drake equation, used for calculating the probability of extraterrestrial civilisation, written by astronomer and astrophysicist Frank Drake in 1961.
By accounting for new knowledge gleaned from the Kepler missions, Frank and Sullivan's calculations were much more accurate.
"Rather than asking how many civilisations may exist now, we ask 'Are we the only technological species that has ever arisen?'" Sullivan said in a statement. "This shifted focus eliminates the uncertainty of the civilization lifetime question and allows us to address what we call the 'cosmic archaeological question' -- how often in the history of the universe has life evolved to an advanced state?" for a more accurate calculation.
We also now know, thanks to Kepler, that approximately one in five stars have planets in the habitable zone, a number that Drake originally had to guess.
Frank and Sullivan calculated that human civilisation is only unique if the odds of a civilisation developing on a habitable planet are less than one in 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," Frank said.
So why haven't we found them yet? Well, it's entirely possible they're just too far away, and that advanced civilisation can only survive a very short time.
Humans have only been around for about 200,000 years, and civilisation as we know it only 6,000 years. And radio transmission technology isn't even 200 years old. By the time we sent a transmission to another planetary system 100 light-years away, it would be another 200 years before we received a reply.
But while we may not be able to communicate with our neighbours, the new equation does have scientific and practical importance.
"From a fundamental perspective the question is 'has it ever happened anywhere before?' Our result is the first time anyone has been able to set any empirical answer for that question," Frank said. "It is astonishingly likely that we are not the only time and place that an advanced civilisation has evolved."