How do we know we are not alone?
If you ask a scientist whether they believe in aliens you might be surprised by the answer. Scientists typically believe in extraterrestrial life - though out there, in space, not buzzing around our planet and scaring the locals. The Search for Extraterrestrial Life (SETI) is a respectable research area. What makes scientists sure that it's a worthwhile cause?
Well, they aren't sure, but they strongly suspect. We're here, for a start, so intelligent life is definitely possible. And while we like to think of ourselves as special, we probably aren't. There's no reason to think intelligent life hasn't evolved in other places too.
SETI instigator Frank Drake put this notion on a more formal footing back in the 1960s with what has become known as the Drake equation.
This equation combines numerical estimates of the factors involved in the evolution of advanced civilisations to arrive at an estimate of the number of detectable civilisations in our galaxy. It includes the rate of star formation, the proportion of stars that have planets, the number of habitable planets those stars have, the likelihood that a planet will develop life, the probability that life, once it appears, will evolve intelligence, the proportion of intelligent civilisations that will emit measurable radio signals and the length of time such civilisations last.
The Drake equation is more of a conceptual guide than a practical formula since many of these factors are difficult to put a figure to.
Some of the more astrophysical factors are amenable to measurement, like the rate of star formation and the proportion of stars with planets. When Drake originally formulated his equation, he estimated that few stars have planets - at the time many people thought our solar system might be rare, or even unique. This has been revised sharply upwards since the discovery that planets are, in fact, common.
But other factors are just guesswork. We only know of one planet (ours) that has developed life and intelligence, and it's hard to generalise from a sample of one. Even if we discover life on Mars, Titan or Europa - supporting the idea that life, too, is common - there are still several factors we have to basically guess. Drake supposed that if the right planetary conditions existed then an advanced civilisation was bound to appear.
Combined with all the other factors, that would suggest there are thousands or even millions of detectable civilisations in our galaxy alone. It's just a hypothesis, but a hypothesis that most scientists agree with. They don't know that it's true - but nothing has so far proven it false.
Which is why scientists aren't unsettled by the paradox posed by physicist Enrico Fermi in the 1950s: if alien life is common, as we expect, why haven't we heard from them yet? It's a good question, but there are good answers, too.
Maybe we're looking in the wrong places, listening on the wrong frequencies, missing signals in the background noise of the universe.
Plus, we've only been listening a few decades. We'll need to listen a lot more before the Fermi paradox trumps the Drake equation.