(A)This is the biggest question in the universe: are we alone? Philosophers have debated the ques-tion for millennium.When 16th-century Italian astronomer and Dominican friar Giordano Bruno declared that the cosmos contained “an infinity of worlds of the same kind as our own”, he was directly contravening religious dogma.He was later burned at the stake during the Inquisition, in part for daring to question Earth's unique status. 

(B)The debate continues,in more restrained fashion, to this day.For some, the sheer size of the uni-verse makes it unlikely that life formed only once.For others, the remarkable complexity of life on Earth is testament to its uniqueness. 

(C)Until recently, vague philosophical answers of this kind were the best science could do.The signs of life were far too ambiguous to pin down for certain, and our nearest potentially habitable worlds were too small and distant to test. 

(D)But for the first time in human history we are reaching the technological sophistication needed to provide a genuine answer.Powerful telescopes are letting us study planets in other solar systems,giving us a glimpse into their atmospheres and a flavour of what type of life might be living on their surfaces.At the same time, improved analysis of our own planet is allowing what life might look like from afar, and is helping us to distinguish the signs of a flourishing alien civilization  from the mere geological rumblings of a lifeless world.With these tools at our disposal, answers are finally within our grasp. 

(E)To understand my optimism, it is worth revisiting the work of astronomer Frank Drake.In 1961,Drake devised a formula to estimate how many advanced civilizations were capable of signaling their presence in the Milky Way.His eponymous equation depends on breaking down that big unknowable quantity into a number of more tractable ones that can be multiplied together, such as the number of stars in the galaxy and the fraction of those likely to have planets. 

(F)Even with pessimistic values, the existence of millions of technological civilizations seems likely.The main bottleneck on that apparent explosion of life, however, is in Drake' s final term:the av-erage lifetime of a communicating civilization.Humans have been broadcasting radio signals that escape into space for only about a century, and, in the current geopolitical climate, who is to say how many more years we have left.If you take the pessimistic assumption that intelligent life de-stroys itself rather quickly, the Drake equation suggests that statistically we are alone in the galaxy.If intelligent civilizations survive for millions, or even billions of years, however, then the Milky Way should be teenung with aliens. 

(G)This calls for optimism, but also caution.After all, if there are millions of alien civilizations out there, then why haven't we seen signs of them akeady? This seeming contradiction is sometimes called the Fermi paradox, afier Italian physicist Enrico Fermi, who gave it its most succinct ex-pression.With a back-of-the-envelope calculation, he showed that a single space-faring civiliza-tion could easily colonize a galaxy within a hundred million years.Because the universe is 13.8 billion years old, and no interstellar colonists have yet appeared on our horizon. 

(H)Perhaps, say some, the aliens are already here, just keeping their identities secret.Perhaps they are deliberately steering clear of Earth, treating it as a sort of cosmic heritage site that deserves their protection.Or alternatively, there are simply no aliens out there.As an astrobiologist, I pre-fer to believe that aliens are out there; we simply haven't communicated with them yet. 

(I)It isn't hard to imagine why this could be the case.Alien civilizations might well be millions of years ahead of us in their technological advancement.Trying to communicate with them using our primitive technology would be as absurd as teaching a ladybird to use a telephone.That hasn't stopped us trying, of course, whether by including artifacts, such as plaques etched with celestial maps and images of humans, on our long-distance spacecraft or by broadcasting target-ed radio messages into the depths of space.So far, no reply. 

(J)All hope is not lost.The Fermi paradox and Drake equation specifically deal with the question of intelligent life, with the ability to communicate, travel and colonize.But only a fraction of the life we know of would be capable of these feats.Today, the vast majority of Earth's biosphere con-sists of microbes.Single-celled organisms dominated the planet's surface for nearly 3 billion years before multicellular life began.What is more, microbial cells not only outnumber human cells on our planet, they even outnumber them on and in your body.If life exists elsewhere in the universe, chances are it is microbial.