Are we alone in the universe?

Measured edge to edge, the universe as we know it stretches some 93 billion light-years across. That unfathomable expanse contains 2 trillion galaxies, each shining with millions of stars and dotted with more planets than you can imagine. Given all that real estate, it seems unlikely we're alone. Yet in all of human history, we've found nothing to suggest otherwise.

Scientists who have spent their careers searching for any sign of an otherworldly civilization concede it’s possible we’ve got the cosmos to ourselves. Still, they highly doubt that’s the case. “To say this is the only place where there’s any intelligence is hubris of a very high order,” says astrophysicist Seth Shostak of the SETI Institute. (The acronym stands for “search for extraterrestrial intelligence.”) Statistically speaking, there are too many locations where life could thrive for humanity to be an anomaly.

Astronomer Frank Drake suggested as much in 1961. He posited that the number of technologically advanced civilizations in our galaxy would be the product of seven variables. They include the number of stars throughout the Milky Way, how many of those bright, burning gas balls illuminate planets, and what percentage of those worlds could support life. His eponymous equation was a thought exercise meant to start a discussion among colleagues, but it has helped frame the topic in the years since he wrote it.

Many of Drake's variables are speculative, making his math little more than conjecture. But astronomers now know for certain that exoplanets, many of which could harbor life, form throughout the Milky Way like dust bunnies under your couch. In the past two decades, researchers have confirmed the existence of more than 4,000 planets in our galaxy, a finding that suggests the cosmos all but brims with them. Astrophysicist Christopher Conselice of the University of Nottingham puts the number at 100 quintillion. That's a one with 20 zeros. Some think there may be far more than that.