Well, scientists don’t like to use the term “aliens,” but that is a decent summary. In SETI—the search for extraterrestrial intelligence—one normally thinks about the actual problem of communication. And so you want to know how many are simultaneously existing.
We are saying that the Kepler mission, which has discovered so many exoplanets around other stars, allowed us to nail down several factors in the Drake Equation so that we could ask a different question: how many technological species like us have ever existed. We now can reliably estimate, for example, that in our Milky Way alone there are about 60 billion habitable planets.
It’s a philosophical thing to be concerned about how many past civilizations have occurred. You could think of it as a sort of cosmic archaeology. If you could travel around to planets not just in our own galaxy but in other galaxies—like an archaeologist goes around the world looking for evidence—we’re saying that it seems likely that you’d find evidence of these guys all over. And wouldn’t that be interesting?
We can now estimate how many habitable planets there are in the universe. But we don’t know, given a habitable planet, how often life begins and evolves to technology. Now if you want to posit that there’s never been a second such civilization in our entire universe, how pessimistic must you be? The answer is that you have to think that, given a habitable planet, the odds of a technological species emerging are worse than one chance in 10 billion trillion. That’s only one chance in 10,000,000,000,000,000,000,000!
This shows the degree of pessimism that you must have to say that we are unique over the entire 13 billion year history of the universe.