The Seti Institute in California uses an array of radio telescopes to look for signals from intelligent extraterrestrials. Meeting as part of its science advisory board last month, I heard director Jill Tarter insist that its work was based on rational science, not “faith”. I disagreed. “You can’t prove yet that alien life exists,” I explained. “But you must have faith that it’s there, despite the lack of evidence; that’s why you keep looking.”

Extraterrestrial life is an example of something we believe ought to exist, even without prior evidence. We’ve recently had success looking for such things – the Higgs boson and gravitational waves to name two. Those searches required fabulously complex and expensive detectors and huge teams of scientists to find things we really weren’t sure were actually there.

More often in astronomy, though, we believe in the things we cannot see, because they affect the things we can see. Indeed, almost all of the nearly 100 scientists at the Seti Institute actually work on these more tangible questions, such as finding planets around other stars and understanding how they might sustain life. We now know there are thousands of stars with systems of planets not too different from our own solar system. Recently, the star nearest to our Sun, Proxima Centauri, has been added to the much shorter list of stars with a planet comparable to Earth.