Aliens

For more than 60 years, scientists have been on the stealthiest stakeout in history. Using state-of-the-art listening devices, they’ve tapped into thousands of homes, waiting patiently for their targets to reveal their presence, and ultimately been rewarded with silence. If this covert activity were occurring in our own country, one might have become impatient and simply walked up and knocked down the door. But the targets are not living in our country, or even on our planet or in our solar system. Our persons of interest are aliens. This is Seti: the Search for Extraterrestrial Intelligence, focused on life on planets orbiting other suns. So far, we’ve found no sign of them.

Last week, a team of astronomers led by the University of Cambridge professor Nikku Madhusudhan announced that they had found tentative evidence for a ‘biosignature’ embedded in the light from a distant planet. Scientists and non-scientists around the world tried to interpret the results. Was this it? Was this the moment when humanity could finally claim it had answered that ancient question: are we alone? As an astrophysicist involved in the search for life beyond Earth, I can tell you that the results were not that moment. But that doesn’t make them any less exciting.

Over the past three decades, astronomers have discovered planets orbiting Sun-like stars throughout the universe. This discovery ended 2,500 years of debate about whether worlds existed beyond our solar system, but it came with a shock. The most common kind of planet in the universe is the type of world that doesn’t exist in our small corner of the cosmos: what astronomers call ‘Super-Earths’ and ‘Sub-Neptunes’, planets with much greater masses than ours and which could, in theory, sustain life. Astronomers concluded a little over a decade ago that every star in the night sky hosts a family of worlds. Importantly for the search for life, one in five of those stars will have a planet in the ‘Goldilocks’ zone (sometimes referred to as the ‘habitable zone’).