The Biggest Questions: Are we alone in the universe?

In 1977, the New York Times published an article titled “Seeking an End to Cosmic Loneliness,” describing physicists’ attempts to pick up radio messages from aliens. The endeavor, known as the Search for Extraterrestrial Intelligence (SETI), was still in its early stages, and its proponents were struggling to persuade their peers and Congress that the idea was worth funding.

The quest to determine if anyone or anything is out there has gained greater scientific footing in the nearly half-century since that article’s publication. Back then, astronomers had yet to spot a single planet outside our solar system. Now we know the galaxy is teeming with a diversity of worlds. Our planet’s oceans were once considered exceptional, whereas evidence today suggests that numerous moons in the outer solar system host subsurface waters.

Our notion of the range of environments where life could exist has also expanded thanks to the discovery on Earth of extremophile organisms that can thrive in places far hotter, saltier, acidic, and more radioactive than previously thought possible, including creatures living around undersea hydrothermal vents.

We’re now getting closer than ever before to learning how common living worlds like ours actually are. New tools, including machine learning and artificial intelligence, could help scientists look past their preconceived notions of what constitutes life. Future instruments will sniff the atmospheres of distant planets and scan samples from our local solar system to see if they contain telltale chemicals in the right proportions for organisms to prosper.

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The current best estimates suggest that anywhere between 10% and 50% of sun-like stars have planets like ours, leading to numbers that make astronomers’ heads swim.

“If it’s 50%, that’s bonkers, right?” says Jessie Christiansen, an astrophysicist at Caltech in Pasadena, California. “There are billions of sun-like stars in the galaxy, and if half of them have Earth-like planets, there could be billions of habitable rocky planets.”