Read: A glowing clue in the search for alien life

Before 51 Pegasi b made itself known, astronomers had long suspected that other stars had planets of their own, guided by the Copernican principle that warns against thinking we’re anything special. The first-ever known exoplanet had been found a few years earlier orbiting a pulsar, a fast-rotating, very energetic stellar object. But researchers were also looking in particular for an object orbiting a sun-like star. Some astronomers had picked up intriguing signals in their telescope data that could have been exoplanets, but, erring on the side of caution, they hedged their findings or dismissed them. Astronomers had made triumphant declarations about exoplanets that were found to be wrong, felled by technical flaws masquerading as alien worlds.

The strange nature of 51 Pegasi b didn’t help. The planet most closely resembled Jupiter in mass, but orbited closer to its star than Mercury does to the sun. Astronomers had thought that gas planets like Jupiter form farther from the sun’s glare, but there was 51 Peg, as its discoverers call it, completing one scorching orbit in just four days. “It was absolutely not expected from theory,” Mayor, a professor emeritus at the University of Geneva, told me in an interview last year.

A different team confirmed the discovery, and the exoplanet bonanza proceeded from there, first with telescopes on the ground and then with spacecraft in orbit above Earth, like the Kepler Space Telescope, which ran out of fuel last year after spotting thousands of exoplanets.

Read: An exoplanet like no other yet found

The exoplanets vary in size, composition, and the time it takes them to loop around their home star. Some are gaseous like Neptune, others rocky with iron cores, like Earth. One class of planets checks off the most exciting boxes on astronomers’ wish list: They are rocky, about the size of Earth, and reside in the habitable zone of their stars, where the temperatures are just right for liquid water.

As the catalog swelled, scientists started to look closely at individual exoplanets, particularly at their atmospheres. The study of exoplanet atmospheres, too, might someday become routine. When two groups of astronomers last month announced the discovery of water vapor in the atmosphere of a distant planet, the juiciest news in the science community wasn’t the detection, but the jostling between the teams to publish their results first. In the past decade, astronomers have gotten quite good at finding a mix of familiar organic molecules around other planets. Some have gotten creative with the possibilities, proposing searches for the distinct signature of vegetation or the glow from ultraviolet-loving organisms.

Astronomers have yet to find the distinct atmospheric cocktail that matches our own, but the prospect no longer seems impossible, especially as more powerful observatories become available. This is how signs of life, if any exist, could be detected by sensitive instruments from Earth. If scientists do detect alien life on another world, the discoveries will likely begin the same way the first detections of exoplanets did—with doubt and hesitation, before someone can finally say, with reasonable certainty, that they have found the signature of another life form, far off in the cosmos.