A Dazzling Quarter Century of Exoplanet Discovery

An artist's illustration of exoplanet 51 Pegasi b
An artist's illustration of exoplanet 51 Pegasi bESO / M. Kornmesser / Nick Risinger

A pair of astrophysicists have won a Nobel Prize for discovering something that—as is often the case in science—at first they couldn’t believe was real.

Michel Mayor and Didier Queloz were awarded the prize in physics today for finding 51 Pegasi b, the first known planet orbiting another sun-like star. (They share the prize with the physicist James Peebles, for his theoretical work on the origins and nature of the universe.)

Mayor and Queloz discovered the gaseous planet in 1995, and in the years since, the field of exoplanets research has ballooned spectacularly. To date, more than 4,000 exoplanets have been found in the Milky Way galaxy. New discoveries are presented in batches, like an impressive haul after a good day of fishing in the universe. In barely a quarter of a century, finding an exoplanet has gone from a riveting achievement to a routine occurrence. There is no doubt today that our solar system is just one of countless others in the cosmos.

Nobel prizes in science are imperfect honors, long criticized for inadequately recognizing the work of some while entirely overlooking the contributions of others. But they do their job of honoring well-accepted science, and that’s what makes the award for Mayor and Queloz so exciting. The detection of exoplanets now seems ordinary to us; what might happen in the next quarter century? Perhaps the next Nobel Prize in this particular field will honor the discoverers of something even more momentous—not only an alien world, but the life that resides there.

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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.

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.