Illustration of 51 Pegasi b circling its star. Twenty years ago it was the first exoplanet detected; this year, it was the first seen directly by its reflected light. (Credit: ESO/M. Kornmesser/Nick Risinger) Twenty years ago today, an invisible object circling an obscure star in the constellation Pegasus overturned everything astronomers knew about planets around other stars. No, the fallout was even bigger than that. The indirect detection of 51 Pegasi b—the first planet ever found around a star similar to the sun—revealed that they had never really known anything to begin with. At the time, even the most adventurous minds blithely assumed that our solar system was more or less typical, a template for all the others. 51 Peg b threw a big splash of reality in their faces. The newfound world was bizarre, a Jupiter-size world skimming the surface of its star in a blistering-fast “year” that lasted just 4.2 days. Its existence ran counter to the standard theories of how planets form and evolve. It answered one big question: Yes, other planetary systems really do exist. But it raised a thousand others. Michel Mayor still sounds giddy as he recalls the profound confusion sparked by his discovery. In the fall of 1994, he and Didier Queloz, his graduate student at the University of Geneva, were trying out a brand-new spectrograph they had built, called ELODIE. It was designed to split up light in a way that can reveal the very precise motions of stars. Mayor and Queloz used ELODIE to search for subtle back-and-forth stellar dance steps caused by the gravitational tug of a planet or brown dwarf (bigger than a planet but not big enough to shine) orbiting around it. They succeeded almost right away, but the signal they detected from the star 51 Peg was like nothing they anticipated.

Left: Michel Mayor and his post-doc, Didier Queloz, in 1995 when they found 51 Pegasi b. Right: The ELODIE spectrograph that ferreted out the planet. Doesn't look like much, but it sure did the job. (Credit: Mayor) “It was so strange that we decided to wait until the next observing season,” Mayor recalls. The motion of the star was far too fast and too extreme to be explained by a planet that was at all like the ones in our solar system…and yet the signal was undeniably there. After another round of observations yielded the exact same result, Mayor and Queloz wrote up a paper—only to have it rejected by one of the reviewers at Nature, who found its claims too outlandish. The editor in chief interceded, Mayor popped a bottle of champagne, and on October 6, 1995, he announced his findings at an Italian astronomy conference, to a chorus of amazement and incredulity. I shouldn’t sound so judgmental. It’s easy to forget now, but twenty years ago there was no convincing evidence—none—of planets around other sunlike stars. There were just science-fiction fantasies, a handful of discredited claims, and some broad-brush inferences based on the only example scientists had to draw on: our own solar system. (The sole clue about the true diversity of planets was the discovery, three years earlier, of three small objects circling a pulsar, the dead remnant of a supernova explosion—a finding so bizarre that researchers are still puzzling over it.)

...and that's how you find a planet without seeing it. At left, a starlight spectrum from ELODIE. At right, the measured motions of the star 51 Pegasi, which betrayed the pull of an invisible planet. (Credit: Mayor/Queloz) Mayor and Queloz opened the floodgates. They showed exactly how to detect planets, and what to look for; they proved, right out the gate, that many of those alien worlds are nothing like the ones in our own solar system. By 2000, astronomers had found dozens of such “exoplanets,” using an expanding set of search techniques. A European satellite called CoRoT pioneered the approach of looking for transits, miniature eclipses created when a planet passes in front of its star. Then in 2009, the Kepler Space Telescope took this technique and pushed it much further, to stunning effect. Since its launch, Kepler has dominated the planet-hunting business, overwhelming the number of worlds that Mayor and his colleagues could find from the ground. The current tally lists more than 5,400 likely exoplanets, the majority from the Kepler dataset, including about 1,600 that have been confirmed by follow-up observations. Those follow-ups are crucial, though, because Kepler is great at detections but lousy at analysis. It is, as Mayor puts it, a machine of “fantastic statistics.” Making sense of those stats is long, hard work. But that hard work has paid off beautifully. Continuing in the tradition of 51 Peg b, the exoplanet catalog now contains a dazzling variety of unfamiliar types of worlds: evaporating planets, puffy planets, diamond planets, backward-orbiting planets, and planets orbiting double stars—just like Tatooine in Star Wars, as many fans have eagerly pointed out. In 2004, Mayor’s team discovered a whole new class of planets called super-Earths: rocky worlds that are up to 10 times as massive as our own, and potentially habitable. Super-Earths turn out to be the single most common kind of planet around other stars. “It is very strange to see this huge population,” Mayor says, “since we do not have any planet in this range in the solar system.” Mayor’s pioneering work in planet hunting has been somewhat obscured over the years by the onslaught of discoveries from Kepler and by a growing number of competing teams. His modest nature and richly accented English surely do not help. But if Mayor feels in any way slighted, he certainly does not show it. When I speak with him, he expounds as effusively about his current research as he does about the initial groping efforts that led him to 51 Peg b.

Nearby "super-Earth" planet HD 219134b passes in front of its stars, casting a slight shadow. This transit (mini-eclipse) will make it possible to study its atmosphere and composition in detail. (Credit: NASA/JPL-Caltech) Just in the past year, Mayor and his team have made two more notable contributions to the exploration of worlds around other stars. In April they revisited 51 Peg b and this time were able to detect the reflected light of the planet itself. That is the first time anyone has observed a direct reflection off the cloudtops of an exoplanet—an incredible achievement given that the nearby star is millions of times brighter. Then in July, Mayor’s collaborators found a planet called HD 219134b, one of the closest super-Earths. It is located just 21 light years away, which will make it a valuable object of study for future space telescopes. What Mayor and his now-sizable mob of colleagues have not yet managed to track down is the thing they want the most: Earth’s twin, another warm, wet, and welcoming planet that can (or does) support life. Filling in the landscape of exoplanets has been the great adventure of the last twenty years. Finding that one special place on the cosmic map is the great quest for the year 2035. How will we do it? I’ll look at the plan of attack in my next post. For more astronomy and space news, follow me on Twitter: @coreyspowell