Twenty years ago this month the universe became a richer, stranger and decidedly less lonely place. For centuries, visionaries ranging from Isaac Newton to Gene Roddenberry had speculated about planets orbiting other suns, analogous to the worlds of our solar system—but it was only speculation. Then in October 1995 Michel Mayor, an astronomer at the University of Geneva, and his graduate student Didier Queloz discovered company: the first known planet orbiting a sunlike star.
Technologically, Mayor and Queloz’s work was a tour de force. They used a sensitive spectrograph to break up the star’s light and measure its minuscule back-and-forth motion due to the gravitational yank of the unseen world circling around it. Conceptually, their work was a white-hot firebomb. The planet, known simply as 51 Pegasi b, is as massive as Jupiter but orbits its star 100 times closer. Its “year” is just 4.2 days long and its cloud tops broil at about 1,000 degrees Celsius. It is utterly unlike anything in our solar system, so strange that it forced a wholesale rethink of where and how planets form.
In the two decades since, Mayor and his colleagues have deployed a variety of techniques to discover nearly 2,000 more exoplanets, worlds of staggering diversity. Researchers now estimate there are tens of billions of planets similar to Earth throughout our galaxy. Although Mayor’s breakthrough work helped kick-start what was then a new field of exploration, it did not exactly make him famous. His work was quickly overshadowed by findings from larger teams and big-budget satellite missions—and by more quotable, native-English-speaking researchers. Nevertheless, when Scientific American caught up with Mayor he was in high spirits, cheerfully describing his historic moment as well as his ongoing, boundary-pushing exoplanetary searches.
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[An edited transcript of the interview follows.]
What were you expecting to find when you started searching for companions around nearby stars two decades ago?
An important thing to realize is that it was a sad time to search for planets. Gordon Walker and Bruce Campbell [at the University of British Columbia] had been searching for 10 years and concluded that there were no Jupiter-type planets orbiting solar-type stars. A second team, Geoff Marcy and Paul Butler [at San Francisco State University], mimicked that study and in August ‘94 they reported the same result. But we were not troubled by these negative results. We had started construction of a new spectrograph in ‘90 and were not about to stop.
Given those dismal results, what made you optimistic you would discover something?
In ’89, with our old spectrograph, we found an interesting object with 11 times the mass of Jupiter and realized, we are not far from detecting planets. Then in ’94 we had “first light” [inaugural observations] with our new spectrograph, ELODIE, at Haute–Provence Observatory in France. It was fantastic; it could measure stellar motions as small as 15 meters per second, 20 times better than we had with our old instrument. We decided to conduct a large survey of 142 single stars.
I applied for telescope time with Antoine Duquennoy, one of my postdocs, and Didier Queloz, one of my graduate students. We started our measurements, but then Antoine died in a car accident. Didier and I continued. After only a few months we had enough measurements of 51 Pegasi to see something very special, a periodic signal [back-and-forth motion of the star] of 150 meters per second. We fed our data into the computer and saw we had something going around 51 Peg with a period of 4.2 days, meaning it had to be in a very close orbit. This was a surprise, because at the time the idea was that giant planets had to be more than five AUs [five times the distance from Earth to sun] from their star. That was in fall of ’94.
Why didn’t you announce your big discovery until a full year later?
It was so unusual, so unexpected, that we decided to wait for the next season of visibility for 51 Peg. We wanted to be sure the amplitude of the variability was the same, the phase was correct and the period was the same [proving that it was a real planet in a stable orbit]. We didn’t have telescope time again until July 1995. We saw that all the parameters matched up—that was the time when we opened a bottle of champagne. On August 25, we submitted our paper to Nature. [Scientific American is part of Nature Publishing Group.]
51 Pegasi b went against all contemporary thinking about what a planet should be. Did you run into a lot of skepticism?
We were going to a conference on solar-type stars in Florence in October. Just before, I received information from Nature that only two of the three referees voted to accept the paper. It was up to the editor at that point. Fortunately, he decided to accept! Some people at the meeting were really intrigued: “Now we have to look for the reason why we have such a short-period planet.” Other colleagues were looking for arguments: “It’s not a real object, you don’t have enough precision.” But we were 100 percent sure of our measurements.
In contrast, it seems like the theorists were fully ready to embrace the idea that a Jupiter-size world could migrate extremely close to its star.
Yes, in fact the answer had already existed for 15 years. The first paper published on orbital migration by Peter Goldreich and Scott Tremaine—two very important men in astronomy—was devoted to the study of a small body embedded in a disk. The body could be a small galaxy in the disk of a large galaxy or a planet in an accretion disk [the dusty structure around a newborn star]. In the abstract of this paper the last sentence read: “Jupiter was not born where it is today.” That was in 1980.
Still, you must have wondered if 51 Pegasi b was a freak, or whether it was normal and our solar system was perhaps the oddball.
We had only 51 Peg, one object with an orbital period of four days. What would be the impact of a single discovery? Absolutely nothing. Things changed when Geoff Marcy went on his telescope to see if 51 Peg b is real. He realized our observation was correct, and then he and Paul Butler reanalyzed a lot of measurements they had accumulated during the previous years. On January 17, 1996, [at the American Astronomical Society meeting] in San Antonio, Texas, they announced two new exoplanets. Several more objects with short periods were discovered in the first six months of ’96. It was only after we discovered a lot of other planets that we realized 51 Peg b is actually a normal object.
And yet the later discoveries by other teams also overshadowed your early work, at least in the English-speaking media.
It’s true, we don’t have so many callers from the U.S. I’m just sorry my English is so bad.
You remained very active in exoplanet research. What were your most exciting discoveries in the years after 51 Pegasi b?
A very important step was our new spectrograph, HARPS, at La Silla [Paranal] Observatory in Chile. It has a precision of about three meters per second [the smallest back-and-forth stellar motion it can measure]. We’ve improved by a factor of 1,000 in 30 years. Starting in 2004, HARPS [for High Accuracy Radial velocity Planet Searcher] has detected a population of super-Earths, objects with a mass between one and 10 times the mass of Earth. We do not have any planets in this range in our solar system but they are extremely frequent around other sunlike stars. We built another version of HARPS on La Palma in the Canary Islands for the northern sky. This past July HARPS–North found HD 219134 b, one of the closest super-Earth planets, just six parsecs [about 20 light-years] away.
What about the ultimate goal: directly observing a true Earth twin around another star?
Detecting an Earth-type planet that’s habitable is really tough. Earth is a small planet. It will take a new generation of instruments on the new, bigger telescopes [like the Giant Magellan Telescope and Thirty Meter Telescope] to have a chance to observe them directly. To learn about the physics of Earth-like planets—atmospheres and such—we need close, bright stars. At some point, people will try to build a spacecraft to make direct images of other Earths. They will have to know which are the interesting stars to look at. My ambition now is to set up such a list. The planet we announced in July—HD 219134 b—will be a very important target for the future.