Geoff Marcy remembers the hair standing up on the back of his neck. Paul Butler remembers being dead tired. The two men had just made history: the first confirmation of a planet orbiting another star.
The groundbreaking discovery had been announced less than a week earlier by the European team of Michel Mayor and Didier Queloz. But the news was met with some initial skepticism in the astronomical community. By a stroke of good luck, Marcy and Butler happened to have previously scheduled observation time on a 120-inch telescope at the Lick Observatory, atop California’s Mount Hamilton.
The scientists, who would become two of the world’s most famous planet hunters, remember driving down the mountainside together in October 1995. They’d spent four straight nights making their observations. And while further processing would be needed to make the scientific case, their data seemed clear and unmistakable – and almost impossible. A huge planet, at least half the size of Jupiter, was not only orbiting its host star more tightly than Mercury hugs the sun. It was racing around that star, making a complete orbit in just four days.
The planet, called 51 Pegasi b, would open a new era in humanity’s exploration of our galactic neighborhood. It would be the first in a series of “hot Jupiters” – giant planets in fast, tight orbits – discovered in rapid succession. The rush of new worlds would propel Marcy, Butler and their research team into the media spotlight, and forever change our view of the cosmos.
But for the moment, on that solemn drive down the mountainside, Marcy and Butler were alone with their world-altering news. “We knew we were the only people on the planet to be sure that 51 Peg, the planet, really did exist,” Marcy said recently. “It was exhilarating. We were absolutely thrilled to know an historic moment in science history was happening before our eyes. It was a truly spine-tingling experience.”
Still, the astronomical pioneers had a few struggles ahead to gain the acceptance of the scientific community. The hunt for extrasolar planets – exoplanets, for short – had a poor track record, with decades’ worth of false detections. Among them was the thrilling discovery of a planet orbiting Barnard’s star in the 1960s; it turned out to be an unnoticed shift of a telescope lens. Once the shift was accounted for, the “planet” disappeared.
The early ‘90s had seen the actual detection of “pulsar planets,” but these seemed too strange to count, orbiting a rapidly spinning, radiation-spewing stellar remnant called a pulsar. Most scientists would reserve the “first” designation for a planet orbiting a normal star.
“The whole field had a snake-oil sort of feel to it,” Butler said in a recent interview. “For the previous fifty years or so, there were many announcements, all proved to be wrong. If we went to a meeting and said we were looking for extrasolar planets, we might as well have said we were looking for little green men.”
Even Marcy greeted the announcement of 51 Peg, made at a scientific conference in Florence, Italy, by Mayor and Queloz, with a bit of a yawn – at first.
“This claim on October 6, 1995, of the first planet ever discovered was sort of business as usual,” he said. “Here’s another false claim. This one is more obviously a false claim. The orbital period is claimed to be 4.3 days. Nobody in their right mind thought planets could orbit so close to a star.”
But the four nights of observations at the Lick Observatory – perfectly coinciding with 51 Peg’s four-day orbit – changed all that. Both the Mayor and Marcy teams had been trying to develop a planet-hunting technique based on wobbling stars. The wobbles, known as the star’s “radial velocity,” were induced by the gravitational tugs of orbiting planets. The starlight wavelength was compressed, then stretched, as the star moved toward and away from astronomers’ telescopes.
Now, Mayor and Queloz had proven that the technique worked. And a few days later, Marcy and Butler validated both the method used by Mayor’s team and their own very similar detection method.
But Marcy and his team realized something more. The only thing that had kept them from beating Mayor’s group to that first detection was a perfectly reasonable assumption: that big planets moved in stately orbits, like the 12 years it took Jupiter to take one lap around the sun.
Either they would have to watch stars for a very long time, or they would have to refine their wobble detector until it could pick up the very tiny shifts in a star’s position caused by small planets in tighter, faster orbits.
They were working on just this type of refinement when Mayor announced his discovery. More importantly, they had been recording observations with their wobble-detecting device, known as a spectrograph. Sure enough, when they took another look, big, star-hugging planets began popping out of their data.
Source: NASA
