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Two teams of astronomers may have found the first earth-like planet in outer space. So who truly discovered Gliese 667C
Artist’s impression of Gliese 667 Cb with the Gliese 667 A/B binary in the background
No one knows what the planet Gliese 667Cc looks like. We know that it is about 22 light-years from Earth, a journey of lifetimes upon lifetimes. But no one can say whether it is a world like ours, with oceans and life, cities and single-malt Scotch. Only a hint of a to-and-fro oscillation in the star it orbits, detectable by Earth’s most sensitive telescopes and spectrographs, lets astronomers say the planet exists at all. The planet is bigger than our world, perhaps made of rocks instead of gas, and within its star’s “habitable zone”—at a Goldilocks distance that ensures enough starlight to make liquid water possible but not so much as to nuke the planet clean.
That’s enough to fill the scientists who hunt for worlds outside our own solar system—so-called exoplanets—with wonder. Gliese 667Cc is, if not a sibling to our world, at least a cousin out there amid the stars. No one knows if it is a place we humans could someday live, breathe, and watch triple sunsets. No one knows whether barely imagined natives are right now pointing their most sensitive and far-seeing technology at Earth, wondering the same things. Yet regardless, to be the person who found Gliese 667Cc is to be the person who changes the quest for life beyond our world, to be remembered as long as humans exist to remember—by the light of the sun or a distant, unknown star.
Which is a problem. Because another thing no one knows about Gliese 667Cc is who should get credit for discovering it.
Gliese 667Cc is at the center of an epic controversy in astronomy—a fight over the validity of data, the nature of scientific discovery, and the ever-important question of who got there first.
In late 1995 Swiss astronomer Michel Mayor and his student Didier Queloz found 51 Pegasi b, the first known exoplanet orbiting a sunlike star. It was orbiting far too close to its sun to allow the formation of water, but the discovery made Mayor’s European team world famous anyway.
Soon, though, they lost their lead in the planet-hunting race to a pair of American researchers, Geoff Marcy and Paul Butler. The two men had been looking for exoplanets for almost a decade; they bagged their first two worlds a couple of months after Mayor’s announcement.
The two teams evolved into fiercely competitive dynasties, fighting to have the most—and most tantalizing—worlds to their names. Their rivalry was good for science; within a decade, each had found on the order of a hundred planets around a wide variety of stars. Soon the hunt narrowed to a bigger prize. The teams went searching for smaller, rocky planets they could crown “Earth-like.”
The Spectral fluctuations of a star with an exoplanet create a sine wave.
Most planet hunters aren’t looking for exoplanets, per se. Those worlds are too small and dim to easily see. They’re looking instead for telltale shifts in the light of a star, “wobbles” in its spectral identity caused by the gravitational pull of an unseen orbiting exoplanet. When that force tugs a star toward Earth, the Doppler effect ever so slightly compresses the waves of light it emits, shifting them toward the blue end of the spectrum. When the star moves away from Earth, its waves of starlight stretch to reach us, shifting toward the red. You can’t see those shifts with the naked eye. Only a spectrograph can, and the more stable and precise it is, the smaller the wobbles—and planets—you can find.
By late 2003 the European team had a very precise instrument, the [ESO] High Accuracy Radial velocity Planet Searcher, or Harps. Mounted to a 3.6-meter telescope on a mountaintop in Chile, Harps could detect wobbles of less than a meter per second. (Earth moves the sun just a tenth that amount.) The Americans had to make do with an older instrument called the [Keck]High Resolution Echelle Spectrometer, or Hires—less precise but paired with a more powerful telescope.
As the two teams continued to fight for preeminence, trouble was brewing among the Americans. Marcy, a natural showman as well as a brilliant scientist, regularly appeared on magazine covers, newspaper front pages, and even David Letterman’s late-night show. His far more taciturn partner, Butler, preferred the gritty tasks of refining data pipelines and calibration techniques. Having devoted years of their lives to the planet-hunting cause, Butler and another member of the team, Marcy’s PhD adviser, Steve Vogt (the mastermind behind Hires), began to feel marginalized and diminished by Marcy’s growing fame. The relationships hit a low in 2005, when Marcy split a $1 million award with their archrival Mayor. Marcy credited Butler and Vogt in his acceptance speech and donated most of the money to his home institutions, the University of California and San Francisco State University, but the damage was done. Two years later, the relationship disintegrated. Butler and Vogt formed their own splinter group; Butler and Marcy have barely spoken since.
It was a risky move. Harps and Hires remained the best planet-hunting spectrographs available, and Butler and Vogt now lacked easy access to fresh data from either one. The American dynasty was shattered, and Marcy was forced to find new collaborators. Meanwhile, the ever-expanding European team continued to wring planets from Harps even though Mayor had formally retired in 2007. The search for Earth 2.0, long seen as a struggle between two teams, became a more crowded and open contest.
Then a seeming breakthrough: In the spring of 2007, the Europeans announced that they’d spotted a potentially habitable world, Gliese 581d.1 It was a blockbuster—a “super-Earth”—on the outer edge of the habitable zone, eight times more massive than our own world.
Three years later, in 2010, Butler and Vogt scored their own big find around the same star—Gliese 581g. It was smack in the center of the habitable zone and only three or four times the bulk of Earth, so idyllic-seeming that Vogt poetically called it Zarmina’s World, after his wife, and said he thought the chances for life there were “100 percent.” Butler beamed too, in his own subdued way, saying “the planet is the right distance from the star to have water and the right mass to hold an atmosphere.” They had beaten Marcy, laid some claim to the first potentially Earth-like world, and bested their European competitors.
But to a chorus of skeptics, Zarmina’s World seemed too good to be true. The European group said the signals the Americans had seen were too weak to be taken seriously. The fight was getting ugly; entire worlds were at stake.
Plotted on a computer screen, a stellar wobble caused by a single planet looks like a sine wave, though real measurements are rarely so clear. A centimeters-per-second wobble in a million-kilometer-wide ball of seething, roiling plasma isn’t exactly a bright beacon across light-years. Spotting it takes hundreds to thousands of observations, spanning years, and even then it registers as a fractional offset of a single pixel in a detector. Sometimes a signal in one state-of-the-art spectrograph will fail to manifest in another. Researchers can chase promising blips for years, only to see their planetary dreams evaporate. Finding a stellar wobble caused by a habitable world requires a volatile mix of scientific acumen and slow-simmering personal obsession.
A Spanish astronomer named Guillem Anglada-Escudé certainly meets that description. Now a lecturer at Queen Mary University in London, he began working with the American breakaways Butler (a friend and collaborator) and Vogt not long after they announced Gliese 581g.
Today, Anglada-Escudé’s name is on the books next to between 20 and 30 exoplanets, many found by scraping public archives in search of weak, borderline wobbles. The European Southern Observatory, which funds Harps, mandates that the spectrograph’s overlords release its data after a proprietary period of a year or two. That gives other researchers access to high-quality observations and potential discoveries that the Harps team might have missed. Scavenging scraps from the European table, it turns out, can be almost as worthwhile as being invited to the meal.
In the summer of 2011, Anglada-Escudé was a 32-year-old postdoc at the end of a fellowship, looking for a steady research position in academia. With Butler’s help he had developed alternative analytic techniques that he used to scour public Harps data. In fact, Anglada-Escudé argued that his approach treated planetary data sets more thoroughly and efficiently, harvesting more significant signals from the noise.
One late night that August, he picked a new target: nearly 150 observations of a star called Gliese 667C2 taken by the Harps team between 2004 and 2008. He sat before his laptop in a darkened room, waiting impatiently as his custom software slowly crunched through possible physically stable configurations of planets within the data.
The first wobble to appear suggested a world in a seven-day orbit—the faster the orbit, the closer to the star the planet must be. A weeklong year is about enough time to get roasted to an inhospitable cinder—and anyway the Harps team had announced that one in 2009, as the planet Gliese 667Cb. But Anglada-Escudé spied what looked suspiciously like structure in the residuals of the stellar sine wave snaking across his screen. He ran his software again and another signal emerged, a strong oscillation with a 91-day period—possibly a planet, possibly a pulsation related to the estimated 105-day rotation period of the star itself.
1 Astronomer Wilhelm Gliese cataloged hundreds of stars in the 1950s. The lowercase letter marks the order in which astronomers discovered the planets orbiting a star.
2 The capital C indicates a trinary system, with A and B stars.
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