by Lee Billings
Marcy was not as incompetent as he believed. His encyclopedic knowledge of astronomy, paired with a quick wit and the skills of a natural storyteller, made even the most abstruse astronomical topics comprehensible to laymen. He soon landed a junior faculty job at San Francisco State University, and in between teaching courses he pondered an RV planet survey, though his plans always seemed half-baked—the spectroscopic signals of planets would be impossible to discern without proper calibration. Things coalesced when he met Paul Butler, a younger student simultaneously pursuing a bachelor’s degree in chemistry and a master’s in astrophysics. Butler shared Marcy’s interest in exoplanets, and they became close friends. Together, they worked to find ideal calibration methods, until Butler came up with a solution: a glass vessel filled with iodine that could be attached to a spectrometer. Light shining through this “absorption cell” would project iodine absorption lines like hashmarks upon a star’s spectrum, allowing small spectral wobbles to be seen. Butler’s iodine cell would become the standard calibration technique for decades of RV planet searches.
In 1987, Marcy and Butler mated the iodine cell with the general-purpose “Hamilton” spectrometer built by Marcy’s former PhD advisor, the UC Santa Cruz astronomer Steve Vogt, and began their planet search. For years they used the spectrometer on various telescopes at Lick Observatory on Mount Hamilton, twenty-five miles east of San Jose, searching to no avail for extrasolar Jupiters around 120 nearby Sun-like stars. Butler left for a time to obtain his PhD at the University of Maryland, but continued to hone the duo’s data-analysis software, eventually sharpening the RV precision of their data from 15 to 5 meters per second. By the autumn of 1995 they were nearing the end of their patience when two University of Geneva astronomers, Michel Mayor and Didier Queloz, announced the discovery of 51 Pegasi b based on another RV survey conducted from the Haute-Provence Observatory in the south of France.
When they heard the news, Marcy and Butler rushed to observe 51 Pegasi for themselves, and within days saw the star’s telltale wobble from its whirling hot Jupiter—a variety of planet they had not conceived of or looked for in all their previous years of searching. Revisiting their old data, they rapidly found two more giant planets around the stars 47 Ursae Majoris and 70 Virginis, retaking the lead in the burgeoning race of discovery and establishing a rivalry that would span decades.
In those early golden years, Marcy and Butler surged ahead in the race, propelled by nearly a decade of experience and their extensive back-catalog of data. By the turn of the millennium, they had discovered nearly forty close-orbiting gas giants. Each announcement was a news event—the discovery of exoplanets had yet to become truly routine. With their research featured on magazine covers and national newscasts, the duo abruptly found themselves in high academic demand, and soon secured more-prestigious positions. Marcy became a UC Berkeley professor, and Butler secured a job as a staff scientist at the Carnegie Institution for Science in Washington, DC. Though geographically separated, they continued their work together, ultimately utilizing their growing fame to expand their team to include Vogt as well as another brilliant planet hunter, the astronomer Debra Fischer. The group gained more funding and access to some of the best astronomy resources in the world, notably another Vogt-built spectrometer, HIRES, operating on the twin 10-meter telescopes of the W. M. Keck Observatory in Mauna Kea, Hawaii. HIRES could reach RV precisions of 3 meters per second, allowing it to discover smaller exoplanets in cooler orbits. But to reach potentially habitable worlds, even more precision would be required. Marcy began closing his intergroup e-mails with the exhortation “OMPSOD!”—One Meter Per Second, Or Death!
“The Swiss,” as Marcy’s and Butler’s Geneva-based competitors are invariably called despite having collaborators around the world, were not sitting idle while their American counterparts surged. They were expanding their team as well, and redoubling their efforts to find more planets. As both teams excelled, their competition became fierce. At a conference in June 1998, the American team announced their discovery of Gliese 876b—the first planet found around a red dwarf star. The following day, the Swiss made an announcement of their own, saying they had detected the same planet days before the American announcement; they claimed to have confirmed their discovery some hours before the conference, but Marcy and Butler had beaten them to the podium. The American team would also beat them to peer-reviewed publication on the planet, and came away with credit for the discovery. In November of 1999, both teams nipped at each other’s heels to share the discovery of the first transiting planet, a hot Jupiter around the star HD 209458, after separately and nearly simultaneously observing the transiting world and submitting papers on the results. The rivalry deepened in 2002, when the Swiss released a paper claiming the detection of twin “hot Saturns” around the star HD 83443. Marcy, Butler, and Vogt also observed the star, but could find no evidence in their data to support both Swiss planets. Butler spearheaded the publication of a paper detailing the case against the Swiss claims, and months afterward the Swiss team retracted one of the planets, placing a dark blemish on their otherwise flawless record. In contrast, while working together the American team never retracted a single world from their tally. The Swiss never forgot about the time when Butler led the American charge against them. They would keep him far from their orbit ever after.
The Swiss, for their part, seized the lead in RV precision in 2004 with the debut of their HARPS spectrometer, developed in collaboration with the European Southern Observatory (ESO). Stabilized in a temperature-controlled vacuum chamber and mounted on a 3.6-meter ESO telescope in Cerro Paranal, Chile, HARPS proved capable of breathtaking RV precisions of slightly below 1 meter per second, giving the Swiss a decisive edge that they used to discover a multitude of smaller planets orbiting on the cusps of habitable zones. Some of the worlds were only a few times the mass of our own, and thus possibly rocky rather than smothered in heavy layers of gas. They were hopefully called “super-Earths.” Keck’s HIRES would receive upgraded detectors that same year, boosting its precision closer to but not equaling that of HARPS. The Americans had more planets to their names, but they knew it was their competitors who were making faster progress toward detecting potentially habitable worlds. The Swiss had been the first to break the 1-meter-per-second barrier, and even with its upgrades HIRES was slightly below HARPS in performance. After their years of dominance, the Americans privately worried that their sudden disadvantages, albeit minor, could lead to the downfall of their team.
As the Swiss were developing HARPS in 2002 and 2003, Fischer, Vogt, Butler, and Marcy made grand plans of their own for what they hoped would be a superior piece of kit: the Automated Planet Finder, a 2.4-meter robotic telescope to be constructed at Lick Observatory and outfitted with a new spectrometer custom-built by Vogt to excel at precision RV measurements of a meter per second or less. Though it would be dwarfed by the light-gathering power of many larger ground-based telescopes, the APF’s advantage would be its singular focus. Almost all world-class telescopes were by necessity workhorses for the entire breadth of astronomy, with only a portion of their time devoted to planet hunting. The APF’s sole task, by contrast, would be to survey bright nearby stars, night after night, steadily accumulating RV signals for any accompanying small, rocky planets. The group selected Vogt to be the APF’s principal investigator. The project hit a snag, however, after the relationship between Marcy and Butler took a sudden turn for the worse.
Over time, the duo’s extreme success had pushed them apart and destabilized their friendship. They were no longer young—each man now sported dark circles beneath the eyes, a salt-and-pepper beard, and a head unburdened by more than a waning crescent of hair. After two decades of working together, much that had once seemed fresh and new now felt tiresome and constraining. Though he had initially been only a graduate student beneath Marcy, Butler’s development of the iodine cell and seminal contributions to RV data-analysis techniques had elevated him to equal stature with
his longtime research partner. Yet Marcy still acted as the group’s de facto leader and manager. Where Butler was taciturn and blunt, preferring the simplicity of actual planet hunting to the delicate nuance of press interviews and academic politicking, Marcy was loquacious, charismatic, and cunning, happy to speak at length in eminently quotable sentences about the team’s work and always careful to offer diplomatic praise for his competitors. With the Swiss, Marcy was cordial, even friendly, though they had for years collectively treated Butler as persona non grata. The almost universal propensity to overlook Butler in favor of Marcy had progressively led to a tangible divergence of the two men’s fortunes, as Marcy accrued the lucrative lion’s share of press mentions and professional awards.
Feeling underappreciated and eclipsed, Butler reached his limit in 2007 and abandoned Marcy to form a new planet-hunting team with Vogt, using instruments at Lick Observatory as well as facilities in Chile and Australia. Their dynasty was splitting into multiple shards. Fischer soon left the team, and her position as a professor at San Francisco State University, for a professorship at Yale, where she founded a planet-hunting group of her own and began building a new spectrometer, CHIRON, meant to rival and surpass HARPS. Marcy, by now a coinvestigator on NASA’s Kepler mission, remained at UC Berkeley and worked closely with a new protégé, the astronomer Andrew Howard, using Keck and HIRES to search for more planets and to study Kepler’s thousands of candidates. Where once there had been only two RV teams in serious contention for finding alien Earths, the disintegration of the powerful Marcy-Butler partnership had given birth to many, with still more upstart groups waiting in the wings to use a new generation of planet-hunting spectrometers being constructed and deployed at observatories across the globe. But along with the unified Swiss, the scattered, grizzled veterans of the crumbled American dynasty still had the best data, the best observatory access, and the best chance of finding rocky planets in stellar habitable zones. Marcy’s group, continuing its existing program while also first in line to drink from Kepler’s firehose, was the odds-on favorite to be first.
On September 29, 2010, Vogt, Butler, and four other collaborators declared they had beaten those odds. Their announcement coincided, perhaps not by chance, with Marcy’s fifty-sixth birthday—a final spiteful gift from former friends. By combining their own old HIRES observations with publicly available HARPS data from the Swiss, Vogt and Butler claimed the RV detection of two planets around the red dwarf star Gliese 581, located some 20 light-years away in the constellation of Libra. One of the planets was between three and four Earth masses, in a 37-day orbit that placed it squarely in the center of the habitable zone. According to convention, its official name was Gliese 581g—the “g” denoted that the world was the sixth planet discovered around that particular star, though it could have equally well stood for “Goldilocks,” since the planet was potentially rocky, and in an orbit very clearly neither too hot nor too cold for life as we know it. Vogt preferred another appellation. He called it “Zarmina’s World,” after his wife, and said in a press conference that he believed the chances for life on the planet were “one hundred percent.” Butler opted for the more conservative statement that “the planet is the right distance from the star to have water and the right mass to hold an atmosphere.”
The announcement left the Swiss team collectively scratching their heads. They had previously found four small planets orbiting Gliese 581, including two borderline-habitable worlds hugging each end of the star’s habitable zone. How had they missed two more planets? When they expanded and reanalyzed their own HARPS observations of the star, they readily confirmed once again the four planets their team had previously detected, but there was no signal of Zarmina’s World or of the other potential planet, Gliese 581f. They argued that Vogt’s less-precise HIRES data had introduced phantom worlds into their higher-quality HARPS observations of Gliese 581. Multiple independent examinations of the public HARPS and HIRES data came to divergent conclusions, with some finding evidence for the new planets and others dismissing them, depending on a shifting host of assumptions. If a certain variety of statistical analysis was used to extract the RV signals, only four emerged, rather than six—the new planets were false alarms! In their dynamical simulations, Vogt and Butler changed the orbits of the planets to nearly circular rather than moderately elongated, and found the results more stable with six worlds than with four—the new planets were real! But, adding up the various ways that the orbits of the four confirmed planets could interact, it appeared they could partially obscure and dilute RV signals of the two disputed worlds.
With uncertainty clouding the case of Gliese 581g, the prize of finding the first undisputed terrestrial world in a habitable zone remained up for grabs. In July of 2012, Vogt, Butler, and another colleague released a rebuttal to their critics, a two-steps forward, one-step back reanalysis of the HARPS data that scarcely mentioned Gliese 581f and modified Zarmina’s World into a smaller 2.2-Earth-mass planet in a shorter 32-day orbit, still within the habitable zone. The potential world, they wrote, had approximately a 4 percent chance of being illusory, though since its signal was so weak much more data would be required for an airtight confirmation. To the Swiss, a 4 percent chance was still too high to bequeath confirmation upon Zarmina’s World. Even a 1 percent chance would be considered borderline for such a planet. Extraordinary claims, they argued, required extraordinary evidence—evidence, they tacitly suggested, that would only come from a spectrometer at least as precise as HARPS.
In the summer of 2011, one of Butler’s occasional collaborators, a thirty-two-year-old Spanish astronomer named Guillem Anglada-Escudé, began developing his own alternative data-analysis software to derive RV signals from HARPS spectral measurements, which the Swiss made public after a two-year proprietary period in accordance with ESO policies. Unlike the analytic methods of the Swiss, which threw away a significant amount of a star’s raw spectrum, Anglada’s software harvested a greater portion of a star’s spectral data, extracting more signal from the noise to further boost RV precision for certain varieties of stars—particularly red dwarfs. Soon he was running his code on samples of the HARPS data, hoping to find borderline planetary signals that the less-precise Swiss analyses might have missed. His postdoc at Carnegie was almost finished, and he was looking for another job in the field—he thought a few planets under his belt could only help his chances. The first dozen data sets he examined came up empty of any new signals.
Late one August evening, after interviewing for a postdoc position at the University of Göttingen in Germany, Anglada returned to his hotel room and looked at another batch of HARPS data, 143 RV measurements taken between 2004 and 2008 for the red dwarf star GJ 667C, part of a triple star system some 22 light-years away from Earth. He fed his reduction of the data into Laughlin’s Systemic software, and waited as the program looked for patterns. It first found the signature of a planet in a 7-day orbit that the Swiss had announced in 2009, but Anglada could see what looked like residual structure in the pointillistic clusters of measurements. He ran the data through Systemic again, and the software found a strong 91-day trend in the data—another possible planet, but also perhaps a cyclic stellar pulsation related to the star’s estimated rotational period of 105 days. Anglada ran Systemic once more, nulling out the 7- and 91-day signals, then with trembling hands lit a cigarette and stared in disbelief at his laptop’s glowing screen. Another sine wave snaked through the measurements, looking for all the world like the signal of a 4.5-Earth-mass planet, likely terrestrial, in a 28-day orbit firmly within GJ 667C’s habitable zone. If the planet proved to be real, it would be assigned the name GJ 667Cc.
“It was very strange to find an unpublished, unclaimed, potentially habitable planet in a three-year-old public dataset,” Anglada recalled to me. “So I looked again at the measurements using [the Swiss] method—the 28-day signal was there, but with a false-alarm probability that looked to be substantially greater than one percent.” Too high, that is,
to cross the HARPS team’s traditionally ultra-stringent thresholds for announcing a discovery. Anglada’s analysis of the 28-day signal, by contrast, yielded a false-alarm probability of three hundredths of 1 percent. He shared his findings with Butler, who excitedly agreed to gather more data on the star. Butler obtained twenty new RV measurements for GJ 667C, and Vogt supplied twenty older measurements from Keck’s HIRES archives, both of which strengthened the 28-day signal. The team had soon modeled the putative system to examine its dynamical stability, and began drawing up a paper to announce the discovery. In the meantime, Anglada had decided he should bolster his case by seeking new data from HARPS, at the time still the best source in the world. Against the warnings of Butler and Vogt, who did not trust the HARPS team and urged him to publish with the data in hand, on September 28, 2011, Anglada submitted a proposal to ESO for twenty nights of HARPS observing time. The proposal did not explicitly announce Anglada’s potential discovery but included GJ 667C on its short target list, as well as a figure discussing the star’s 7-day, 91-day, and 28-day signals.
