Do 3 Habitable Super-Earths Really Orbit a Nearby Star?

From its position 22 light-years away in the constellation of Scorpius, the red M dwarf star Gliese 667 C doesn’t look like much. Its dim light is lost to the naked eye, washed out by two brighter companion stars. Yet this tiny, exceedingly average star could play a crucial role in establishing that small, potentially Earth-like planets are common throughout our galaxy. Researchers have announced that seven planets orbit that star—and, if their mathematical analyses are correct, three of them could be habitable.

Previous surveys of Gliese 667 C had turned up two planets, including a potentially rocky "super-Earth" orbiting in the star's habitable zone, the region in which a planet might possess liquid water on its surface. Dubbed Gliese 667 C c, this world could be a "Goldilocks" planet like Earth, with a "just right" temperature neither too hot nor too cold for life as we know it. Now, after years of hints that more planets lurk in the data, an international team of astronomers led by Guillem Anglada-Escudé of the University of Göttingen in Germany and Mikko Tuomi of the University of Hertfordshire in England have announced their discovery of between three and five additional worlds around the star. Two of these additional bodies could be super-Earths orbiting in the habitable zone, raising the possibility that the star harbors three Goldilocks worlds. The journal Astronomy & Astrophysics published their study (pdf) online June 26.

Unlike our own solar system, with its spacious arrangement of small inner planets and large outer worlds orbiting a G-type yellow dwarf star, all the purported planets around Gliese 667 C are of intermediate mass, somewhere between that of Earth and Uranus. Stranger still, all but one are huddled interior to the orbit of Mercury, the closest planet to our sun. Such a system is said to be "dynamically packed," for its planets are jammed cheek by jowl in every available island of stability around the star. In recent years, as torrents of data streamed in from major planet surveys such as NASA's Kepler mission, astronomers were shocked to discover that such compact systems seem to be the default planetary arrangement in our galaxy. "We knew from Kepler that dynamically packed systems were prevalent around Sun-type stars, and now we have another around an M dwarf," Anglada says. The result suggests that many more compact systems—and potentially habitable planets—reside around nearby M dwarfs than previously thought.

Finding those planets has not been easy, because small, potentially habitable worlds are usually barely discernible against a noisy background of stellar jitter. Unlike most of the more than 3,000 likely planets found by NASA's Kepler mission, which were discovered by their transits—the shadows they cast toward Earth when they happen to cross the faces of their stars—Gliese 667 C's planets were detected via a more indirect technique, by the back and forth wobble their bulk induces on the star as they whip to and fro in their orbits. For the Gliese 667 C system, each planet's orbital tug only shifts the entire star's position by about one meter per second—walking speed—yet the star's seething surface swarms with stellar activity that at any moment can swamp this faint signal.

Discerning meter-per-second planetary wobbles across the light-years is a bit like listening for faint music emerging from washes of static on a poorly tuned radio. A lone planet's signal is like the sound of a single, steadily strummed guitar string, pure and repeated, almost immediately recognizable. Multiple planets, however, are much tougher to decipher: their overlapping wobbles are more akin to an out-of-tune orchestra playing all at once; only by listening for long periods can you hope to decipher any signals from the noise.

Early exoplanet hints

The first clear hints of a large multiplanet system around Gliese 667 C emerged last year, through the work of Philip Gregory, an astronomer at the University of British Columbia in Vancouver. Gregory was analyzing public data from the European Southern Observatory's HARPS spectrograph, a world-class planet-hunting instrument in La Silla, Chile. He noticed several previously unreported, potentially planetary wobbles, including one that looked like a 2.5-Earth-mass planet in a 39-day orbit—that is, another rocky planet within the star's habitable zone in addition to the already discovered—Gliese 667 C c. Gregory wrote up his findings and submitted them to a journal, but he stopped just short of claiming that he had found new planets.

As Gregory wrote his paper, Anglada and his colleagues were also glimpsing the wobbly evidence of Gliese 667 C's wealth of worlds by combining the HARPS measurements with data from two other telescopes. They analyzed the combined data using two independent and distinct statistical methods. Both methods strongly supported the presence of the two previously announced planets as well as three "new" planets with orbits and masses essentially identical to what Gregory reported in 2012. One of the two methods also found tentative evidence for two additional small planets, one in a hot 17-day orbit and another in a frigid 256-day orbit. Several rounds of further simulations only increased their confidence the planets were real.

Gregory praises the group's work as "a very significant step forward," and notes that although his paper "served to draw attention to the possibility of multiple planets in the habitable zone," the Anglada study contains "more definitive results."

Wobbly statistics

Even so, doubts remain. According to Xavier Bonfils, the leader of the HARPS team's M-dwarf survey, Anglada's team took various statistical "shortcuts" that made their analyses easier to perform but less robust. A key point, Bonfils contends, is that the team assumed Gliese 667C's planets reside in near-circular orbits, a notion supported more by dynamical simulations than actual data from the star. More elongated "eccentric" orbits would make such a close-packed system unstable. So, if the new planets are real, most must trace low-eccentricity orbits. Or perhaps there are simply fewer planets than claimed. "The analysis they propose seems mathematically correct, but it is a less conservative approach than what is usually done," Bonfils says, hastening to add that he hopes the planets prove to be genuine. "The signals are there, but that doesn't mean they are all planets.” Hundreds or thousands of additional costly, time-consuming measurements could be required to confirm the planetary provenance of Gliese 667 C's meter-scale wobbles, Bonfils says.

This is not the first time Anglada, Tuomi and their collaborators have made similar claims, notes Sara Seager, a prominent exoplanet researcher at the Massachusetts Institute of Technology who was not involved with the group's study. In recent years the group has also announced small planets—including potentially habitable ones—around a few other stars, although many of those claims remain unconfirmed.

The issue, Seager explains, is not necessarily that these planets aren't real, but rather that the statistical techniques used to reveal their presence are so abstruse that there are few clear precedents and outside experts to properly judge the claims. "They use highly sophisticated, specialized methods to pull very weak signals out of noisy data," Seager says. "Only a handful of other teams in the world can reproduce this kind of data analysis."

If the Anglada results hold up, though, they could help reshape the future of planet-hunting. Multiple-star systems and red dwarfs like Gliese 667 are the most common types in the Milky Way, and if most of them harbor packed planetary systems, the closest habitable worlds outside the solar system could be quite nearby indeed.

“The clichéd response to this is that 'extraordinary claims require extraordinary proof,'" says Greg Laughlin, another exoplanet expert at the University of California, Santa Cruz, who was unaffiliated with the study. "But you can't really consider this to be an extraordinary claim, because even though it's not at all like our own solar system, what's being proposed is an extraordinarily ordinary planetary arrangement.” He adds that the Kepler mission “has clearly indicated that systems like Gliese 667 C, rather than systems like ours, are the default mode of planet formation in the galaxy."

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