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."