Berkeley, Calif.—Astronomers say they may have detected a second planet around Proxima Centauri, our solar system’s nearest neighboring star.
Announced at Breakthrough Discuss, an annual invitation-only interdisciplinary meeting held by the Breakthrough Initiatives (a scientific research organization primarily bankrolled by the Silicon Valley billionaire Yuri Milner), the planet’s existence remains unconfirmed—for now. Dubbed Proxima c, it would be a so-called super-Earth, with a minimum mass roughly six times that of our planet’s. Its approximately 1900-day orbit would likely make it a frigid, inhospitable place, orbiting some 1.5 times the Earth-sun distance from Proxima Centauri—which is a red dwarf star some four light-years away that is much smaller and dimmer than our familiar yellow sun. If confirmed, the newfound world would join Proxima b, a roughly Earth-mass planet discovered in 2016 in a more clement orbit around Proxima Centauri.
According to the scientists making the presentation—Mario Damasso of the Astrophysical Observatory of Turin and Fabio Del Sordo of the University of Crete—the tentative detection is based upon the same expansive multi-year dataset that first revealed Proxima b, with the addition of more than 60 further measurements of the star taken in 2017. Primarily gathered through the European Southern Observatory’s (ESO) HARPS instrument, the measurements look for planets by the telltale wobbling such worlds induce upon their host stars. The strength of such wobbles provides an estimate of a world’s mass; the wobble’s period yields a planet’s orbit. Among other incidental evidence, the wobble of Proxima c—a subtle swerve in the position of Proxima Centauri by slightly more than a meter per second—appeared in earlier observations to be of borderline significance, but was pushed into firmer territory by the last few years of additional measurements. The search for Proxima Centauri's planets has been spearheaded by the international Pale Red Dot planet-hunting team. The results are summarized in a paper that has been submitted to a peer-reviewed journal.
“It is only a candidate,” Damasso said during the presentation. “This is very important to underline.” Del Sordo offered similar cautions in his remarks, comparing the candidate world to a “castle in the air,” one that “we should keep working to put even stronger foundations under.” (Neither Damasso nor Del Sordo would make further comments on the record outside of their presentation, citing concerns about the embargo policies of the journal to which they submitted their paper.)
Further measurements with HARPS, the pair said, could ultimately confirm the planetary nature of Proxima c, as could follow-up studies with other facilities on the ground and in space. ESO’s next-generation planet-hunting ESPRESSO instrument on the Very Large Telescope in Chile, for example, would be able to detect the wobble caused by the candidate world with even higher fidelity. But most promising would be observations from the European Space Agency’s Milky-Way-mapping Gaia satellite, which is monitoring the motions and positions of more than a billion stars in our galaxy—including, it turns out, Proxima Centauri. Gaia could detect the planet’s presence by watching for wobbles, too. By the conclusion of its nominal five-year mission later this year, Del Sordo said, Gaia could provide “a decisive answer” as to whether or not Proxima c is real.
Beyond mere detection, the candidate planet would offer exciting opportunities for follow-up studies to characterize its nature, the presenting scientists said. According to Del Sordo, Proxima c would be “a spectacular laboratory for direct imaging”—astronomers’ parlance for snapping a planet’s picture across the vast gulfs of interstellar space. Proxima b has been discussed as a fruitful target for direct imaging as well. But because Proxima c is further out from the star than b, it should be easier to see. Potentially within reach of future space observatories such as NASA’s James Webb Space Telescope and Webb’s planned successor, the Wide-Field Infrared Survey Telescope, the planet could become the first world beyond the solar system imaged in reflected light. (Previous direct images of planets have been in infrared light, where the glare of a planet’s star is less overwhelming.)
Any image of Proxima c – presuming the planet proves genuine – would likely reveal a chilly, gas-dominated orb, but could still prove extremely useful for astronomers struggling to understand what super-Earths are actually like. Despite being the most common known variety of planet in the Milky Way, super-Earths are entirely absent from our own solar system. Midway in mass and size between Earth and Neptune, super-Earths may either be mostly gassy planets offering slim chances for life as we know it, or instead super-sized versions of our own habitable, rocky world.
Images of planets in the Proxima Centauri system might also help resolve lingering debates over the potential for red dwarf stars to harbor habitable planets; such stars are often more active than solar-type stars, blasting accompanying worlds with showers of high-speed particles and hard radiation that can strip away atmospheres like so much sand-blasted paint. Pictures could resolve the fates of such worlds—provided, that is, astronomers manage to secure time on Earth’s most powerful telescopes to go look.