Three planets have been found 14 light-years away, circling the red dwarf Wolf 1061. One has an orbit compatible with liquid water – and potentially life – making it the closest undisputed planet to Earth in this category.
An M-type star with a quarter the Sun's mass and less than a hundredth its light, Wolf 1061 requires a small telescope to see. Even though the outermost planet has an orbit lasting 67.3 days (shorter than Mercury's), it receives less warmth than Mars. The innermost planet orbits in 4.9 days, and is far too hot for life.
Appropriately, Wolf 1061c is in the “Goldilocks zone,” a “just right” distance. With a mass at least 4.3 times that of Earth, it could be rocky, and its 17.9-day orbit means an Earth-like average temperature.
First author of The Astrophysical Journal Letters paper announcing the discovery, Dr. Duncan Wright of the University of New South Wales told IFLScience that the age and orbital distance indicate Wolf 1061c is probably “tidally locked.” One side of Wolf 1061c may always point towards its parent star, while the other experiences perpetual darkness. In between, a narrow strip may form a sort of localized Goldilocks zone of intermediate temperature.
Tidal locking “causes complexity for habitability,” Wright told IFLScience. “It doesn't preclude the possibility of life, but may make it harder.” On the other hand, he added, “there are always some unknowns, you would expect Mercury to be tidally locked and it didn't happen.”
Co-author Professor Chris Tinney added to IFLScience: “It is indeed probably the most exciting system we've yet found amongst the 50-odd planets we've found,” referring to his own team and not the overall count of exoplanets, which stands in the thousands.
Planetary candidates have been reported around closer stars, some in habitable zones. Three years ago, Tinney announced evidence of five planets around the star Tau Ceti, two light-years closer than Wolf 1061, and our nearest Sun-like star. However, the evidence for Tau Ceti's planets is ambiguous. “In the catalogues, they are usually marked with an asterisk,” Wright said. Sadly for science fiction fans, the hypothesized Tau Ceti planets are probably ill-suited to advanced life, even if real.
Reports of a planet around Kapteyn's star are even more questionable, Wright added. Wolf 1061, however, is a very stable star with a slow rotation period, eliminating the noise that often interferes with planetary searches. Consequently, Wright said, “These planetary signals are very clear. There's not really anything else they could be.”
He added: “Any effort to estimate the star's age would have big error bars. But slow rotation does suggest a very old star,” offering plenty of time for life to evolve.
Besides its orbital distance and mass, we know nothing about Wolf 1061c. Wright said telescope time has been booked to investigate whether any of the planets transit their star, as seen from Earth. Transits would enable us to learn their size, determine mass more precisely, and measure any atmospheres. The authors calculate transit prospects at 14 percent, but Wright told IFLScience there are reasons to think it is higher than this.