GJ 667 C f is a proposed planet whose existence is disputed. It was reported in 2013 as one of up to seven planets around GJ 667 C, a red dwarf about 24 light-years away in a triple star system. In that model it orbits every 39 days, just outside its sibling GJ 667 C c, and has a minimum mass of about 2.7 Earth masses.
The proposal came from a team led by Guillem Anglada-Escudé, who combined measurements from the HARPS, PFS and HIRES spectrographs and used the radial velocity method, which detects the slight back-and-forth motion a planet's gravity gives its star. They found that six signals described the data better than two: the known planets b and c, three few-Earth-mass candidates at 92, 62 and 39 days, called d, e and f, and a colder planet at 260 days. They concluded that three of the planets, c, f and e, lie in the star's habitable zone, the range of distances where liquid water could last on a surface.
Other astronomers have not been able to confirm it. In 2014 a team led by Paul Robertson corrected the same kind of data for the star's magnetic activity, which can create signals that look like planets. They confirmed b and c, explained the 92-day signal as an effect of the star's rotation, and could not confirm the other proposed periods, including f's. GJ 667 C f is still listed in the NASA Exoplanet Archive, but it should be treated as uncertain until new measurements settle the question. Like the system's other candidates, it has never been seen passing in front of the star.