Just 23.6 light-years away, GJ 667 C c orbits the faintest star of a triple-star system in the constellation Scorpius. It was found by the radial velocity method: as a planet orbits, it tugs its star back and forth, slightly shifting the star's light. Two teams reported it in 2012. One, led by Guillem Anglada-Escudé, combined data from the HARPS, Magellan and Keck telescopes. The HARPS team, led by Xavier Delfosse, published its own analysis.
The planet circles its small star every 28 days, inside the habitable zone, the range of distances where liquid water could exist. Delfosse's team estimated it gets about 10 percent less energy than Earth receives from the Sun. Planets this close to such a star are expected to be tidally locked, with one side always facing it. Its mass is at least about 3.8 Earth masses. Radial velocity gives only a minimum mass, because the tilt of the orbit is unknown. The planet has never been seen crossing its star, so its width has not been measured; the radius on this page is an estimate.
The system became a lesson in false alarms. In 2013 Anglada-Escudé's team argued for six or perhaps seven planets, including three super-Earths in the habitable zone, and ESO announced the result. Later studies disagreed. Farhan Feroz and Mike Hobson found that once noise from the star itself is modelled properly, the data strongly support only two planets, b and c. Paul Robertson and colleagues showed that one claimed planet, at about 90 days, was really a signal from the star's rotation and magnetic activity. Both studies kept planet c, so its existence is not in doubt.
TerraForge's model, working from minimum mass, orbit and star, classifies GJ 667 C c as a water world at about 303 K with a high habitability score. Observations cannot test that yet. With no measured size, density or atmosphere, nobody knows whether it is rocky, watery or wrapped in gas, so the score reflects its promising orbit rather than evidence of water.