GJ 180 c is a proposed super-Earth whose existence has been questioned. It would orbit GJ 180, a red dwarf about 39 light-years away, every 24.3 days, with a minimum mass of about 6.4 Earth masses.
It was reported in 2014 by a team led by Mikko Tuomi, which reanalysed archived measurements of 41 nearby red dwarfs from the UVES and HARPS spectrographs. They used the radial velocity method, which detects the slight back-and-forth motion a planet's gravity gives its star, and a statistical approach designed to separate weak planetary signals from noise and stellar activity. They reported eight new planet candidates, including two around GJ 180, at about 17 and 24 days.
In 2020 a team led by Fabo Feng combined the same kind of data with measurements from the Keck and Magellan telescopes. They supported the 17-day planet, GJ 180 b, and found a new one, GJ 180 d, on a 106-day orbit in the habitable zone. But they did not find the 24-day signal of GJ 180 c to be significant, and treated GJ 180 as a two-planet system. GJ 180 c remains in the NASA Exoplanet Archive, but its existence is uncertain until further measurements settle it. It has never been seen passing in front of its star, so its size is unknown.