Kapteyn c is a disputed planet around Kapteyn's Star, an old red dwarf. The star, about 13 light-years away, belongs to the Milky Way's halo, an ancient population of stars that orbit the galaxy differently from the Sun. In 2014 a team led by Guillem Anglada-Escudé reported two super-Earths around it: Kapteyn b, on a 48-day orbit in the habitable zone, and Kapteyn c, a colder planet on a 120-day orbit with a minimum mass of about seven Earth masses.
They used the radial velocity method, which finds planets from the slight back-and-forth motion their gravity gives a star, with data from three instruments: HARPS, HIRES and PFS. The catch is that activity on a star's surface, such as spots carried around as it spins, can produce similar signals. In 2015 a team led by Paul Robertson argued that Kapteyn b was really an effect of the star's rotation, which they put at 143 days, three times b's period. Anglada-Escudé's team replied that the evidence for that rotation period and for the correlations was weak, and that two planets remained the simplest explanation.
In 2021 a team led by Anna Bortle reanalysed the data with a different technique that models a star's rotation as a signal that repeats only loosely. They found a rotation period of about 125 days and concluded that both reported planets, b and c, are artifacts of the star's rotation and activity. Kapteyn c remains in the NASA Exoplanet Archive's list of planets, but its existence is in serious doubt. If it is real, its distant orbit around a dim star would make it very cold.