YZ Ceti c is the heaviest of three Earth-mass planets packed tightly around YZ Ceti, a small red dwarf about 12 light-years away. Its minimum mass is about 1.14 Earth masses, and it orbits every 3.06 days. It was discovered together with YZ Ceti b and d.
A team led by N. Astudillo-Defru reported the three in 2017 from measurements with the HARPS spectrograph in Chile, using the radial velocity method: a planet's gravity makes its star move slightly toward and away from us, and that motion shows up as a tiny shift in the star's light. At 3.6 parsecs, they noted, YZ Ceti was then the nearest system with more than one planet found this way. They also saw that the inner two planets were close to a 2 to 3 ratio in their orbits, a near-resonance.
Its orbital period became the subject of a debate. Because astronomers can only observe at night and at certain times of year, a signal can be matched by more than one period, a problem called aliasing. In 2018 Paul Robertson argued that YZ Ceti c's 3.06-day period was probably an alias, and that its true period was 0.75 days, which would have made it lighter and more likely to pass in front of the star. In 2020 a CARMENES team led by S. Stock added 229 new measurements and resolved the aliases in favour of the original 3.06 days. No transits have been seen by NASA's TESS satellite, so its size remains unknown. Orbiting this close, it is too hot for liquid water. Its inner sibling, YZ Ceti b, is a candidate for magnetic interaction with the star detected through radio waves.