TOI-789 c is the smallest of the three confirmed planets around TOI-789, a red dwarf about 140 light-years away, at about 1.15 times Earth's width. It orbits every 8.04 days, between its siblings TOI-789 b and d, which were validated together with it in 2026.
A team led by Juan F. Pagés Navarrete studied about seven years of data from NASA's TESS satellite, which spots planets by the dips in starlight as they cross their star, along with ground-based photometry and high-resolution imaging. They validated the three planets statistically, and then searched the data again for weaker signals. That search turned up a possible fourth planet, on a 19.7-day orbit, smaller than Earth at about 0.88 times its width. It is too faint a signal to confirm, so it remains a candidate.
The team found that the four objects together span the transition between Venus-like and Earth-like conditions, with two of the confirmed planets falling in the temperate range by their estimated equilibrium temperature, the temperature set by starlight alone. TOI-789 c, at about 390 kelvin according to the NASA Exoplanet Archive, receives about five and a half times the light Earth gets from the Sun. Its mass has not been measured; the archive's figure of about 3 Earth masses is an estimate from its size. The team described TOI-789 as an example of the most common kind of system around red dwarfs: compact groups of small planets, best studied through their orbits when the star is too faint for atmospheric study.