TOI-789 d is the outermost and largest of three confirmed planets around TOI-789, a red dwarf about 140 light-years away. It is about 1.4 times Earth's width and orbits every 12.97 days. It was validated in 2026 together with TOI-789 b and c.
All three were found by NASA's TESS satellite from the small dips in starlight as they cross the star. A team led by Juan F. Pagés Navarrete analysed about seven years of TESS data with ground-based follow-up and high-resolution imaging, and confirmed the planets statistically, since their masses have not been measured. Running computer simulations of the system's orbits, the team found the planets close to simple ratios of each other's periods, and the system stable only if the orbits are very nearly circular. Beyond TOI-789 d they found a possible fourth planet on a 19.7-day orbit, which remains an unconfirmed candidate.
TOI-789 d is the coolest of the three. According to the NASA Exoplanet Archive, it receives about three times the light Earth gets from the Sun, with an estimated equilibrium temperature, set by starlight alone, of about 340 kelvin, which places it among the system's more temperate worlds. That is still more light than Earth receives, and nothing is known about any atmosphere it may have; the team noted the star is too faint for its planets' atmospheres to be studied with current telescopes. The archive's mass of about 6.6 Earth masses is an estimate from its size, not a measurement.