TOI-789 b is the innermost of three Earth-sized planets packed close around TOI-789, a red dwarf about 140 light-years away. It is about 1.18 times Earth's width and orbits every 5.45 days. It was validated in 2026 together with its siblings, TOI-789 c and d.
The planets were found by NASA's TESS satellite, which caught the small dips in starlight as each crossed the star. A team led by Juan F. Pagés Navarrete used about seven years of TESS data, 42 sectors in all, together with ground-based observations from the LCOGT and MEarth telescope networks and high-resolution images from the Gemini and SOAR telescopes. Planets this small cannot easily be weighed, so they confirmed all three statistically, with a tool called TRICERATOPS, showing that the signals are far more likely to come from planets than from other sources, after ruling out nearby stars with their own observations.
TOI-789 b is the hottest of the system's planets: according to the NASA Exoplanet Archive, it receives about nine and a half times the light Earth gets from the Sun, far too much for liquid water. The team's simulations found the system dynamically calm, with the planets near simple ratios of each other's periods, but stable only if their orbits are nearly circular. Its mass has not been measured; the archive's figure of about 3.7 Earth masses is an estimate from its size. The team noted that the star is too faint for current telescopes to study these planets' atmospheres.