Barnard d is the innermost known planet of Barnard's Star, the closest single star to the Sun at about six light-years. It races around the star every 2.34 days at about 0.019 astronomical units, roughly a twentieth of Mercury's distance from the Sun. Its minimum mass is about a quarter of Earth's, 0.263 Earth masses. It is one of four small planets found together around this star, alongside Barnard b, c and e.
Like its siblings, it was detected by the radial velocity method, which picks up the slight back-and-forth motion a planet's gravity gives its star. A team led by Jonay González Hernández first saw its signal in 2024, in four years of ESPRESSO measurements from the European Southern Observatory's Very Large Telescope, and listed it as a candidate next to the confirmed Barnard b. In 2025 a team led by Ritvik Basant at the University of Chicago observed the star independently with MAROON-X on the Gemini North telescope, and those data alone confirmed it. Two separate instruments seeing the same signal is what turned it from a candidate into a planet.
It is the hottest of the four. The 2025 paper estimates an equilibrium temperature, set by starlight alone and assuming the planet reflects none of it, of about 483 kelvin, roughly 210 degrees Celsius. That is far closer in than the star's habitable zone, where liquid water could last on a surface. The four planets all orbit within about a week, making this one of the most compact systems known around a small red dwarf. None of them crosses in front of the star, so Barnard d's size is unknown, and its true mass could be higher than the minimum measured.