Barnard's Star b, or Barnard b, is a sub-Earth-mass exoplanet closely orbiting Barnard's Star, a nearby red dwarf star six light-years from Earth. The planet was discovered using radial velocity observations from the ESPRESSO spectrograph on the Very Large Telescope, and was announced on 1 October 2024.^[3] It is the second from its star of four known planets in its system.^[2]

The designation "Barnard's Star b" was first used for a different planetary candidate announced in 2018,^[4] the existence of which was later refuted.^[5]

Barnard b orbits close to its star, completing an orbit every 3.15 days at a distance of 0.023 AU (3.4 million km; 2.1 million mi).^[1] It orbits closer to the star than the habitable zone and so is too hot to be potentially habitable, with an estimated equilibrium temperature of 438 K (165 °C; 329 °F). Its orbital eccentricity is near zero, meaning its orbit is nearly circular.^[2]

Barnard b is a sub-Earth, with a minimum mass of 0.3 times the mass of Earth,^[2] and is thus likely a rocky planet. Its true mass is uncertain since its orbital inclination is unknown.^[1] The radius of Barnard b is also unknown, and TESS observations show no evidence that it transits its host star, which would otherwise allow its radius to be measured. Based on mass-radius relationships, its radius is predicted to be about three-quarters that of Earth. The lack of a transit sets an upper limit of 87.9° on the orbital inclination.^[6]

The discovery paper of Barnard b also found evidence for three additional planetary candidates,^[1] which were confirmed in 2025. These are all low-mass planets in close orbits, similar to Barnard b.^[2]

In November 2018, an international team of astronomers led by Ignasi Ribas announced the detection by radial velocity of a candidate super-Earth orbiting Barnard's Star, which was referred to as Barnard's Star b.^[4][7] However, the existence of this planet was refuted in 2021, when the radial velocity signal was found to originate from long-term activity on the star itself, related to its rotation.^[5] Further studies in the following years confirmed this result.^[8][1]

This planet was thought to orbit every 233 days at 0.4 AU, near the stellar system's snow line, and to have a minimum mass of 3.2 M_E. The planet would have most likely been frigid, with an estimated equilibrium temperature of about 105 K (−168 °C; −271 °F), placing it outside its host star's presumed habitable zone.^[4]

On 1 October 2024, the discovery of the planet now known as Barnard b was announced by a team of astronomers led by Jonay González Hernández, using radial velocity data from the ESPRESSO spectrograph on the Very Large Telescope. This constituted the first convincing evidence for a planet orbiting Barnard's Star. Additionally, three other candidate low-mass planets were proposed in this study, all orbiting closer to the star than the habitable zone.^[1][3] Barnard's Star b (or Barnard b) is a re-use of the designation originally used for the refuted super-Earth candidate.

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