The designation "Barnard's Star b" was first used for a different planetary candidate announced in 2018,[3] the existence of which was later refuted.[4]
Characteristics
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). 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 400 K (127 °C; 260 °F). Its orbital eccentricity is unknown, but is constrained to be less than 0.16, or less than 0.01 in a four-planet model of the system.[1]
Barnard b is a sub-Earth, with a minimum mass of 0.37 times the mass of Earth, 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.[5]
While Barnard b is the only confirmed planet orbiting Barnard's Star, its discovery paper found evidence for three additional planetary candidates. If confirmed, these would all be low-mass planets in close orbits, similar to Barnard b.[1]
History of observations
Refuted 2018 candidate
In November 2018, an international team of astronomers led by Ignasi Ribas of Spain announced the detection by radial velocity of a candidate super-Earth orbiting Barnard's Star, which was referred to as Barnard's Star b.[3][6] 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.[4] Further studies in the following years confirmed this result.[7][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 ME. 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.[3]
2024 confirmation
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 ESPRESSOspectrograph 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][2] 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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