Barnard's Star b
Sub-Earth exoplanet orbiting Barnard's Star
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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]
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Artist's impression of Barnard b | |
| Discovery[1] | |
|---|---|
| Discovered by | González Hernández et al. |
| Discovery site | ESPRESSO (VLT) |
| Discovery date | October 2024 |
| Radial velocity | |
| Designations | |
| Barnard b, GJ 699 b | |
| Orbital characteristics[2] | |
| 0.0229±0.0003 AU | |
| Eccentricity | 0.03+0.03 −0.02 |
| 3.1542±0.0004 d | |
| Semi-amplitude | 0.440±0.036 m/s |
| Star | Barnard's Star |
| Physical characteristics[2] | |
| Mass | ≥0.299±0.026 M🜨 |
| Temperature | 438 K (165 °C; 329 °F, equilibrium) |
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]
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).[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]
History of observations
Refuted 2018 candidate
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🜨. 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]
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 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.