Barnard's Star b

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Barnard's Star b
Artist's impression of a sub-Earth-mass planet orbiting Barnard's star (eso2414a).jpg
Artist's impression of Barnard b
Discovery [1]
Discovered by González Hernández et al.
Discovery site ESPRESSO (VLT)
Discovery dateOctober 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)

    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]

    Contents

    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 [2] 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. [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  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. [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.

    See also

    References

    1. 1 2 3 4 5 6 González Hernández, J. I.; Suárez Mascareño, A.; et al. (1 October 2024). "A sub-Earth-mass planet orbiting Barnard's star". Astronomy & Astrophysics . 690: A79. arXiv: 2410.00569 . Bibcode:2024A&A...690A..79G. doi: 10.1051/0004-6361/202451311 .
    2. 1 2 3 4 5 6 Basant, Ritvik; Luque, Rafael; et al. (March 2025). "Four Sub-Earth Planets Orbiting Barnard's Star from MAROON-X and ESPRESSO". The Astrophysical Journal Letters . 982: L1. arXiv: 2503.08095 . doi: 10.3847/2041-8213/adb8d5 .
    3. 1 2 "Scientists discover planet orbiting closest single star to our Sun". European Southern Observatory. 1 October 2024. Retrieved 1 October 2024.
    4. 1 2 3 Ribas, I.; Tuomi, M.; et al. (November 2018). "A candidate super-Earth planet orbiting near the snow line of Barnard's star". Nature . 563 (7731): 365–368. arXiv: 1811.05955 . Bibcode:2018Natur.563..365R. doi:10.1038/s41586-018-0677-y. PMID   30429552.
    5. 1 2 Lubin, Jack; Robertson, Paul; et al. (August 2021). "Stellar Activity Manifesting at a One-year Alias Explains Barnard b as a False Positive". The Astronomical Journal . 162 (2): 61. arXiv: 2105.07005 . Bibcode:2021AJ....162...61L. doi: 10.3847/1538-3881/ac0057 .
    6. ↑ Stefanov, A. K.; González Hernández, J. I.; et al. (January 2025). "A sub-Earth-mass planet orbiting Barnard's star: No evidence of transits in TESS photometry". Astronomy & Astrophysics . 693: L3. arXiv: 2410.00577 . Bibcode:2025A&A...693L...3S. doi:10.1051/0004-6361/202452450.
    7. ↑ "Super-Earth Orbiting Barnard's Star". European Southern Observatory. 14 November 2018. Retrieved 1 October 2024.
    8. ↑ Artigau, Étienne; Cadieux, Charles; et al. (September 2022). "Line-by-line Velocity Measurements: an Outlier-resistant Method for Precision Velocimetry". The Astronomical Journal . 164 (3): 84. arXiv: 2207.13524 . Bibcode:2022AJ....164...84A. doi: 10.3847/1538-3881/ac7ce6 .
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