Barnard's Star
Barnard's Star is a small red dwarf star in the constellation of Ophiuchus. At a distance of from Earth, it is the fourth-nearest-known individual star to the Sun after the three components of the Alpha Centauri system, and is the closest star in the northern celestial hemisphere. Its stellar mass is about 16% of the Sun's, and it has 19% of the Sun's diameter. Despite its proximity, the star has a dim apparent visual magnitude of +9.5 and is invisible to the unaided eye; it is much brighter in the infrared than in visible light.
Barnard's Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator. Historically, research on Barnard's Star has focused on measuring its stellar characteristics, its astrometry, and also refining the limits of possible extrasolar planets. Although Barnard's Star is ancient, it still experiences stellar flare events, one being observed in 1998.
Barnard's Star hosts a system of four close-orbiting, sub-Earth-mass planets. Multiple claims for a planetary system had been proposed since the beginning of the twentieth century, notably by Peter van de Kamp in the 1960s, but none were supported by follow-up studies, until the now known four-planet system was discovered by two independent teams of astronomers in 2024–2025.
Discovery and naming
The star is named after Edward Emerson Barnard, an American astronomer who in 1916 measured its proper motion as 10.3 arcseconds per year relative to the Sun, the highest known for any star. The star had previously appeared on Harvard University photographic plates in 1888 and 1890.In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue and standardize proper names for stars. The WGSN approved the name Barnard's Star for this star on 1 February 2017 and it is now included in the List of IAU-approved Star Names.
Description
Barnard's Star is a red dwarf of the dim spectral type M4 and is too faint to see without a telescope; its apparent magnitude is 9.5.At 7–12 billion years of age, Barnard's Star is considerably older than the Sun, which is 4.5 billion years old, and it might be among the oldest stars in the Milky Way galaxy. Barnard's Star has lost a great deal of rotational energy; the periodic slight changes in its brightness indicate that it rotates once in 130 days. Given its age, Barnard's Star was long assumed to be quiescent in terms of stellar activity. In 1998, astronomers observed an intense stellar flare, showing that Barnard's Star is a flare star. Barnard's Star has the variable star designation V2500 Ophiuchi. In 2003, Barnard's Star presented the first detectable change in the radial velocity of a star caused by its motion. Further variability in the radial velocity of Barnard's Star was attributed to its stellar activity.
The proper motion of Barnard's Star corresponds to a relative lateral speed of 90km/s. The 10.3 arcseconds it travels in a year amount to a quarter of a degree in a human lifetime, roughly half the angular diameter of the full Moon.
The radial velocity of Barnard's Star is, as measured from the blueshift due to its motion toward the Sun. Combined with its proper motion and distance, this gives a "space velocity" of. Barnard's Star will make its closest approach to the Sun around 11,800 CE, when it will approach to within about 3.75 light-years.
Proxima Centauri is the closest star to the Sun at a position currently 4.24 light-years distant from it. However, despite Barnard's Star's even closer pass to the Sun in 11,800 CE, it will still not then be the nearest star, since by that time Proxima Centauri will have moved to a yet-nearer proximity to the Sun. At the time of the star's closest pass by the Sun, Barnard's Star will still be too dim to be seen with the naked eye, since its apparent magnitude will only have increased by one magnitude to about 8.5 by then, still being 2.5 magnitudes short of visibility to the naked eye.
Barnard's Star has a mass of about 0.16 solar masses, and a radius about 0.2 times that of the Sun. Thus, although Barnard's Star has roughly 150 times the mass of Jupiter, its radius is only roughly 2 times larger, due to its much higher density. Its effective temperature is about 3,220 kelvin, and it has a luminosity of only 0.0034 solar luminosities. Barnard's Star is so faint that if it were at the same distance from Earth as the Sun is, it would appear only 100 times brighter than a full moon, comparable to the brightness of the Sun at 80 astronomical units.
Barnard's Star has 10–32% of the solar metallicity. Metallicity is the proportion of stellar mass made up of elements heavier than helium and helps classify stars relative to the galactic population. Barnard's Star seems to be typical of the old, red dwarf population II stars, yet these are also generally metal-poor halo stars. While sub-solar, Barnard's Star's metallicity is higher than that of a halo star and is in keeping with the low end of the metal-rich disk star range; this, plus its high space motion, have led to the designation "intermediate population II star", between a halo and disk star. However, some recently published scientific papers have given much higher estimates for the metallicity of the star, very close to the Sun's level, between 75 and 125% of the solar metallicity.
Planetary system
In August 2024, by using data from ESPRESSO spectrograph of the Very Large Telescope, the existence of an exoplanet with a minimum mass of and orbital period of 3.15 days was confirmed. 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 of these planets orbit closer to the star than the habitable zone. The confirmed planet is designated Barnard's Star b, a re-use of the designation originally used for the refuted super-Earth candidate. An examination of TESS photometry revealed no planetary transits, implying that the system is not viewed edge-on.In March 2025, an independent follow-up study confirmed all four planets. The data ruled out planets with masses greater than in the habitable zone of Barnard's Star with 99% confidence. With a minimum mass of only, Barnard's Star e is the least massive exoplanet yet detected by the radial velocity method. The best-fit orbital solution implies the planets have slightly eccentric orbits, but simulations suggest that these orbits would be unstable while circular orbits remain stable, so the eccentricities may be overestimated.
Previous planetary claims
Barnard's Star has been subject to multiple claims of planets that were later disproven. From the early 1960s to the early 1970s, Peter van de Kamp argued that planets orbited Barnard's Star. His specific claims of large gas giants were refuted in the mid-1970s after much debate. In November 2018, a candidate super-Earth planetary companion was reported to orbit Barnard's Star. It was believed to have a minimum mass of and orbit at. However, work presented in July 2021 refuted the existence of this planet.Astrometric planetary claims
For a decade from 1963 to about 1973, a substantial number of astronomers accepted a claim by Peter van de Kamp that he had detected, by using astrometry, a perturbation in the proper motion of Barnard's Star consistent with its having one or more planets comparable in mass with Jupiter. Van de Kamp had been observing the star from 1938, attempting, with colleagues at the Sproul Observatory at Swarthmore College, to find minuscule variations of one micrometre in its position on photographic plates consistent with orbital perturbations that would indicate a planetary companion; this involved as many as ten people averaging their results in looking at plates, to avoid systemic individual errors.Van de Kamp's initial suggestion was a planet having about at a distance of 4.4AU in a slightly eccentric orbit, and these measurements were apparently refined in a 1969 paper. Later that year, Van de Kamp suggested that there were two planets of 1.1 and.
Image:RedDwarfPlanet.jpg|thumb|right|Artist's conception of a planet in orbit around a red dwarf
Other astronomers subsequently repeated Van de Kamp's measurements, and two papers in 1973 undermined the claim of a planet or planets. George Gatewood and Heinrich Eichhorn, at a different observatory and using newer plate measuring techniques, failed to verify the planetary companion. Another paper published by John L. Hershey four months earlier, also using the Swarthmore observatory, found that changes in the astrometric field of various stars correlated to the timing of adjustments and modifications that had been carried out on the refractor telescope's objective lens; the claimed planet was attributed to an artifact of maintenance and upgrade work. The affair has been discussed as part of a broader scientific review.
Van de Kamp never acknowledged any error and published a further claim of two planets' existence as late as 1982; he died in 1995. Wulff Heintz, Van de Kamp's successor at Swarthmore and an expert on double stars, questioned his findings and began publishing criticisms from 1976 onwards. The two men were reported to have become estranged because of this.
Refuted 2018 planetary claim
In November 2018, an international team of astronomers announced the detection by radial velocity of a candidate super-Earth orbiting in relatively close proximity to Barnard's Star. Led by Ignasi Ribas of Spain their work, conducted over two decades of observation, provided strong evidence of the planet's existence. However, the existence of the 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. Further studies in the following years confirmed this result.Dubbed Barnard's Star b, the planet was thought to be near the stellar system's snow line, which is an ideal spot for the icy accretion of proto-planetary material. It was thought to orbit at 0.4AU every 233 days and had a proposed minimum mass of. The planet would have most likely been frigid, with an estimated surface temperature of about, and lie outside Barnard Star's presumed habitable zone. Direct imaging of the planet and its tell-tale light signature would have been possible in the decade after its discovery. Further faint and unaccounted-for perturbations in the system suggested there may be a second planetary companion even farther out.