Editing Barnard's Star (section)

==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 {{val|0.37|0.05|ul=Earth mass}} 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]].<ref>{{cite web
 | title=Scientists discover planet orbiting closest single star to our sun
 | website=phys.org | date=October 1, 2024
 | url=https://phys.org/news/2024-09-scientists-planet-orbiting-closest-star.html#google_vignette
 | access-date=2024-10-01
}}</ref><ref name="GonzálezHernández2024"/> The confirmed planet is designated [[Barnard's Star b]] (or Barnard b), a re-use of the designation originally used for the refuted super-Earth candidate. An examination of [[TESS]] photometry revealed no [[planetary transit]]s, implying that the system is not viewed edge-on.<ref name="Stefanov2025"/>

In March 2025, an independent follow-up study confirmed all four planets. The data ruled out planets with masses greater than {{val|0.57|ul=Earth mass}} in the habitable zone of Barnard's Star with 99% confidence. With a minimum mass of only {{val|0.193|u=Earth mass}}, 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 [[orbital eccentricity|eccentric]] orbits, but simulations suggest that these orbits would be unstable while circular orbits remain stable, so the eccentricities may be overestimated.<ref name="Basant2025"/><ref name="AASNova2025"/>

{{Orbitbox planet begin
| table_ref = <ref name="Basant2025"/>
}}
{{Orbitbox planet
| exoplanet = d
| mass_earth = {{val|0.263|0.024|p=≥}}
| semimajor = {{val|0.0188|0.0003}}
| period = {{val|2.3402|0.0003}}
| eccentricity = {{val|0.04|0.05|0.03}}
| inclination = >20
}}
{{Orbitbox planet
| exoplanet = [[Barnard's Star b|b]]
| mass_earth = {{val|0.299|0.026|p=≥}}
| semimajor = {{val|0.0229|0.0003}}
| period = {{val|3.1542|0.0004}}
| eccentricity = {{val|0.03|0.03|0.02}}
| inclination = 20-87.9<ref name="Stefanov2025"/>
}}
{{Orbitbox planet
| exoplanet = c
| mass_earth = {{val|0.335|0.030|p=≥}}
| semimajor = {{val|0.0274|0.0004}}
| period = {{val|4.1244|0.0006}}
| eccentricity = {{val|0.08|0.06|0.05}}
| inclination = >20
}}
{{Orbitbox planet
| exoplanet = e
| mass_earth = {{val|0.193|0.033|p=≥}}
| semimajor = {{val|0.0381|0.0005}}
| period = {{val|6.7392|0.0028}}
| eccentricity = {{val|0.04|0.04|0.03}}
| inclination = >20
}}
{{Orbitbox end}}

=== 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 giant]]s 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 {{earth mass|3.2|link=y}} and orbit at {{val|0.4|ul=AU}}.<ref name=Ribas/> However, work presented in July 2021 refuted the existence of this planet.<ref name=Lubin2021/>

==== 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 [[Methods of detecting exoplanets#Astrometry|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 plate]]s consistent with [[perturbation (astronomy)|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.<ref name=Blunder>{{cite web|url=http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=1635|title=The Barnard's Star Blunder|date=July 2005|work=Astrobiology Magazine|access-date=26 January 2014 |url-status=usurped |archive-url=https://web.archive.org/web/20110804214004/http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=1635 |archive-date=2011-08-04}}</ref> 
Van de Kamp's initial suggestion was a planet having about {{Jupiter mass|1.6}} at a distance of 4.4{{nbsp}}AU in a slightly eccentric orbit,<ref>{{cite journal|last=van de Kamp|first=Peter|year=1963|title=Astrometric study of Barnard's star from plates taken with the 24-inch Sproul refractor|journal=[[The Astronomical Journal]]|volume=68|issue=7|page=515|bibcode=1963AJ.....68..515V|doi=10.1086/109001}}</ref> and these measurements were apparently refined in a 1969 paper.<ref>{{cite journal|last=van de Kamp|first=Peter|year=1969|title=Parallax, proper motion acceleration, and orbital motion of Barnard's Star|journal=[[The Astronomical Journal]]|volume=74|issue=2|page=238|bibcode=1969AJ.....74..238V|doi=10.1086/110799|doi-access=free}}</ref> Later that year, Van de Kamp suggested that there were two planets of 1.1 and {{Jupiter mass|0.8}}.<ref>{{cite journal|last=van de Kamp|first=Peter|date=August 1969|title=Alternate dynamical analysis of Barnard's star|journal=The Astronomical Journal|volume=74|issue=8|pages=757–759|bibcode=1969AJ.....74..757V|doi=10.1086/110852}}</ref>

[[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.<ref>{{cite journal|last1=Gatewood|first1=George|last2=Eichhorn|first2=H.|name-list-style=amp|year=1973|title=An unsuccessful search for a planetary companion of Barnard's star (BD +4 3561)|bibcode=1973AJ.....78..769G|journal=[[The Astronomical Journal]]|volume=78|issue=10|page=769|doi=10.1086/111480|doi-access=free}}</ref> 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;<ref>{{cite journal|first=John L.|last=Hershey|date=June 1973|title=Astrometric analysis of the field of AC +65 6955 from plates taken with the Sproul 24-inch refractor|journal=The Astronomical Journal|volume=78|issue=6|pages=421–425|bibcode=1973AJ.....78..421H|doi=10.1086/111436|doi-access=free}}</ref> 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.<ref name=Bell>{{cite web|first=George H.|last=Bell|url=http://www.public.asu.edu/~sciref/exoplnt.htm|date=April 2001|title=The Search for the Extrasolar Planets: A Brief History of the Search, the Findings and the Future Implications|at=Section 2|publisher=Arizona State University|access-date=10 August 2006|archive-url=https://web.archive.org/web/20060813111219/http://www.public.asu.edu/~sciref/exoplnt.htm|archive-date=13 August 2006|url-status=live}} (Full description of the Van de Kamp planet controversy.)</ref>

Van de Kamp never acknowledged any error and published a further claim of two planets' existence as late as 1982;<ref>{{cite journal|last=Van de Kamp|first=Peter|year=1982|title=The planetary system of Barnard's star|journal=Vistas in Astronomy|volume=26|issue=2|page=141|bibcode=1982VA.....26..141V|doi=10.1016/0083-6656(82)90004-6}}</ref> he died in 1995. [[Wulff Heintz]], Van de Kamp's successor at Swarthmore and an expert on [[double star]]s, questioned his findings and began publishing criticisms from 1976 onwards. The two men were reported to have become estranged because of this.<ref name=Swathmore>{{cite web|first=Bill|last=Kent|url=http://media.swarthmore.edu/bulletin/wp-content/archived_issues_pdf/Bulletin_2001_03.pdf|title=Barnard's Wobble|pages=28–31|date=March 2001|work=Swarthmore College Bulletin|publisher=Swarthmore College|access-date=2 June 2010|url-status=dead|archive-url=https://web.archive.org/web/20110719124154/http://media.swarthmore.edu/bulletin/wp-content/archived_issues_pdf/Bulletin_2001_03.pdf|archive-date=19 July 2011}}</ref>

==== Refuted 2018 planetary claim ====
[[File:Artist’s impression of the surface of a super-Earth orbiting Barnard’s Star.jpg|thumb|left|Artist's impression of the surface of a [[super-Earth]] orbiting Barnard's Star<ref>{{cite web|title=Super-Earth Orbiting Barnard's Star – Red Dots campaign uncovers compelling evidence of exoplanet around closest single star to Sun|url=https://www.eso.org/public/news/eso1837/|website=eso.org|access-date=15 November 2018}}</ref>]]
In November 2018, an international team of astronomers announced the detection by [[Doppler spectroscopy|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.<ref name=Ribas/><ref>{{cite web|url=https://www.eso.org/public/news/eso1837/|title=Super-Earth Orbiting Barnard's Star|publisher=[[European Southern Observatory]]|date=14 November 2018|access-date=14 November 2018}}</ref> 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.<ref name=Lubin2021 /> Further studies in the following years confirmed this result.<ref name="Artigau2022"/><ref name="GonzálezHernández2024"/>

Dubbed Barnard's Star b, the planet was thought to be near the stellar system's [[Frost line (astrophysics)|snow line]], which is an ideal spot for the icy accretion of proto-planetary material. It was thought to orbit at 0.4{{nbsp}}AU every 233 days and had a proposed [[minimum mass]] of {{earth mass|3.2|link=y}}. The planet would have most likely been frigid, with an estimated surface temperature of about {{Convert|-170|C}}, and lie outside Barnard Star's presumed [[habitable zone]]. Direct imaging of the planet and its tell-tale [[Spectral signature|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.<ref name=sciam>{{cite web|url=https://www.scientificamerican.com/article/a-frozen-super-earth-may-orbit-barnards-star/|title=A Frozen Super-Earth May Orbit Barnard's Star|last=Billings|first=Lee|work=Scientific American|date=14 November 2018|access-date=19 November 2018}}</ref>

==== Refining planetary boundaries ====
For the more than four decades between van de Kamp's rejected claim and the eventual announcement of a planet candidate, Barnard's Star was carefully studied and the mass and orbital boundaries for possible planets were slowly tightened. [[M dwarf]]s such as Barnard's Star are more easily studied than larger stars in this regard because their lower masses render perturbations more obvious.<ref>{{cite journal|last1=Endl|first1=Michael|last2=Cochran|first2=William D.|last3=Tull|first3=Robert G.|last4=MacQueen|first4=Phillip J.|year=2003|title=A Dedicated M Dwarf Planet Search Using the Hobby-Eberly Telescope|journal=[[The Astronomical Journal]]|volume=126|issue=12|pages=3099–3107|bibcode=2003AJ....126.3099E|arxiv=astro-ph/0308477|doi=10.1086/379137|s2cid=17353771}}</ref>

Null results for planetary companions continued throughout the 1980s and 1990s, including [[interferometric]] work with the [[Hubble Space Telescope]] in 1999.<ref name=Hubble99>{{cite journal|last1=Benedict|first1=G. Fritz|title=Interferometric Astrometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: Detection Limits for Substellar Companions|year=1999|journal=[[The Astronomical Journal]]|volume=118|issue=2|pages=1086–1100|arxiv=astro-ph/9905318|doi=10.1086/300975 |bibcode=1999AJ....118.1086B|last2=McArthur|first2=Barbara|last3=Chappell|first3=D. W.|last4=Nelan|first4=E.|last5=Jefferys|first5=W. H.|last6=Van Altena|first6=W.|last7=Lee|first7=J.|last8=Cornell|first8=D.|last9=Shelus|first9=P. J.|last10=Hemenway|first10=P. D.|last11=Franz|first11=Otto G.|last12=Wasserman|first12=L. H.|last13=Duncombe|first13=R. L.|last14=Story|first14=D.|last15=Whipple|first15=A. L.|last16=Fredrick|first16=L. W.|s2cid=18099356}}</ref> Gatewood was able to show in 1995 that planets with {{Jupiter mass|10}} were impossible around Barnard's Star,<ref name=Bell/> in a paper which helped refine the negative certainty regarding planetary objects in general.<ref name=Gatewood95>{{cite journal|last=Gatewood|first=George D.|title=A study of the astrometric motion of Barnard's star|journal=Astrophysics and Space Science|volume=223|issue=1|year=1995|pages=91–98|doi=10.1007/BF00989158|bibcode=1995Ap&SS.223...91G|s2cid=120060893}}</ref> In 1999, the Hubble work further excluded planetary companions of {{Jupiter mass|0.8}} with an orbital period of less than 1,000 days (Jupiter's orbital period is 4,332 days),<ref name=Hubble99/> while Kuerster determined in 2003 that within the [[habitable zone]] around Barnard's Star, planets are not possible with an "''M'' sin ''i''" value<ref group=note>"''M'' sin ''i''" means the mass of the planet times the sine of the angle of inclination of its orbit, and hence provides the minimum mass for the planet.</ref> greater than 7.5 times the mass of the Earth ({{Earth mass|link=y}}), or with a mass greater than 3.1 times the mass of Neptune (much lower than van de Kamp's smallest suggested value).<ref name=Kurster/>

In 2013, a research paper was published that further refined planet mass boundaries for the star. Using radial velocity measurements, taken over a period of 25 years, from the [[Lick Observatory|Lick]] and [[W. M. Keck Observatory|Keck]] Observatories and applying [[Monte Carlo analysis]] for both circular and eccentric orbits, upper masses for planets out to 1,000-day orbits were determined. Planets above two Earth masses in orbits of less than 10 days were excluded, and planets of more than ten Earth masses out to a two-year orbit were also confidently ruled out. It was also discovered that the habitable zone of the star seemed to be devoid of roughly Earth-mass planets or larger, save for face-on orbits.<ref name="no-earth-dreams">{{cite web|url=https://www.centauri-dreams.org/2012/08/16/barnards-star-no-earth-mass-planets-found/|title=Barnard's Star: No Sign of Planets|website=Centauri Dreams|last=Gilster|first=Paul|date=16 August 2012|access-date=11 April 2018}}</ref><ref name=Choi2013>{{Cite journal|arxiv=1208.2273|last1=Choi|first1=Jieun|title=Precise Doppler Monitoring of Barnard's Star|journal=The Astrophysical Journal|volume=764|issue=2|pages=131|last2=McCarthy|first2=Chris|last3=Marcy|first3=Geoffrey W|last4=Howard|first4=Andrew W|last5=Fischer|first5=Debra A|last6=Johnson|first6=John A|last7=Isaacson|first7=Howard|last8=Wright|first8=Jason T|year=2012|doi=10.1088/0004-637X/764/2/131|bibcode=2013ApJ...764..131C|s2cid=29053334}}</ref>

Even though this research greatly restricted the possible properties of planets around Barnard's Star, it did not rule them out completely as [[terrestrial planet]]s were always going to be difficult to detect. [[NASA]]'s [[Space Interferometry Mission]], which was to begin searching for extrasolar Earth-like planets, was reported to have chosen Barnard's Star as an early search target,<ref name=SolStation/> however the mission was shut down in 2010.<ref name=SIM>{{cite web|first=James|last=Marr|url=http://planetquest.jpl.nasa.gov/SIM/projectNews/projectManagerUpdates/|archive-url=https://web.archive.org/web/20110302135037/http://planetquest.jpl.nasa.gov/SIM/projectNews/projectManagerUpdates/|url-status=dead|archive-date=2 March 2011|title=Updates from the Project Manager|date=8 November 2010|publisher=NASA|access-date=26 January 2014}}</ref> [[ESA]]'s similar [[Darwin (ESA)|Darwin]] interferometry mission had the same goal, but was stripped of funding in 2007.<ref>{{cite web|url=http://www.esa.int/esaSC/SEMZ0E1A6BD_index_0.html|title=Darwin factsheet: Finding Earth-like planets|publisher=[[European Space Agency]]|date=23 October 2009|access-date=12 September 2011|url-status=dead|archive-url=https://web.archive.org/web/20080513085904/http://www.esa.int/esaSC/SEMZ0E1A6BD_index_0.html|archive-date=13 May 2008}}</ref>

The analysis of radial velocities that eventually led to the announcement of a candidate super-Earth orbiting Barnard's Star was also used to set more precise upper mass limits for possible planets, up to and within the habitable zone: a maximum of {{earth mass|0.7|link=y}} up to the inner edge and {{earth mass|1.2}} on the outer edge of the optimistic habitable zone, corresponding to orbital periods of up to 10 and 40 days respectively. Therefore, it appears that Barnard's Star indeed does not host Earth-mass planets or larger, in hot and temperate orbits, unlike other M-dwarf stars that commonly have these types of planets in close-in orbits.<ref name=Ribas/><!--It was much easier for the Kepler telescope to detect close-in larger planets than further-out planets, or smaller close-in planets, so M-dwarfs may not in reality commonly host close-in larger planets. It might all just be down to an observational bias inherent to the way Kepler detects planets, and just an illusion.--->