Editing Barnard's Star (section)

==== 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.--->