Editing 61 Cygni (section)

== Claims of a planetary system ==
On different occasions, it has been claimed that 61 Cygni might have unseen low-mass companions, planets or a [[brown dwarf]]. [[Kaj Aage Gunnar Strand|Kaj Strand]] of the Sproul Observatory, under the direction of [[Peter van de Kamp]], made the first such claim in 1942 using observations to detect tiny but systematic variations in the orbital motions of 61 Cygni A and B. These [[Perturbation (astronomy)|perturbations]] suggested that a third body of about 16 Jupiter masses must be orbiting 61 Cygni A.<ref>{{cite journal
 | last=Strand | first=K. Aa.
 | title=61 Cygni as a Triple System
 | journal=Publications of the Astronomical Society of the Pacific
 | date=1943 | volume=55 | issue=322 | pages=29–32
 | bibcode=1943PASP...55...29S
 | doi=10.1086/125484 | doi-access=free}}</ref> Reports of this third body served as inspiration for [[Hal Clement]]'s 1953 science fiction novel ''[[Mission of Gravity]]''.<ref>{{Cite book |url=https://books.google.com/books?id=v6DNXIN3UqEC&q=third+body+61+cygni+mission+of+gravity&pg=PA44 |title=Science Fiction Voices #1 |last1=Darrell Schweitzer |last2=Theodore Sturgeon |last3=Alfred Bester |date=2009 |publisher=Wildside Press LLC |isbn=978-1-4344-0784-9 |page=64 |access-date=3 February 2019}}</ref> In 1957, van de Kamp narrowed his uncertainties, claiming that the object had a mass of eight times that of Jupiter, a calculated orbital period of 4.8&nbsp;years, and a semi-major axis of 2.4&nbsp;AU, where 1&nbsp;AU is the average distance from the Earth to the Sun.<ref>{{cite journal
 | last=Strand | first=K. Aa.
 | title=The orbital motion of 61 Cygni
 | journal=The Astronomical Journal
 | date=1957 | volume=62 | page=35
 | bibcode=1957AJ.....62Q..35S
 | doi=10.1086/107588 | doi-access=free
 }}</ref> In 1977, Soviet astronomers at the [[Pulkovo Observatory]] near [[Saint Petersburg]] suggested that the system included three planets: two giant planets with six and twelve [[Jupiter masses]] around 61 Cyg A,<ref>{{cite journal
 | author=Cumming, A.
 | author2=Marcy, G. W.
 | author3=Butler, R. P.
 | date=1999 | title=The Lick planet search: detectability and mass thresholds | doi=10.1086/308020
 | journal=Astrophysical Journal | volume=526
 | issue=2
 | pages=890–915
| bibcode=1999ApJ...526..890C
|arxiv = astro-ph/9906466 | s2cid=12560512
 }}</ref> and one giant planet with seven Jupiter masses around 61 Cygni B.<ref>{{cite journal
 | last=Deich | first=A. N.
 | title=Invisible companions of the binary star 61 Cygni
 | journal= [[Soviet Astronomy]]
 | date=1977 | volume=21 | pages=182–188
 | bibcode=1977SvA....21..182D}}</ref>

In 1978, [[Wulff-Dieter Heintz]] of the [[Sproul Observatory]] proved that these claims were spurious, as they were unable to detect any evidence of such motion down to six percent of the Sun's mass—equivalent to about 60 times the mass of [[Jupiter]].<ref>{{cite journal
 | last=Heintz | first=W. D.
 | title=Reexamination of suspected unresolved binaries
 | journal=The Astrophysical Journal
 | date=1978 | volume=220 | pages=931–934
 | bibcode=1978ApJ...220..931H
 | doi=10.1086/155982 | doi-access=free
 }}</ref><ref>{{cite journal
 | author=Walker, G. A. H.
 | author2=Walker, A. R.
 | author3=Irwin, A. W.
 | author4=Larson, A. M.
 | author5=Yang, S. L. S.
 | author6=Richardson, D. C.
 | date=1995 | title=A search for Jupiter-mass companions to nearby stars | journal=Icarus | volume=116 | issue=2
 | pages=359–375 | doi=10.1006/icar.1995.1130 | bibcode=1995Icar..116..359W}}</ref>

In 2018, analysis of the DR2 data gathered by the [[Gaia (spacecraft)|Gaia space telescope]] revealed significant proper motion anomalies in the orbits of the binary stars around each other; the stars were not quite orbiting around their centre of mass with 61 Cygni B also orbiting too slowly for its assumed mass. These anomalies taken together are indicative of the possible presence of a perturbing third object in orbit around 61 Cygni B.<ref>{{cite journal|last1=Kervella|first1=Pierre|last2=Arenou|first2=Frédéric|last3=Mignard|first3=François|last4=Thévenin|first4=Frédéric|display-authors=2|title=Stellar and substellar companions of nearby stars from Gaia DR2|journal=Astronomy & Astrophysics|volume=623|year=2019|pages=A72|issn=0004-6361|arxiv=1811.08902 |bibcode=2019A&A...623A..72K |doi=10.1051/0004-6361/201834371|s2cid=119491061|quote="This PMa offset between 61 Cyg A and B points at the possible presence of a third body in the system, likely orbiting around 61 Cyg B."}}</ref>

The [[habitable zone]] for 61 Cygni A, defined as the locations where liquid water could be present on an Earth-like planet, is 0.26–0.58&nbsp;[[astronomical unit|AU]]. For 61 Cygni B, the habitable zone is 0.24–0.50 AU.<ref>{{Citation |last1=Cantrell |first1=Justin R. |title=The Solar Neighborhood XXIX: The Habitable Real Estate of Our Nearest Stellar Neighbors |date=October 2013 |journal=The Astronomical Journal |volume=146 |issue=4 |page=99 |arxiv=1307.7038 |bibcode=2013AJ....146...99C |doi=10.1088/0004-6256/146/4/99|last2=Henry |first2=Todd J. |last3=White |first3=Russel J.|s2cid=44208180 }}</ref>

=== Refining planetary boundaries ===
Since no certain planetary object has been detected around either star so far, [[McDonald Observatory]] team has set limits to the presence of one or more planets around 61 Cygni A and 61 Cygni B with masses between 0.07 and 2.1&nbsp;Jupiter masses and average separations spanning between 0.05 and 5.2&nbsp;AU.<ref>{{cite journal
  | author=Wittenmyer, R. A.
 | author2=Endl, M.
 | author3=Cochran, W.D.
 | author4=Hatzes, A.
 | author5=Walker, G. A. H.
 | author6=Yang, S. L. S.
 | author7=Paulson, D. B.
 | title=Detection Limits from the McDonald Observatory Planet Search Program | journal=[[The Astronomical Journal]]
 |date=May 2006 | volume=132 | issue=1
 | pages=177–188 | doi=10.1086/504942 | bibcode=2006AJ....132..177W
 | arxiv=astro-ph/0604171| s2cid=16755455
 }}</ref>

Because of the proximity of this system to the Sun, it is a frequent target of interest for astronomers. Both stars were selected by [[NASA]] as "Tier 1" targets for the proposed optical [[Space Interferometry Mission]].<ref>{{cite web
 |url         = http://tauceti.sfsu.edu/~chris/SIM/t1.html
 |title       = SIM Planet Search Tier 1 Target Stars
 |first       = Christopher
 |last        = McCarthy
 |year        = 2005
 |publisher   = [[San Francisco State University]]
 |access-date  = 23 July 2007
 |archive-url  = https://web.archive.org/web/20070804210039/http://tauceti.sfsu.edu/~chris/SIM/t1.html
 |archive-date = 4 August 2007
 }}</ref> This mission is potentially capable of detecting planets with as little as 3&nbsp;times the mass of the Earth at an orbital distance of 2&nbsp;AU from the star.

Measurements of this system appeared to have detected an [[infrared excess|excess of far infrared radiation]], beyond what is emitted by the stars. Such an excess is sometimes associated with a [[debris disk|disk of dust]], but in this case it lies sufficiently close to one or both of the stars that it has not been resolved with a telescope.<ref>{{cite journal
 | author=Kuchner, Marc J.
 | author2=Brown, Michael E.
 | author3=Koresko, Chris D.
 | title=An 11.6 Micron Keck Search for Exo-Zodiacal Dust
 | journal=The Publications of the Astronomical Society of the Pacific
 | date=1998 | volume=110 | issue=753 | pages=1336–1341
 | bibcode=1998PASP..110.1336K
 | doi=10.1086/316267 |arxiv = astro-ph/0002040 | s2cid=119479494
 }}</ref> A 2011 study using the [[W. M. Keck Observatory|Keck Interferometer Nuller]] failed to detect any [[exozodiacal dust]] around 61 Cygni A.<ref>{{Citation |last1=Millan-Gabet, R. |title=Exozodiacal Dust Levels for Nearby Main-sequence Stars: A Survey with the Keck Interferometer |date=June 2011 |journal=The Astrophysical Journal |volume=734 |issue=1 |page=67 |postscript=. |arxiv=1104.1382 |bibcode=2011ApJ...734...67M |doi=10.1088/0004-637X/734/1/67 |last2=Serabyn, E. |last3=Mennesson, B. |last4=Traub, W. A. |last5=Barry, R. K. |last6=Danchi, W. C. |last7=Kuchner, M. |last8=Stark, C. C. |last9=Ragland, S. |last10=Hrynevych, M. |last11=Woillez, J. |last12=Stapelfeldt, K. |last13=Bryden, G. |last14=Colavita, M. M. |last15=Booth, A. J.|s2cid=118614703 }} See Table 5, p. 58.</ref>

=== Object for biosignature research ===
The two stars are among five (all nearby star) paradigms listed among those K-type stars of a type in a 'sweet spot' between Sun-analog stars and M stars for the likelihood of evolved life, per analysis of Giada Arney from NASA's Goddard Space Flight Center.<ref name="bill steigerwald-2019" />