Editing Oort cloud (section)

== Stellar perturbations and stellar companion hypotheses ==
Besides the [[galactic tide]], the main trigger for sending comets into the inner Solar System is thought to be interaction between the Sun's Oort cloud and the gravitational fields of nearby stars<ref name=Morbidelli2006 /> or giant [[molecular cloud]]s.<ref name=julio /> The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively [[List of nearest stars#Future and past|close proximity to other stellar systems]]. For example, it is hypothesized that 70,000 years ago [[Scholz's Star]] passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect).<ref>{{cite journal | last1 = de la Fuente Marcos | first1 = Carlos | last2 = de la Fuente Marcos | first2 = Raúl | last3 = Aarseth | first3 = Sverre J. | title = Where the Solar System meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies | journal = Monthly Notices of the Royal Astronomical Society: Letters | volume = 476 | issue = 1 | pages = L1–L5 | year = 2018 | doi = 10.1093/mnrasl/sly019 | doi-access = free | url = https://academic.oup.com/mnrasl/article/476/1/L1/4914453 | arxiv = 1802.00778 }}</ref><ref>{{cite journal
 |last1        = Mamajek
 |first1       = Eric E.
 |last2        = Barenfeld
 |first2       = Scott A.
 |last3        = Ivanov
 |first3       = Valentin D.
 |title        = The Closest Known Flyby of a Star to the Solar System
 |journal      = [[The Astrophysical Journal]]
 |volume       = 800
 |issue        = 1
 |date         = 2015
 |doi          = 10.1088/2041-8205/800/1/L17
 |arxiv        = 1502.04655
 |bibcode      = 2015ApJ...800L..17M
 |page         = L17
 |s2cid        = 40618530
 |url          = https://authors.library.caltech.edu/55650/1/2041-8205_800_1_L17.pdf
 |access-date  = 2018-08-02
 |archive-date = 2017-08-16
 |archive-url  = https://web.archive.org/web/20170816112840/http://authors.library.caltech.edu/55650/1/2041-8205_800_1_L17.pdf
 |url-status   = live
}}</ref> During the next 10&nbsp;million years the known star with the greatest possibility of perturbing the Oort cloud is [[Gliese 710]].<ref name=algol /> This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution.<ref name="algol">{{cite conference|author=L. A. Molnar|author2=R. L. Mutel|date=1997|title=Close Approaches of Stars to the Oort Cloud: Algol and Gliese 710|conference=American Astronomical Society 191st meeting|publisher=[[American Astronomical Society]]|bibcode=1997AAS...191.6906M}}<!--|access-date = 2014-01-19--></ref><ref>{{cite journal
 |author=A. Higuchi
 |author2=E. Kokubo
 |author3=T. Mukai
 |name-list-style=amp
 |date=2006
 |title=Scattering of Planetesimals by a Planet: Formation of Comet Cloud Candidates
 |journal=[[Astronomical Journal]]
 |volume=131
 |pages=1119–1129
 |bibcode=2006AJ....131.1119H
 |doi=10.1086/498892
 |issue=2
 |url=https://zenodo.org/record/896851
 |doi-access=free
 |access-date=2019-08-25
 |archive-date=2020-10-01
 |archive-url=https://web.archive.org/web/20201001163305/https://zenodo.org/record/896851
 |url-status=live
 }}</ref>

In 1984, [[physicist]] [[Richard A. Muller]] postulated that the Sun has an as-yet undetected companion, either a [[brown dwarf]] or a [[red dwarf]], in an elliptical orbit within the Oort cloud.<ref>{{cite journal | last1 = Davis | first1 = M. | last2 = Hut | first2 = P. | last3 = Muller | first3 = R. A. | title = Extinction of species by periodic comet showers | journal = Nature | volume = 308 | issue = 5954 | pages = 715–717 | year = 1984 | doi = 10.1038/308715a0 | bibcode = 1984Natur.308..715D | url = https://www.nature.com/articles/308715a0 }}</ref> This object, known as [[Nemesis (hypothetical star)|Nemesis]], was hypothesized to pass through a portion of the Oort cloud approximately every 26&nbsp;million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis has been found, and many lines of evidence (such as [[crater counting|crater counts]]), have thrown its existence into doubt.<ref>{{cite journal
 |author= J. G. Hills
 |date=1984
 |title=Dynamical constraints on the mass and perihelion distance of Nemesis and the stability of its orbit
 |journal=[[Nature (journal)|Nature]]
 |volume=311 |issue= 5987 |pages=636–638
 |bibcode = 1984Natur.311..636H
 |doi=10.1038/311636a0
|s2cid=4237439
 }}</ref><ref>{{cite web|title=Nemesis is a myth|publisher=Max Planck Institute|url=http://www.mpg.de/4372308/nemsis_myth?page=1|date=2011|access-date=2011-08-11|archive-date=2011-11-05|archive-url=https://web.archive.org/web/20111105170009/http://www.mpg.de/4372308/nemsis_myth?page=1|url-status=live}}</ref> Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.<ref name="Tyche2011-060" /> Thus, the Nemesis hypothesis is no longer needed to explain current assumptions.<ref name="Tyche2011-060">{{cite web
  |date=February 18, 2011
  |title=Can WISE Find the Hypothetical 'Tyche'?
  |publisher=NASA/JPL
  |url=https://www.jpl.nasa.gov/news/news.php?feature=2921
  |access-date=2011-06-15
  |archive-date=2020-12-05
  |archive-url=https://web.archive.org/web/20201205102115/https://www.jpl.nasa.gov/news/news.php?feature=2921
  |url-status=live
  }}</ref>

A somewhat similar hypothesis was advanced by astronomer [[John J. Matese]] of the [[University of Louisiana at Lafayette]] in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the postulated Oort cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause would be a [[Jupiter]]-mass object in a distant orbit.<ref>{{cite conference
 |author=John J. Matese
 |author2=Jack J. Lissauer
 |name-list-style=amp
 |chapter=Continuing Evidence of an Impulsive Component of Oort Cloud Cometary Flux
 |title=Proceedings of Asteroids, Comets, Meteors – ACM 2002. International Conference, 29 July – 2 August 2002, Berlin, Germany
 |journal=Asteroids
 |pages=309–314
 |chapter-url=http://www.ucs.louisiana.edu/~jjm9638/acm2002/acm2002_05_06.pdf
 |publisher=[[University of Louisiana at Lafayette]], and [[NASA]] [[Ames Research Center]]
 |date=2002-05-06
 |volume=500
 |access-date=2008-03-21
 |bibcode=2002ESASP.500..309M
 |archive-date=2012-10-21
 |archive-url=https://web.archive.org/web/20121021071740/http://www.ucs.louisiana.edu/~jjm9638/acm2002/acm2002_05_06.pdf
 |url-status=live
 }}</ref> This hypothetical [[gas giant]] was nicknamed [[Tyche (hypothetical planet)|Tyche]]. The [[WISE mission]], an [[all-sky survey]] using [[parallax]] measurements in order to clarify local star distances, was capable of proving or disproving the Tyche hypothesis.<ref name="Tyche2011-060" /> In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.<ref name="NASA-20140307">{{cite journal
 |journal=[[The Astrophysical Journal]]
 |last=K. L. |first=Luhman
 |title=A Search For A Distant Companion To The Sun With The Wide-field Infrared Survey Explorer
 |date=7 March 2014
 |volume=781 |number=1 |page=4
 |doi=10.1088/0004-637X/781/1/4
 |bibcode = 2014ApJ...781....4L
|s2cid=122930471 }}</ref>