Editing Oort cloud (section)

== Tidal effects ==
{{Main|Galactic tide}}
Most of the comets seen close to the Sun seem to have reached their current positions through gravitational perturbation of the Oort cloud by the [[tidal force]] exerted by the [[Milky Way]]. Just as the [[Moon]]'s tidal force deforms Earth's oceans, causing the tides to rise and fall, the galactic tide also distorts the orbits of bodies in the [[outer Solar System]].<ref>{{cite journal | last1 = Heisler | first1 = J. | last2 = Tremaine | first2 = S. | title = The influence of the Galactic tidal field on the Oort comet cloud | journal = Icarus | volume = 65 | issue = 1 | pages = 13–26 | year = 1986 | doi = 10.1016/0019-1035(86)90060-6 | bibcode = 1986Icar...65...13H | url = https://dx.doi.org/10.1016/0019-1035%2886%2990060-6 }}</ref> In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun, but in the outer reaches of the system, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational [[Galactic Center]] compresses it along the other two axes; these small perturbations can shift orbits in the Oort cloud to bring objects close to the Sun.<ref>{{cite journal
|author=Marc Fouchard
|author2=Christiane Froeschlé
|author3=Giovanni Valsecchi
|author4=Hans Rickman
|date=2006
|title=Long-term effects of the galactic tide on cometary dynamics
|journal=[[Celestial Mechanics and Dynamical Astronomy]]
|volume=95 |issue=1–4 |pages=299–326
|bibcode=2006CeMDA..95..299F
|doi=10.1007/s10569-006-9027-8
|s2cid=123126965
}}</ref> The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 AU, and marks the outer boundary of the Oort cloud.<ref name=book />

Some scholars theorize that the galactic tide may have contributed to the formation of the Oort cloud by increasing the [[Perihelion and aphelion|perihelia]] (smallest distances to the Sun) of [[planetesimal]]s with large aphelia (largest distances to the Sun).<ref>{{cite journal
|author=Higuchi A.
|author2=Kokubo E.
|author3=Mukai, T.
|name-list-style=amp
|date=2005
|title=Orbital Evolution of Planetesimals by the Galactic Tide
|journal=[[Bulletin of the American Astronomical Society]]
|volume=37 |page=521
|bibcode=2005DDA....36.0205H
}}</ref> The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.<ref>{{cite journal
|author=Nurmi P.
|author2=Valtonen M.J. |author3=Zheng J.Q.
 |date=2001
|title=Periodic variation of Oort Cloud flux and cometary impacts on the Earth and Jupiter
|journal=[[Monthly Notices of the Royal Astronomical Society]]
|volume=327 |issue=4 |pages=1367–1376
|bibcode=2001MNRAS.327.1367N
|doi=10.1046/j.1365-8711.2001.04854.x
|doi-access=free
}}</ref> Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.<ref>{{cite journal
|author=John J. Matese
|author2=Jack J. Lissauer
|name-list-style=amp
|date=2004
|title=Perihelion evolution of observed new comets implies the dominance of the galactic tide in making Oort Cloud comets discernible
|journal=[[Icarus (journal)|Icarus]]
|volume=170
|issue=2
|pages=508–513
|bibcode=2004Icar..170..508M
|doi=10.1016/j.icarus.2004.03.019
|citeseerx=10.1.1.535.1013
|url=http://www.ucs.louisiana.edu/~jjm9638/dps2003/I08821w.pdf
|access-date=2018-08-02
|archive-date=2016-03-09
|archive-url=https://web.archive.org/web/20160309162822/http://www.ucs.louisiana.edu/~jjm9638/dps2003/I08821w.pdf
|url-status=live
}}</ref>