Editing Solar System (section)

== Boundary region and uncertainties ==
{{See also|Planets beyond Neptune|Planet Nine|List of Solar System objects by greatest aphelion}}

[[File:Kuiper oort-en.svg|thumb|An [[artist's impression]] of the [[Oort cloud]], a region still well within the [[Sphere of influence (astrodynamics)|sphere of influence]] of the Solar System, including a depiction of the much further inside [[Kuiper belt]] (inset); the sizes of objects are over-scaled for visibility.]]

{{Anchor|Farthest regions|Boundaries}}Much of the outer Solar System is still unknown. The region beyond 100 AU away is virtually unexplored and learning about this region of space is difficult. Study of this region depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets.<ref>{{Cite journal |last1=Bernardinelli |first1=Pedro H. |last2=Bernstein |first2=Gary M. |last3=Montet |first3=Benjamin T. |last4=Weryk |first4=Robert |last5=Wainscoat |first5=Richard |last6=Aguena |first6=M. |last7=Allam |first7=S. |last8=Andrade-Oliveira |first8=F. |last9=Annis |first9=J. |last10=Avila |first10=S. |last11=Bertin |first11=E. |display-authors=3 |date=1 November 2021 |title=C/2014 UN 271 (Bernardinelli-Bernstein): The Nearly Spherical Cow of Comets |journal=[[The Astrophysical Journal Letters]] |volume=921 |issue=2 |page=L37 |arxiv=2109.09852 |bibcode=2021ApJ...921L..37B |doi=10.3847/2041-8213/ac32d3 |issn=2041-8205 |s2cid=237581632 |doi-access=free}}</ref> Many objects are yet to be discovered in the Solar System's outer region.<ref>{{Cite web |last=Loeffler |first=John |date=1 October 2021 |title=Our solar system may have a hidden planet beyond Neptune – no, not that one |url=https://www.msn.com/en-us/news/technology/our-solar-system-may-have-a-hidden-planet-beyond-neptune-no-not-that-one/ar-AAP3b0l?ocid=msedgntp |url-status=live |archive-url=https://web.archive.org/web/20211001203656/https://www.msn.com/en-us/news/technology/our-solar-system-may-have-a-hidden-planet-beyond-neptune-no-not-that-one/ar-AAP3b0l?ocid=msedgntp |archive-date=1 October 2021 |access-date=7 April 2022 |website=MSN}}</ref>

{{Anchor|Oort cloud}}The [[Oort cloud]] is a theorized spherical shell of up to a trillion icy objects that is thought to be the source for all long-period comets.<ref name=":5">{{Cite journal |vauthors=Stern SA, Weissman PR |date=2001 |title=Rapid collisional evolution of comets during the formation of the Oort cloud |journal=Nature |volume=409 |issue=6820 |pages=589–591 |bibcode=2001Natur.409..589S |doi=10.1038/35054508 |pmid=11214311 |s2cid=205013399}}</ref><ref name=":6">{{Cite web |last=Arnett |first=Bill |date=2006 |title=The Kuiper Belt and the Oort Cloud |url=http://www.nineplanets.org/kboc.html |url-status=live |archive-url=https://web.archive.org/web/20190807064224/http://nineplanets.org/kboc.html |archive-date=7 August 2019 |access-date=23 June 2006 |website=Nine Planets}}</ref> No direct observation of the Oort cloud is possible with present imaging technology.<ref>{{Cite web |title=Oort Cloud |url=https://solarsystem.nasa.gov/solar-system/oort-cloud/overview |url-status=live |archive-url=https://web.archive.org/web/20230630162050/https://solarsystem.nasa.gov/solar-system/oort-cloud/overview/ |archive-date=30 June 2023 |access-date=1 July 2023 |website=NASA Solar System Exploration|date=20 June 2023 }}</ref> It is theorized to surround the Solar System at roughly 50,000&nbsp;AU (~0.9&nbsp;[[Light-year|ly]]) from the Sun and possibly to as far as 100,000&nbsp;AU (~1.8&nbsp;ly). The Oort cloud is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the [[galactic tide]], the [[tidal force]] exerted by the Milky Way.<ref name=":5" /><ref name=":6" />

As of the 2020s, a few astronomers have hypothesized that [[Planet Nine]] (a planet beyond Neptune) might exist, based on statistical variance in the orbit of [[extreme trans-Neptunian object]]s.<ref name="P9H2019">{{cite journal |last1=Batygin |first1=Konstantin |last2=Adams |first2=Fred C. |last3=Brown |first3=Michael E. |last4=Becker |first4=Juliette C. |date=2019 |title=The Planet Nine Hypothesis |journal=[[Physics Reports]] |volume=805 |pages=1–53 |arxiv=1902.10103 |bibcode=2019PhR...805....1B |doi=10.1016/j.physrep.2019.01.009 |s2cid=119248548}}</ref> Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted, suggesting that a large planet might be influencing their orbit over millions of years.<ref name="Sheppard2014">{{cite journal |last1=Trujillo |first1=Chadwick A. |author-link=Chad Trujillo |last2=Sheppard |first2=Scott S. |author-link2=Scott S. Sheppard |date=2014 |title=A Sedna-like Body with a Perihelion of 80 Astronomical Units |url=http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf |url-status=dead |journal=[[Nature (journal)|Nature]] |volume=507 |issue=7493 |pages=471–474 |bibcode=2014Natur.507..471T |doi=10.1038/nature13156 |pmid=24670765 |s2cid=4393431 |url-access=subscription |archive-url=https://web.archive.org/web/20141216183818/http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf |archive-date=16 December 2014 |access-date=20 January 2016}}</ref><ref name="nodes-2021">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=1 September 2021 |title=Peculiar orbits and asymmetries in extreme trans-Neptunian space |url=https://academic.oup.com/mnras/article-abstract/506/1/633/6307523 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=506 |issue=1 |pages=633–649 |arxiv=2106.08369 |bibcode=2021MNRAS.506..633D |doi=10.1093/mnras/stab1756 |doi-access=free |access-date=20 April 2024 |archive-date=19 October 2021 |archive-url=https://web.archive.org/web/20211019195919/https://academic.oup.com/mnras/article-abstract/506/1/633/6307523 |url-status=live }}</ref><ref name="nodes-2022">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=1 May 2022 |title=Twisted extreme trans-Neptunian orbital parameter space: statistically significant asymmetries confirmed |url=https://academic.oup.com/mnrasl/article-abstract/512/1/L6/6524836 |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=512 |issue=1 |pages=L6–L10 |arxiv=2202.01693 |bibcode=2022MNRAS.512L...6D |doi=10.1093/mnrasl/slac012 |doi-access=free |access-date=20 April 2024 |archive-date=9 April 2023 |archive-url=https://web.archive.org/web/20230409170426/https://academic.oup.com/mnrasl/article-abstract/512/1/L6/6524836 |url-status=live }}</ref> However, some astronomers said that this observation might be credited to observational biases or just sheer coincidence.<ref>{{cite journal |last=Napier |first=K. J. |date=2021 |title=No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects |journal=The Planetary Science Journal |volume=2 |issue=2 |page=59 |arxiv=2102.05601 |bibcode=2021PSJ.....2...59N |doi=10.3847/PSJ/abe53e |doi-access=free}}</ref> An alternative hypothesis has a close flyby of another star disrupting the outer Solar System.<ref>{{cite journal | title=Trajectory of the stellar flyby that shaped the outer Solar System | first1=Susanne | last1=Pfalzner | first2=Amith | last2=Govind | first3=Simon Portegies | last3=Zwart | journal=Nature Astronomy | date=September 2024 | volume=8 | issue=11 | pages=1380–1386 | doi=10.1038/s41550-024-02349-x | arxiv=2409.03342 | bibcode=2024NatAs...8.1380P }}</ref>

The Sun's gravitational field is estimated to [[Sphere of influence (astrodynamics)|dominate the gravitational forces of surrounding stars]] out to about two light-years ({{val|125,000|fmt=commas|u=AU}}). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than {{val|50,000|fmt=commas|u=AU}}.<ref name="Encrenaz_et_al_2004">{{Cite book |last1=Encrenaz |first1=T. |author-link=Thérèse Encrenaz |title=The Solar System |last2=Bibring |first2=J. P. |last3=Blanc |first3=M. |last4=Barucci |first4=M. A. |last5=Roques |first5=F. |last6=Zarka |first6=P. H. |date=2004 |publisher=Springer |edition=3rd |page=1}}</ref> Most of the mass is orbiting in the region between 3,000 and {{val|100,000|fmt=commas|u=AU}}.<ref>{{Cite journal |last1=Torres |first1=S. |last2=Cai |first2=M. X. |last3=Brown |first3=A. G. A. |last4=Portegies Zwart |first4=S. |date=September 2019 |title=Galactic tide and local stellar perturbations on the Oort cloud: creation of interstellar comets |journal=Astronomy & Astrophysics |volume=629 |page=13 |arxiv=1906.10617 |bibcode=2019A&A...629A.139T |doi=10.1051/0004-6361/201935330 |s2cid=195584070 |id=A139}}</ref> The furthest known objects, such as [[Comet West]], have aphelia around {{val|70,000|fmt=commas|u=AU}} from the Sun.<ref>{{Cite web |last=Norman |first=Neil |date=May 2020 |title=10 great comets of recent times |url=https://www.skyatnightmagazine.com/space-science/greatest-comets-of-recent-times |url-status=live |archive-url=https://web.archive.org/web/20220125042109/https://www.skyatnightmagazine.com/space-science/greatest-comets-of-recent-times |archive-date=25 January 2022 |access-date=10 April 2022 |website=[[BBC Sky at Night|BBC Sky at Night Magazine]] |language=en}}</ref> The Sun's [[Hill sphere]] with respect to the galactic nucleus, the effective range of its gravitational influence, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.<ref name="Littmann">{{Cite book |last=Littmann |first=Mark |url=https://archive.org/details/planetsbeyonddis00mlit |title=Planets Beyond: Discovering the Outer Solar System |date=2004 |publisher=Courier Dover Publications |isbn=978-0-486-43602-9 |pages=[https://archive.org/details/planetsbeyonddis00mlit/page/n92 162]–163 |url-access=limited}}</ref> It was calculated by [[Gleb Alexandrovich Chebotarev|G. A. Chebotarev]] to be 230,000&nbsp;AU.<ref name=Chebotarev />

[[File:Interstellar medium annotated.jpg|thumb|center|upright=2.5|The Solar System (left) within the [[interstellar medium]], with the different regions and their distances on a [[logarithmic scale]]]]