Editing Oort cloud (section)

== Comets ==
{{See|Halley-type comet|List of Halley-type comets}}
{{See|Jupiter-family comet|List of periodic comets#List of unnumbered Jupiter-Family comets}}
{{See|List of centaurs (small Solar System bodies)}}
[[Comet]]s are remnants from the formation of the Solar system around 4 billion years ago, stored in two separate areas, the [[Kuiper belt]] and the Oort cloud.<ref>{{Cite journal |last=Alan Stern |first=S. |date=August 2003 |title=The evolution of comets in the Oort cloud and Kuiper belt |url=https://www.nature.com/articles/nature01725 |journal=Nature |language=en |volume=424 |issue=6949 |pages=639–642 |doi=10.1038/nature01725 |issn=0028-0836}}</ref> Short-period comets (those with orbits of up to 200&nbsp;years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune's orbit at 30&nbsp;AU and jointly extending out beyond 100&nbsp;AU. Very long-period comets, such as [[C/1999 F1 (Catalina)]], whose orbits last for millions of years, are thought to originate directly from the outer Oort cloud.<ref name="C1999F1"/> Other comets modeled to have come directly from the outer Oort cloud include [[C/2006 P1 (McNaught)]], [[C/2010 X1 (Elenin)]], [[Comet ISON]], [[C/2013 A1 (Siding Spring)]], [[C/2017 K2]], and [[C/2017 T2 (PANSTARRS)]]. The orbits within the Kuiper belt are relatively stable, so very few comets are thought to originate there. The scattered disc, however, is dynamically active and is far more likely to be the place of origin for comets.<ref name=book /> Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as [[centaur (minor planet)|centaurs]].<ref>{{cite book
 |author=Harold E. Levison
 |author2=Luke Dones
 |name-list-style=amp
 |date=2007
 |chapter=Chapter 31: Comet Populations and Cometary Dynamics
 |title=Encyclopedia of the Solar System
 |pages=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 575–588]
 |doi=10.1016/B978-012088589-3/50035-9
 |isbn=978-0-12-088589-3
 |bibcode=2007ess..book..575L
 |chapter-url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575
 }}</ref><ref>{{cite journal | last = Jewitt | first = David | title = The Active Centaurs | journal = The Astronomical Journal | volume = 137 | issue = 5 | pages = 4296–4312 | year = 2009 | doi = 10.1088/0004-6256/137/5/4296 | arxiv = 0902.4687 | bibcode = 2009AJ....137.4296J | url = https://iopscience.iop.org/article/10.1088/0004-6256/137/5/4296 }}</ref> These centaurs are then sent farther inward to become the short-period comets.<ref>{{cite journal
 |author  = J Horner
 |author2 = NW Evans
 |author3 = ME Bailey
 |author4 = DJ Asher
 |date    = 2003
 |title   = The Populations of Comet-like Bodies in the Solar System
 |journal = Monthly Notices of the Royal Astronomical Society
 |volume  = 343
 |issue   = 4
 |pages   = 1057–1066
 |doi     = 10.1046/j.1365-8711.2003.06714.x
|doi-access = free
 |arxiv= astro-ph/0304319
 |bibcode= 2003MNRAS.343.1057H
 |s2cid = 2822011
 }}</ref>

There are two main types of short-period comets: Jupiter-family comets (with orbits smaller than 5 AU) and Halley-family comets. Halley-family comets, named after [[Halley's Comet]], are distinct because, even though they are short-period comets, they are thought to come from the Oort Cloud rather than the scattered disc.<ref>{{cite journal | last1 = Levison | first1 = H. F. | last2 = Duncan | first2 = M. J. | title = From the Kuiper Belt to Jupiter-Family Comets: The Spatial Distribution of Ecliptic Comets | journal = Icarus | volume = 127 | issue = 1 | pages = 13–32 | year = 1997 | doi = 10.1006/icar.1996.5637 | bibcode = 1997Icar..127...13L | url = https://www.sciencedirect.com/science/article/abs/pii/S0019103596956377 | url-access = subscription }}</ref><ref>{{cite journal | last1 = Wang | first1 = J.-H. | last2 = Brasser | first2 = R. | title = An Oort Cloud origin of the Halley-type comets | journal = Astronomy & Astrophysics | volume = 563 | pages = A122 | year = 2014 | doi = 10.1051/0004-6361/201322508 | arxiv = 1402.2027 | bibcode = 2014A&A...563A.122W | url = https://www.aanda.org/articles/aa/full_html/2014/03/aa22508-13/aa22508-13.html }}</ref> Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.<ref name=dave /> This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.<ref name="emel2007" />

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved.<ref>{{cite journal | last = Neslušan | first = L. | title = Fading of long-period comets in near-parabolic orbits | journal = Astronomy & Astrophysics | volume = 351 | pages = 767–772 | year = 1999 | doi = 10.1051/aas:1999605 | doi-broken-date = 23 March 2025 | url = https://www.aanda.org/articles/aa/full/1999/47/aa9605/aa9605.html }}</ref> No dynamical process is known to explain the smaller number of observed comets than Oort estimated. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all [[Volatile (astrogeology)|volatiles]], rendering some comets invisible, or the formation of a non-volatile crust on the surface.<ref>{{cite book
 |author=Luke Dones
 |author2=Paul R Weissman
 |author3=Harold F Levison
 |author4=Martin J Duncan
 |chapter=Oort Cloud Formation and Dynamics
 |chapter-url=http://www.lpi.usra.edu/books/CometsII/7031.pdf
 |editor=Michel C. Festou
 |editor2=H. Uwe Keller
 |editor3=Harold A. Weaver
 |date=2004
 |title=Comets II
 |url=http://www.uapress.arizona.edu/books/bid1580.htm
 |publisher=University of Arizona Press
 |pages=153–173
 |access-date=2008-03-22
 |archive-date=2017-08-24
 |archive-url=https://web.archive.org/web/20170824213414/http://www.uapress.arizona.edu/Books/bid1580.htm
 |url-status=live
 }}</ref> Dynamical studies of hypothetical Oort cloud comets have estimated that their occurrence in the [[outer planets|outer-planet]] region would be several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction of [[Jupiter]], which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with [[Comet Shoemaker–Levy 9]] in 1994.<ref name=julio>{{cite journal
|author=Julio A. Fernández
|date=2000
|title=Long-Period Comets and the Oort Cloud
|journal=[[Earth, Moon, and Planets]]
 |volume=89 |issue=1–4 |pages=325–343
|bibcode = 2002EM&P...89..325F
|doi=10.1023/A:1021571108658
|s2cid=189898799
}}</ref> An example of a typical dynamically old comet with an origin in the Oort cloud could be C/2018 F4.<ref name="Oort_Cloud_origin">{{cite journal
|last1=Licandro |first1=Javier
|last2=de la Fuente Marcos |first2=Carlos
|last3=de la Fuente Marcos |first3=Raúl
|last4=de Leon |first4=Julia
|last5=Serra-Ricart |first5=Miquel
|last6=Cabrera-Lavers|first6=Antonio
|title=Spectroscopic and dynamical properties of comet C/2018 F4, likely a true average former member of the Oort cloud
|journal=Astronomy and Astrophysics  |volume=625 |pages=A133 (6 pages) 
|date=28 May 2019
|doi=10.1051/0004-6361/201834902
|bibcode=2019A&A...625A.133L
|arxiv = 1903.10838 |s2cid=85517040
}}</ref>