Editing Comet (section)

=== Oort cloud and Hills cloud ===
[[File:Small objects in the Solar System ESA25188647.jpg|thumb|upright=1.5|The [[Oort cloud]] thought to surround the Solar System. Showed with Kuiper Belt and Asteroid Belt for comparison.]]
{{Main|Oort cloud|Hills cloud}}
The Oort cloud is thought to occupy a vast space starting from between {{convert|2000|and|5000|AU|ly|2|abbr=on}}<ref name=book>{{cite book |chapter=Comet Populations and Cometary Dynamics |title=Encyclopedia of the Solar System |publisher=Academic Press |first1=Harold F. |last1=Levison |first2=Luke |last2=Donnes |name-list-style=amp |editor1-first=Lucy-Ann Adams |editor1-last=McFadden |editor2-first=Torrence V. |editor2-last=Johnson |editor3-first=Paul Robert |editor3-last=Weissman |edition=2nd |pages=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 575–588] |date=2007 |isbn=978-0-12-088589-3 |chapter-url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 }}</ref> to as far as {{convert|50000|AU|ly|2|abbr=on}}<ref name=Morbidelli2006/> from the Sun. This cloud encases the celestial bodies that start at the middle of the Solar System—the Sun, all the way to outer limits of the Kuiper Belt. The Oort cloud consists of viable materials necessary for the creation of celestial bodies. The Solar System's planets exist only because of the planetesimals (chunks of leftover space that assisted in the creation of planets) that were condensed and formed by the gravity of the Sun. The eccentric made from these trapped planetesimals is why the Oort Cloud even exists.<ref>{{Cite web|url=https://solarsystem.nasa.gov/solar-system/oort-cloud/in-depth|title=In Depth {{!}} Oort Cloud|website=NASA Solar System Exploration|date=14 November 2017 |access-date=2019-12-01}}</ref> Some estimates place the outer edge at between {{convert|100000|and|200000|AU|ly|2|abbr=on}}.<ref name= book /> The region can be subdivided into a spherical outer Oort cloud of {{convert|20000|-|50000|AU|ly|2|abbr=on}}, and a doughnut-shaped inner cloud, the Hills cloud, of {{convert|2000|-|20000|AU|ly|2|abbr=on}}.<ref>{{cite book |first=Lisa |last=Randall |title=Dark matter and the dinosaurs: The astounding interconnectedness of the universe |date=2015 |publisher=Harper Collins Publishers |isbn=978-0-06-232847-2 |pages=115}}</ref> The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of [[Neptune]].<ref name=Morbidelli2006/> The inner Oort cloud is also known as the Hills cloud, named after [[Jack G. Hills]], who proposed its existence in 1981.<ref name="hills1981" /> Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;<ref name="hills1981">{{cite journal |first=Jack G. |last=Hills |date=1981 |title=Comet showers and the steady-state infall of comets from the Oort Cloud |journal=[[The Astronomical Journal]] |volume=86 |pages=1730–1740 |bibcode=1981AJ.....86.1730H |doi=10.1086/113058|doi-access=free }}</ref><ref name="levison2001">{{cite journal |title=The Origin of Halley-Type Comets: Probing the Inner Oort Cloud |journal=[[The Astronomical Journal]] |first1=Harold F. |last1=Levison |first2=Luke |last2=Dones |first3=Martin J. |last3=Duncan |display-authors=1 |volume=121 |issue=4 |pages=2253–2267 |date=2001 |bibcode=2001AJ....121.2253L |doi=10.1086/319943|doi-access=free }}</ref><ref name="Donahue1991">{{cite book |editor-first=Thomas M. | editor-last=Donahue |others=Trivers, Kathleen Kearney and Abramson, David M. |date=1991 |title=Planetary Sciences: American and Soviet Research, Proceedings from the U.S.–U.S.S.R. Workshop on Planetary Sciences |url=http://books.nap.edu/openbook.php?record_id=1790&page=R1 |publisher=National Academy Press |page=251 |isbn=0-309-04333-6 |access-date=18 March 2008|doi=10.17226/1790 | bibcode=1991psas.conf.....D }}</ref> it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.<ref name="Julio1997">{{cite journal |first=Julio A. |last=Fernéndez |date=1997 |title=The Formation of the Oort Cloud and the Primitive Galactic Environment |url=http://www.gps.caltech.edu/classes/ge133/reading/oort.pdf |journal=[[Icarus (journal)|Icarus]] |volume=219 |issue=1 |pages=106–119 |access-date=18 March 2008 |bibcode=1997Icar..129..106F |doi=10.1006/icar.1997.5754 |archive-date=24 July 2012 |archive-url=https://web.archive.org/web/20120724192955/http://www.gps.caltech.edu/classes/ge133/reading/oort.pdf |url-status=dead }}</ref>