Editing Oort cloud (section)

== Structure and composition ==
[[File:Oort cloud Sedna orbit.svg|thumb|upright=1.25|The presumed distance of the Oort cloud compared to the rest of the Solar System]]

The Oort cloud is thought to occupy a vast space somewhere between {{convert|2000|and|5000|AU|ly|2|abbr=on}}<ref name=book /> from the Sun to as far out as {{convert|50000|AU|ly|2|abbr=on}} or even {{convert|100000|to|200000|AU|ly|2|abbr=on}}.<ref name=Morbidelli2006 /><ref name= book /> The region can be subdivided into a spherical outer Oort cloud with a radius of some {{convert|20000|-|50000|AU|ly|2|abbr=on}} and a [[torus]]-shaped inner Oort cloud with a radius of {{convert|2000|-|20000|AU|ly|2|abbr=on}}.

The inner Oort cloud is sometimes known as the Hills cloud, named for [[Jack G. Hills]], who proposed its existence in 1981.<ref name="hills1981" /> Models predict the inner cloud to be much the denser of the two, having tens or hundreds of times as many cometary nuclei as the outer cloud.<ref name="hills1981" /><ref name="levison2001" /><ref name="Donahue1991" /> The Hills cloud is thought to be necessary to explain the continued existence of the Oort cloud after billions of years.<ref name="Julio1997" />

Because it lies at the interface between the dominion of Solar and galactic gravitation, the objects comprising the outer Oort cloud are only weakly bound to the Sun. This in turn allows small perturbations from nearby stars or the Milky Way itself to inject long-period (and possibly [[List of Halley-type comets|Halley-type]]) comets inside the orbit of [[Neptune]].<ref name="Morbidelli2006" /> This process ought to have depleted the sparser, outer cloud and yet long-period comets with orbits well above or below the ecliptic continue to be observed. The Hills cloud is thought to be a secondary reservoir of cometary nuclei and the source of replenishment for the tenuous outer cloud as the latter's numbers are gradually depleted through losses to the inner Solar System.<ref>{{cite journal | last1 = Kaib | first1 = Nathan A. | last2 = Volk | first2 = Kathryn | title = Dynamical Population of Comet Reservoirs | journal = The Planetary Science Journal | volume = 3 | issue = 6 | pages = 123 | year = 2022 | doi = 10.3847/PSJ/ac6e66 | doi-broken-date = 23 March 2025 | doi-access = free | arxiv = 2206.00010 }}</ref>

The outer Oort cloud may have trillions of objects larger than {{convert|1|km|1|abbr=on}},<ref name="Morbidelli2006" /> and billions with diameters of {{convert|20|km|0|adj=on}}. This corresponds to an [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] of more than 11.<ref>Absolute magnitude is a measure of how bright an object would be if it were 1 au from the Sun and Earth; as opposed to [[apparent magnitude]], which measures how bright an object appears from Earth. Because all measurements of absolute magnitude assume the same distance, absolute magnitude is in effect a measurement of an object's brightness. The lower an object's absolute magnitude, the brighter it is.</ref> On this analysis, "neighboring" objects in the outer cloud are separated by a significant fraction of 1 AU, tens of millions of kilometres.<ref name="emel2007" /><ref>{{cite web |title=The Oort Cloud |date=1998 |author=Paul R. Weissman |work=[[Scientific American]] |url=http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ISSUEID_CHAR=8DB2FB44-6B4B-47AF-B46B-791A911764D&ARTICLEID_CHAR=B294C211-98B8-4374-92AB-158C4866AB1 |access-date=2007-05-26 |archive-url=https://web.archive.org/web/20121111021915/http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ISSUEID_CHAR=8DB2FB44-6B4B-47AF-B46B-791A911764D&ARTICLEID_CHAR=B294C211-98B8-4374-92AB-158C4866AB1 |archive-date=2012-11-11 |url-status=dead }}</ref> The outer cloud's total mass is not known, but assuming that [[Halley's Comet]] is a suitable proxy for the nuclei composing the outer Oort cloud, their combined mass would be roughly {{convert|3E25|kg|lb}}, or five Earth masses.<ref name="Morbidelli2006" /><ref>{{cite journal |author=Paul R. Weissman |date=1983 |title=The mass of the Oort Cloud |journal=[[Astronomy and Astrophysics]] |volume=118 |issue=1 |pages=90–94 |bibcode=1983A&A...118...90W }}</ref>
Formerly the outer cloud was thought to be more massive by two orders of magnitude, containing up to 380 Earth masses,<ref>{{cite web |author=Sebastian Buhai |title=On the Origin of the Long Period Comets: Competing theories |url=http://www.tinbergen.nl/~buhai/pictures/UCU/Physics_AppliedMathematics/Astrophysics/long_period_comets.pdf |archive-url=https://web.archive.org/web/20060930193158/http://www.tinbergen.nl/~buhai/pictures/UCU/Physics_AppliedMathematics/Astrophysics/long_period_comets.pdf |archive-date=2006-09-30 |publisher=Utrecht University College |access-date=2008-03-29 }}</ref>
but improved knowledge of the size distribution of long-period comets has led to lower estimates. No estimates of the mass of the inner Oort cloud have been published as of 2023.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of [[Volatile (astrogeology)|ices]] such as [[ice|water]], [[methane]], [[ethane]], [[carbon monoxide]] and [[hydrogen cyanide]].<ref>{{cite journal |author=E. L. Gibb |author2=M. J. Mumma |author3=N. Dello Russo |author4=M. A. DiSanti |author5=K. Magee-Sauer |name-list-style=amp |date=2003 |title=Methane in Oort Cloud comets |journal=[[Icarus (journal)|Icarus]] |volume=165 |issue=2 |pages=391–406 |bibcode=2003Icar..165..391G |doi=10.1016/S0019-1035(03)00201-X }}</ref>
However, the discovery of the object {{mpl|1996 PW}}, an object whose appearance was consistent with a [[D-type asteroid]]<ref>{{cite journal |last= Rabinowitz|first= D. L. |date= August 1996|title= 1996 PW|bibcode= 1996IAUC.6466....2R|journal= [[IAU Circular]]|issue= 6466|pages= 2}}<!--|access-date=30 October 2014--></ref><ref>{{cite journal |last1= Davies|first1= John K. |display-authors= 4 |last2= McBride|first2= Neil |last3= Green|first3=Simon F. |last4=Mottola|first4= Stefano |last5= Carsenty|first5= Uri |last6=Basran|first6= Devinder |last7=Hudson |first7=Kathryn A. |last8= Foster|first8 = Michael J. |date= April 1998 |title= The Lightcurve and Colors of Unusual Minor Planet 1996 PW |journal= [[Icarus (journal)|Icarus]] |volume= 132 |issue= 2 |pages= 418–430 |doi= 10.1006/icar.1998.5888 |bibcode=1998Icar..132..418D  }}</ref> in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of roughly one to two percent asteroids.<ref>{{cite journal |author=Paul R. Weissman |author2=Harold F. Levison |date=1997 |title=Origin and Evolution of the Unusual Object 1996 PW: Asteroids from the Oort Cloud? |journal=[[Astrophysical Journal]] |volume= 488|issue= 2|pages=L133–L136 |doi=10.1086/310940 |bibcode = 1997ApJ...488L.133W |doi-access=free }}</ref> Analysis of the carbon and nitrogen [[isotope]] ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from the original protosolar cloud,<ref>{{cite journal |author=D. Hutsemekers |author2=J. Manfroid |author3=E. Jehin |author4=C. Arpigny |author5=A. Cochran |author6=R. Schulz |author7=J.A. Stüwe |author8=J.M. Zucconi |name-list-style=amp |date=2005 |title=Isotopic abundances of carbon and nitrogen in Jupiter-family and Oort Cloud comets |journal=[[Astronomy and Astrophysics]] |volume=440 |issue=2 |pages=L21–L24 |arxiv=astro-ph/0508033 |bibcode=2005A&A...440L..21H |doi=10.1051/0004-6361:200500160 |s2cid=9278535 }}</ref> a conclusion also supported by studies of granular size in Oort-cloud comets<ref>{{cite journal |author=Takafumi Ootsubo |author2=Jun-ichi Watanabe |author3=Hideyo Kawakita |author4=Mitsuhiko Honda |author5=Reiko Furusho |name-list-style=amp |date=2007 |title=Grain properties of Oort Cloud comets: Modeling the mineralogical composition of cometary dust from mid-infrared emission features |volume=55 |issue=9 | pages=1044–1049 |journal=Highlights in Planetary Science, 2nd General Assembly of Asia Oceania Geophysical Society |bibcode=2007P&SS...55.1044O |doi=10.1016/j.pss.2006.11.012 }}</ref> and by the recent impact study of Jupiter-family comet [[Tempel 1]].<ref>{{cite journal |author=Michael J. Mumma |author2=Michael A. DiSanti |author3=Karen Magee-Sauer |display-authors=etal |date=2005 |title=Parent Volatiles in Comet 9P/Tempel 1: Before and After Impact |journal=[[Science (journal)|Science Express]] |volume=310 |issue=5746 |pages=270–274 |bibcode=2005Sci...310..270M |doi=10.1126/science.1119337 |pmid=16166477 |s2cid=27627764 |url=https://authors.library.caltech.edu/52069/7/Mumma.SOM.pdf |access-date=2018-08-02 |archive-date=2018-07-24 |archive-url=https://web.archive.org/web/20180724035203/https://authors.library.caltech.edu/52069/7/Mumma.SOM.pdf |url-status=live }}</ref>

In 2020 researchers discovered an unexpected spiral-shaped pattern in the [[Hills cloud|inner part]] of the Oort Cloud. This discovery sheds light on how gravity shapes the outer edges of the Solar System.<ref>{{cite journal | last1 = Smullen | first1 = Rachel A. | last2 = Kaib | first2 = Nathan A. | title = Spiral Structure in the Inner Oort Cloud | journal = The Astrophysical Journal Letters | volume = 904 | issue = 1 | pages = L10 | year = 2020 | doi = 10.3847/2041-8213/abc7c5 | doi-broken-date = 23 March 2025 | doi-access = free | url = https://iopscience.iop.org/article/10.3847/2041-8213/abc7c5 }}</ref>