View source for Ring system

{{short description|Ring of cosmic dust orbiting an astronomical object}}
{{hatnote group|
{{Redirect|Planet ring|other uses|Planet Ring (disambiguation){{!}}Planet Ring}}
{{For|ring systems in chemistry|Ring system (chemistry)}}
}}
[[File:PIA07712 - F ring animation videoquality 6 framerate 5.ogv|thumb|upright=1.5|The [[natural satellite|moons]] [[Prometheus (moon)|Prometheus]] (right) and [[Pandora (moon)|Pandora]] (left) orbit just inside and outside, respectively, the [[Rings of Saturn#F Ring|F ring]] of [[Saturn]], but only Prometheus is thought to function as a [[shepherd moon]]. ]]

A '''ring system''' is a disc or [[torus]] orbiting an [[astronomical object]] that is composed of numerous solid bodies such as [[cosmic dust|dust]] particles, [[meteoroid]]s, [[planetoid]]s, [[moonlet]]s, or stellar objects.

Ring systems are best known as planetary rings, common components of [[satellite system (astronomy)|satellite system]]s around [[giant planet]]s such as the [[Rings of Saturn|rings of Saturn]], or [[circumplanetary disk]]s. But they can also be [[Ring galaxy|galactic ring]]s and [[circumstellar disc]]s, belts of planetoids, such as the [[asteroid belt]] or [[Kuiper belt]], or rings of [[interplanetary dust]], such as around the [[Sun]] at distances of [[Mercury (planet)|Mercury]], [[Venus]], and [[Earth]], in [[mean motion resonance]] with these planets.<ref name="EA-20190312" /><ref name="pokorny2019"/><ref name="NS2023"/> Evidence suggests that ring systems may also be found around other types of astronomical objects, including moons and [[brown dwarf]]s.

In the [[Solar System]], all four giant planets ([[Rings of Jupiter|Jupiter]], [[Rings of Saturn|Saturn]], [[Rings of Uranus|Uranus]], and [[Rings of Neptune|Neptune]]) have ring systems. Ring systems around [[minor planet]]s have also been discovered via [[Occultation|occultations]]. Some studies even theorize that the Earth may have had a ring system during the mid-late [[Ordovician]] period.<ref name="Tomkins2024" />

== Formation ==
There are three ways that thicker planetary rings have been proposed to have formed: from material originating from the [[protoplanetary disk]] that was within the [[Roche limit]] of the planet and thus could not coalesce to form moons, from the debris of a [[natural satellite|moon]] that was disrupted by a large impact, or from the debris of a moon that was disrupted by [[tidal stress]]es when it passed within the planet's Roche limit. Most rings were thought to be unstable and to dissipate over the course of tens or hundreds of millions of years, but it now appears that Saturn's rings might be quite old, dating to the early days of the Solar System.<ref name="NASA2007"/>

Fainter planetary rings can form as a result of meteoroid impacts with moons orbiting around the planet or, in the case of Saturn's E-ring, the ejecta of [[cryovolcano|cryovolcanic]] material.<ref name="Spahn"/><ref name="Helfenstein2006"/>

Ring systems may form around [[centaur (minor planet)|centaurs]] when they are [[Tidal force|tidally disrupted]] in a close encounter (within 0.4 to 0.8 times the [[Roche limit]]) with a giant planet. For a [[Planetary differentiation|differentiated body]] approaching a giant planet at an initial relative velocity of 3−6&nbsp;km/s with an initial rotational period of 8 hours, a ring mass of 0.1%−10% of the centaur's mass is predicted. Ring formation from an undifferentiated body is less likely. The rings would be composed mostly or entirely of material from the parent body's icy mantle. After forming, the ring would spread laterally, leading to satellite formation from whatever portion of it spreads beyond the centaur's Roche Limit. Satellites could also form directly from the disrupted icy mantle. This formation mechanism predicts that roughly 10% of centaurs will have experienced potentially ring-forming encounters with giant planets.<ref name="Hyodo2016"/>

== Ring systems of planets ==
[[File:Saturn in natural colors (captured by the Hubble Space Telescope).jpg|thumb|upright=1.5|The ring orbiting [[Saturn]] consists mostly of chunks of ice and dust. The small dark spot on Saturn is the shadow from Saturn's moon [[Enceladus]].]]

The composition of planetary ring particles varies, ranging from silicates to icy dust. Larger rocks and boulders may also be present, as seen in 2007 when [[Tide|tidal]] effects from eight moonlets only a few hundred meters across were detected within Saturn's rings. The maximum size of a ring particle is determined by the [[specific strength]] of the material it is made of, its density, and the tidal force at its altitude. The tidal force is proportional to the average density inside the radius of the ring, or to the mass of the planet divided by the radius of the ring cubed. It is also inversely proportional to the square of the orbital period of the ring.

{{anchor|shepherds}}Some planetary rings are influenced by [[Shepherd moon|''shepherd moons'']], small [[Natural satellite|moons]] that orbit near the inner or outer edges of a ringlet or within gaps in the rings. The [[gravity]] of shepherd moons serves to maintain a sharply defined edge to the ring; material that drifts closer to the shepherd moon's orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself.

It is also predicted that [[Phobos (moon)|Phobos]], a moon of Mars, will break up and form into a planetary ring in about 50 million years. Its low orbit, with an orbital period that is shorter than a Martian day, is decaying due to [[tidal deceleration]].<ref name="Holsapple2001"/><ref>{{Cite book |last1=Gürtler |first1=Joachim |title=Das Sonnensystem |last2=Dorschner |first2=Johann |date=1993 |publisher=Barth |isbn=978-3-335-00281-9 |location=Leipzig ; Berlin ; Heidelberg}}</ref>

=== Jupiter ===
{{main|Rings of Jupiter}}
Jupiter's ring system was the third to be discovered, when it was first observed by the ''[[Voyager 1]]'' probe in 1979,<ref name="smith1979"/> and was observed more thoroughly by the ''Galileo'' orbiter in the 1990s.<ref name="ockert-bell1999"/> Its four main parts are a faint thick torus known as the "halo"; a thin, relatively bright main ring; and two wide, faint "gossamer rings".<ref name="esposito2002"/> The system consists mostly of dust.<ref name="smith1979"/><ref name="showalter1987"/>

===Saturn===
{{main|Rings of Saturn}}
Saturn's rings are the most extensive ring system of any planet in the Solar System, and thus have been known to exist for quite some time. [[Galileo Galilei]] first observed them in 1610, but they were not accurately described as a disk around Saturn until [[Christiaan Huygens]] did so in 1655.<ref name="solarviews"/> The rings are not a series of tiny ringlets as many think, but are more of a disk with varying density.<ref name="tiscareno2013"/> They consist mostly of water ice and trace amounts of [[Rock (geology)|rock]], and the particles range in size from micrometers to meters.<ref name="ciclops"/>

===Uranus===
{{main|Rings of Uranus}}
Uranus's ring system lies between the level of complexity of Saturn's vast system and the simpler systems around Jupiter and Neptune. They were discovered in 1977 by [[James L. Elliot]], Edward W. Dunham, and [[Jessica Mink]].<ref name="elliot1977"/> In the time between then and 2005, observations by ''[[Voyager 2]]''<ref name="smith1986"/> and the [[Hubble Space Telescope]]<ref name="showalter2006"/> led to a total of 13 distinct rings being identified, most of which are opaque and only a few kilometers wide. They are dark and likely consist of water ice and some radiation-processed [[Organic compound|organics]]. The relative lack of dust is due to [[aerodynamic drag]] from the extended [[exosphere]]-[[Uranus|corona]] of Uranus.

===Neptune===
{{main|Rings of Neptune}}
The system around Neptune consists of five principal rings that, at their densest, are comparable to the low-density regions of Saturn's rings. However, they are faint and dusty, much more similar in structure to those of Jupiter. The very dark material that makes up the rings is likely organics processed by [[radiation]], like in the rings of Uranus.<ref name="smith1989"/> 20 to 70 percent of the rings are [[Cosmic dust|dust]], a relatively high proportion.<ref name="smith1989"/> Hints of the rings were seen [[Rings of Neptune|for decades]] prior to their conclusive discovery by ''Voyager 2'' in 1989.

=== Prehistoric ring systems ===

==== Earth ====
{{main|Rings of Earth}}

A 2024 study suggests that Earth may have had a ring system for a period of 40 million years, starting from the middle of the [[Ordovician]] period (around 466 million years ago). This ring system may have originated from a large asteroid that passed by Earth at this time and had a significant amount of debris stripped by Earth's gravitational pull, forming a ring system. Evidence for this ring comes from impact craters from the [[Ordovician meteor event]] appearing to cluster in a distinctive band around the Earth's equator at that time. The presence of this ring may have led to significant shielding of Earth from sun's rays and a severe cooling event, thus causing the [[Andean-Saharan glaciation|Hirnantian glaciation]], the coldest known period of the last 450 million years.<ref name="Tomkins2024">{{Cite journal |last1=Tomkins |first1=Andrew G. |last2=Martin |first2=Erin L. |last3=Cawood |first3=Peter A. |date=November 2024 |title=Evidence suggesting that earth had a ring in the Ordovician |url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X24004230 |journal=[[Earth and Planetary Science Letters]] |language=en |volume=646 |bibcode=2024E&PSL.64618991T |doi=10.1016/j.epsl.2024.118991 |issn=0012-821X |article-number=118991|doi-access=free }}</ref>

==Rings systems of minor planets and moons==
Reports in March 2008 suggested that Saturn's moon [[Rhea (moon)|Rhea]] may have [[Rings of Rhea|its own tenuous ring system]], which would make it the only moon known to have a ring system.<ref name="NASArhea"/><ref name="Jones_2008"/><ref name="LakdawallaE"/> A later study published in 2010 revealed that imaging of Rhea by the [[Cassini–Huygens|''Cassini'' spacecraft]] was inconsistent with the predicted properties of the rings, suggesting that some other mechanism is responsible for the magnetic effects that had led to the ring hypothesis.<ref name="tiscareno2010"/>

Prior to the arrival of ''[[New Horizons]]'', some astronomers hypothesized that [[Pluto]] and [[Charon (moon)|Charon]] might have a circumbinary ring system created from dust ejected off of Pluto's small outer moons in impacts. A dust ring would have posed a considerable risk to the ''New Horizons'' spacecraft.<ref name="Steffl 2006"/> However, this possibility was ruled out when ''New Horizons'' failed to detect any dust rings around Pluto.

===Chariklo===
[[File:Chariklo occultation 2017 Jul 23 Morgado et al. 2021 Fig 4.jpg|thumb|Silhouette of Chariklo and its rings on 23 July 2017, revealed by occultation observations from various locations on Earth]]
{{main|Rings of Chariklo}}
[[10199 Chariklo]], a [[Centaur (minor planet)|centaur]], was the first minor planet discovered to have rings. It has [[Rings of Chariklo|two rings]], perhaps due to a collision that caused a chain of debris to orbit it. The rings were discovered when astronomers observed Chariklo passing in front of the star UCAC4 248-108672 on June 3, 2013 from seven locations in South America. While watching, they saw two dips in the star's apparent brightness just before and after the occultation. Because this event was observed at multiple locations, the conclusion that the dip in brightness was in fact due to rings is unanimously the leading hypothesis. The observations revealed what is likely a {{convert|19|km|mi|abbr=off|sp=us|adj=mid|-wide}} ring system that is about 1,000 times closer than the Moon is to Earth. In addition, astronomers suspect there could be a moon orbiting amidst the ring debris. If these rings are the leftovers of a collision as astronomers suspect, this would give fodder to the idea that moons (such as the Moon) form through collisions of smaller bits of material. Chariklo's rings have not been officially named, but the discoverers have nicknamed them Oiapoque and Chuí, after two rivers near the northern and southern ends of Brazil.<ref name="UTchariklo"/>

===Chiron===
{{main|Rings of Chiron}}
A second centaur, [[2060 Chiron]], has a constantly evolving disk of rings.<ref name="Lakdawalla2015"/><ref name = "Ortiz2015"/><ref name="Sickafoose2023"/> Based on stellar-occultation data that were initially interpreted as resulting from jets associated with Chiron's comet-like activity, the rings are proposed to be {{val|324|10|u=km}} in radius, though their evolution does change the radius somewhat. Their changing appearance at different viewing angles can explain the long-term variation in Chiron's brightness over time.<ref name = "Ortiz2015"/> Chiron's rings are suspected to be maintained by orbiting material ejected during seasonal outbursts, as a third partial ring detected in 2018 had become a full ring by 2022, with an outburst in between in 2021.<ref name="Ortiz2023"/>

===Haumea===
{{main|Rings of Haumea}}
[[File:Haumea with rings (37641832331).jpg|thumb|Artist's depiction of Haumea's ring system]]
A ring around [[Haumea]], a [[dwarf planet]] and [[Resonant trans-Neptunian object|resonant Kuiper belt member]], was revealed by a stellar occultation observed on 21 January 2017. This makes it the first [[trans-Neptunian object]] found to have a ring system.<ref name="Sickafoose2017"/><ref name="Ortiz2017"/> The ring has a radius of about {{val|2287|u=km|fmt=commas}}, a width of ≈{{val|70|u=km}} and an opacity of 0.5.<ref name="Ortiz2017"/> The ring plane coincides with Haumea's equator and the orbit of its larger, outer moon [[Moons of Haumea|Hi’iaka]]<ref name="Ortiz2017"/> (which has a semimajor axis of ≈{{val|25657|u=km|fmt=commas}}). The ring is close to the 3:1 [[orbital resonance|resonance]] with Haumea's rotation, which is located at a radius of {{val|2285|8|u=km|fmt=commas}}.<ref name="Ortiz2017"/> It is well within Haumea's [[Roche limit]], which would lie at a radius of about {{val|4400|u=km|fmt=commas}} if Haumea were spherical (being nonspherical pushes the limit out farther).<ref name="Ortiz2017"/>

=== Quaoar ===
{{main|Rings of Quaoar}}
[[File:Quaoar-Weywot_orbit_diagram_projected.png|thumb|upright=1.2|A diagram of Quaoar, its moon [[Weywot (moon)|Weywot]], and its two known rings.]]
In 2023, astronomers announced the discovery of a widely separated ring around the dwarf planet and Kuiper belt object [[50000 Quaoar|Quaoar]].<ref name="Devlin"/><ref name="morgado2023"/> Further analysis of the occultation data uncovered a second inner, fainter ring.<ref name="Pereira2023">{{Cite Q|Q117802048|display-authors=1}}</ref>

Both rings display unusual properties. The outer ring orbits at a distance of {{val|4057|6|u=km|fmt=commas}}, approximately 7.5 times the radius of Quaoar and more than double the distance of its Roche limit. The inner ring orbits at a distance of {{val|2520|20|u=km|fmt=commas}}, approximately 4.6 times the radius of Quaoar and also beyond its Roche limit.<ref name="Pereira2023"/> The outer ring appears to be inhomogeneous, containing a thin, dense section as well as a broader, more diffuse section.<ref name="morgado2023"/>

== Rings around exoplanets ==
[[File:Ring formation around extrasolar planet.webm|thumb|Ring formation around extrasolar planet]]
Because all [[giant planet]]s of the Solar System have rings, the existence of [[exoplanet]]s with rings is plausible. Although particles of [[ice]], the material that is predominant in the [[rings of Saturn]], can only exist around planets beyond the [[Frost line (astrophysics)|frost line]], within this line rings consisting of rocky material can be stable in the long term.<ref name="schlichting2011"/> Such ring systems can be detected for planets observed by the [[transit method]] by additional reduction of the light of the central star if their opacity is sufficient. As of 2024, two candidate extrasolar ring systems have been found by this method, around [[HIP 41378 f]]<ref name="akinsanmi2020"/> and [[K2-33b]].<ref name="Ohno2022"/>

[[Fomalhaut b]] was found to be large and unclearly defined when detected in 2008. This was hypothesized to either be due to a cloud of dust attracted from the dust disc of the star, or a possible ring system,<ref name="kalas2008"/> though in 2020 Fomalhaut b itself was determined to very likely be an expanding debris cloud from a collision of asteroids rather than a planet.<ref name="PNAS-20200420"/> Similarly, [[Proxima Centauri c]] has been observed to be far brighter than expected for its low mass of 7 Earth masses, which may be attributed to a ring system of about 5 {{Jupiter radius|link=y}}.<ref name="Gratton2020"/>

A 56-day-long sequence of dimming events in the star [[V1400 Centauri]] observed in 2007 was interpreted as a [[substellar object]] with a [[circumstellar disk]] or massive rings transiting the star.<ref name="Kenworthy2015"/> This substellar object, dubbed "[[J1407b]]", is most likely a free-floating [[brown dwarf]] or [[rogue planet]] several times the mass of Jupiter.<ref name="Kenworthy2020"/> The circumstellar disk or ring system of J1407b is about {{convert|0.6|AU|km mi|abbr=out|lk=in}} in radius.<ref name="Kenworthy2015"/> J1407b's transit of V1400 Centauri revealed gaps and density variations within its disk or ring system, which has been interpreted as hints of exomoons or exoplanets forming around J1407b.<ref name="Kenworthy2015"/>

== Visual comparison ==
[[File:Main Ring Galeleo forward PIA00538.jpg|thumb|400px|left|A ''[[Galileo spacecraft|Galileo]]'' image of [[Jupiter]]'s main ring.]]
{{wide image|Saturn's rings dark side mosaic.jpg|1100px|A ''[[Cassini spacecraft|Cassini]]'' mosaic of [[rings of Saturn|Saturn's rings]].}}
[[File:FDS 26852.19 Rings of Uranus.png|thumb|left|250px|A ''[[Voyager 2]]'' image of [[Uranus]]'s rings.]]
[[File:PIA02202 Neptune's full rings.jpg|thumb|320px|A pair of ''[[Voyager 2]]'' images of [[Neptune]]'s rings.]]
{{clear}}

==See also==
* [[Shepherd moon]]
* [[Circumplanetary disk]]
* [[Circumstellar disc]]
* [[Accretion disk]]
* [[Lists of astronomical objects]]

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== External links ==
{{Commons category|Planetary rings}}
* [https://planetarynames.wr.usgs.gov/append8.html USGS/IAU Ring and Ring Gap Nomenclature]
* [http://www.bridgingthegaps.ie/2017/04/planetary_ring_systems_with_dr_mark_showalter/ Everything a Curious Mind Should Know About Planetary Ring Systems with Dr Mark Showalter], [http://www.bridgingthegaps.ie/ Bridging the Gaps: A Portal for Curious Minds]
* [http://creatacad.org/?id=24&lng=eng Physical Chemistry of Evolution of Planetary Systems]
* Gladyshev G. P. ''Thermodynamics and Macrokinetics of Natural Hierarchical Processes'', p.&nbsp;217. Nauka, Moscow, 1988 (in Russian).
{{Magnetospherics}}
{{Planetary rings|state=uncollapsed}}
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