Ring system

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The moons Prometheus (right) and Pandora (left) orbit just inside and outside, respectively, the 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 solid material such as gas, dust, meteoroids, planetoids or moonlets and stellar objects.

Contents

Ring systems are best known as planetary rings, common components of satellite systems around giant planets such as of Saturn, or circumplanetary disks. But they can also be galactic rings and circumstellar discs, 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, Venus, and Earth, in mean motion resonance with these planets. [1] [2] [3] Evidence suggests that ring systems may also be found around other types of astronomical objects, including moons and brown dwarfs.

In the Solar System, all four giant planets (Jupiter, Saturn, Uranus, and Neptune) have ring systems. Ring systems around minor planets have also been discovered via occultations. Some studies even theorize that the Earth may have had a ring system during the mid-late Ordovician period. [4]

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 moon that was disrupted by a large impact, or from the debris of a moon that was disrupted by tidal stresses 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. [5]

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 cryovolcanic material. [6] [7]

Ring systems may form around centaurs when they are tidally disrupted in a close encounter (within 0.4 to 0.8 times the Roche limit) with a giant planet. For a differentiated body approaching a giant planet at an initial relative velocity of 3−6 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. [8]

Ring systems of planets

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. Saturn in natural colors (captured by the Hubble Space Telescope).jpg
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, and in 2007 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.

Some planetary rings are influenced by shepherd moons, small 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, 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. [9] [10]

Jupiter

Jupiter's ring system was the third to be discovered, when it was first observed by the Voyager 1 probe in 1979, [11] and was observed more thoroughly by the Galileo orbiter in the 1990s. [12] 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". [13] The system consists mostly of dust. [11] [14]

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. [15] The rings are not a series of tiny ringlets as many think, but are more of a disk with varying density. [16] They consist mostly of water ice and trace amounts of rock, and the particles range in size from micrometers to meters. [17]

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. [18] In the time between then and 2005, observations by Voyager 2 [19] and the Hubble Space Telescope [20] 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 organics. The relative lack of dust is due to aerodynamic drag from the extended exosphere-corona of Uranus.

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. [21] 20 to 70 percent of the rings are dust, a relatively high proportion. [21] Hints of the rings were seen for decades prior to their conclusive discovery by Voyager 2 in 1989.

Prehistoric ring systems

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 Hirnantian glaciation, the coldest known period of the last 450 million years. [4]

Rings systems of minor planets and moons

Reports in March 2008 suggested that Saturn's moon Rhea may have its own tenuous ring system, which would make it the only moon known to have a ring system. [22] [23] [24] A later study published in 2010 revealed that imaging of Rhea by the 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. [25]

Prior to the arrival of New Horizons , some astronomers hypothesized that Pluto and 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. [26] However, this possibility was ruled out when New Horizons failed to detect any dust rings around Pluto.

Chariklo

10199 Chariklo, a centaur, was the first minor planet discovered to have rings. It has 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 19-kilometer (12-mile)-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. [27]

Chiron

A second centaur, 2060 Chiron, has a constantly evolving disk of rings. [28] [29] [30] 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 324±10 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. [29] 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. [31]

Haumea

Artist's depiction of Haumea's ring system Haumea with rings (37641832331).jpg
Artist's depiction of Haumea's ring system

A ring around Haumea, a dwarf planet and 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. [32] [33] The ring has a radius of about 2,287 km, a width of ≈70 km and an opacity of 0.5. [33] The ring plane coincides with Haumea's equator and the orbit of its larger, outer moon Hi’iaka [33] (which has a semimajor axis of ≈25,657 km). The ring is close to the 3:1 resonance with Haumea's rotation, which is located at a radius of 2,285±8 km. [33] It is well within Haumea's Roche limit, which would lie at a radius of about 4,400 km if Haumea were spherical (being nonspherical pushes the limit out farther). [33]

Artist impression of Quaoar and its ring Artist impression of Quaoar and its ring ESA24681885.jpeg
Artist impression of Quaoar and its ring

Quaoar

In 2023, astronomers announced the discovery of a widely separated ring around the dwarf planet and Kuiper belt object Quaoar. [34] [35] Further analysis of the occultation data uncovered a second inner, fainter ring. [36]

Both rings display unusual properties. The outer ring orbits at a distance of 4,057±6 km, 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 2,520±20 km, approximately 4.6 times the radius of Quaoar and also beyond its Roche limit. [36] The outer ring appears to be inhomogeneous, containing a thin, dense section as well as a broader, more diffuse section. [35]

Rings around exoplanets

Ring formation around extrasolar planet

Because all giant planets of the Solar System have rings, the existence of exoplanets 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, within this line rings consisting of rocky material can be stable in the long term. [37] 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 [38] and K2-33b. [39]

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, [40] 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. [41] 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 RJ. [42]

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. [43] This substellar object, dubbed "J1407b", is most likely a free-floating brown dwarf or rogue planet several times the mass of Jupiter. [44] The circumstellar disk or ring system of J1407b is about 0.6 astronomical units (90,000,000 km; 56,000,000 mi) in radius. [43] 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. [43]

Visual comparison

A Galileo image of Jupiter's main ring. Main Ring Galeleo forward PIA00538.jpg
A Galileo image of Jupiter's main ring.
Saturn's rings dark side mosaic.jpg
A Cassini mosaic of Saturn's rings.
A Voyager 2 image of Uranus's rings. FDS 26852.19 Rings of Uranus.png
A Voyager 2 image of Uranus's rings.
A pair of Voyager 2 images of Neptune's rings. PIA02202 Neptune's full rings.jpg
A pair of Voyager 2 images of Neptune's rings.

See also

Related Research Articles

<span class="mw-page-title-main">Kuiper belt</span> Area of the Solar System beyond the planets, comprising small bodies

The Kuiper belt is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia, and water. The Kuiper belt is home to most of the objects that astronomers generally accept as dwarf planets: Orcus, Pluto, Haumea, Quaoar, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

<span class="mw-page-title-main">Planet</span> Large, round non-stellar astronomical object

A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the term: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion.

<span class="mw-page-title-main">Solar System</span> The Sun and objects orbiting it

The Solar System is the gravitationally bound system of the Sun and the objects that orbit it. It formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc. The Sun is a typical star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core, releasing this energy from its outer photosphere. Astronomers classify it as a G-type main-sequence star.

<span class="mw-page-title-main">Natural satellite</span> Astronomical body that orbits a planet

A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.

<span class="mw-page-title-main">2060 Chiron</span> Large, ringed 200km centaur/comet with 50-year orbit

2060 Chiron is a ringed small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies orbiting between the asteroid belt and the Kuiper belt. Chiron is named after the centaur Chiron in Greek mythology.

<span class="mw-page-title-main">Centaur (small Solar System body)</span> Type of Solar System object

In planetary astronomy, a centaur is a small Solar System body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Centaurs generally have unstable orbits because of this; almost all their orbits have dynamic lifetimes of only a few million years, but there is one known centaur, 514107 Kaʻepaokaʻawela, which may be in a stable orbit. Centaurs typically exhibit the characteristics of both asteroids and comets. They are named after the mythological centaurs that were a mixture of horse and human. Observational bias toward large objects makes determination of the total centaur population difficult. Estimates for the number of centaurs in the Solar System more than 1 km in diameter range from as low as 44,000 to more than 10,000,000.

<span class="mw-page-title-main">Haumea</span> Dwarf planet in the Solar System

Haumea is a dwarf planet located beyond Neptune's orbit. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory, and formally announced in 2005 by a team headed by José Luis Ortiz Moreno at the Sierra Nevada Observatory in Spain, who had discovered it that year in precovery images taken by the team in 2003. From that announcement, it received the provisional designation 2003 EL61.

HD 69830 is a yellow dwarf star located 41.0 light-years away in the constellation of Puppis. In 2005, the Spitzer Space Telescope discovered a narrow ring of warm debris orbiting the star. The debris ring contains substantially more dust than the Solar System's asteroid belt. In 2006, three extrasolar planets with minimum masses comparable to Neptune were confirmed in orbit around the star, located interior to the debris ring.

<span class="mw-page-title-main">10199 Chariklo</span> Ringed centaur in the outer Solar System

10199 Chariklo is the largest confirmed centaur, a class of minor planet in the outer Solar System. It orbits the Sun between Saturn and Uranus, grazing the orbit of Uranus. On 26 March 2014, astronomers announced the discovery of two rings around Chariklo by observing a stellar occultation, making it the first minor planet known to have rings.

<span class="nowrap">(55636) 2002 TX<sub>300</sub></span> Kuiper Belt object

(55636) 2002 TX300 is a bright Kuiper belt object in the outer Solar System estimated to be about 286 kilometres (178 mi) in diameter. It is a large member of the Haumea family that was discovered on 15 October 2002 by the Near-Earth Asteroid Tracking (NEAT) program.

<span class="mw-page-title-main">Shepherd moon</span> Satellite associated with a planetary ring

A shepherd moon is a small natural satellite that clears a gap in planetary ring material or keeps particles within a ring contained. The name is a result of their limiting the "herd" of the ring particles as a shepherd.

<span class="mw-page-title-main">Dwarf planet</span> Small planetary-mass object

A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like the eight classical planets of the Solar System. The prototypical dwarf planet is Pluto, which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.

<span class="mw-page-title-main">Quaoar</span> Ringed dwarf planet in the Kuiper belt

Quaoar is a large, ringed dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. It has an elongated ellipsoidal shape with an average diameter of 1,090 km (680 mi), about half the size of the dwarf planet Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory on 4 June 2002. Quaoar's surface contains crystalline water ice and ammonia hydrate, which suggests that it might have experienced cryovolcanism. A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects.

<span class="mw-page-title-main">Retrograde and prograde motion</span> Relative directions of orbit or rotation

Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object. It may also describe other motions such as precession or nutation of an object's rotational axis. Prograde or direct motion is more normal motion in the same direction as the primary rotates. However, "retrograde" and "prograde" can also refer to an object other than the primary if so described. The direction of rotation is determined by an inertial frame of reference, such as distant fixed stars.

<span class="mw-page-title-main">Weywot</span> Moon of dwarf planet Quaoar

Weywot is a natural satellite or moon of the trans-Neptunian dwarf planet Quaoar. It was discovered by Michael Brown and Terry-Ann Suer using images taken by the Hubble Space Telescope on 14 February 2006. Named after the Tongva sky god and son of Quaoar, Weywot is thought to be a fragment of Quaoar that was ejected into an eccentric orbit around the dwarf planet by a major impact event billions of years ago. The moon is nearly 200 km (120 mi) in diameter and it orbits Quaoar every 12.4 days at an average distance of 13,300 km (8,300 mi). Weywot is thought to play a role in maintaining Quaoar's outer ring by gravitationally influencing it in an orbital resonance.

<span class="mw-page-title-main">J1407b</span> Free-floating substellar object with a dust disk

J1407b is a substellar object, either a free-floating planet or brown dwarf, with a massive circumplanetary disk or ring system. It was first detected by automated telescopes in 2007 when its disk eclipsed the star V1400 Centauri, causing a series of dimming events for 56 days. The eclipse by J1407b was not discovered until 2010, by Mark Pecaut and Eric Mamajek, and was announced in 2012. J1407b's disk spans a radius of about 90 million kilometers and consists of many rings and gaps which may indicate moons are forming in orbit around the object. It was initially thought to be orbiting V1400 Centauri, but later studies suggest J1407b is likely an unbound object that coincidentally passed in front of the star. J1407b may have been observed via high-resolution imaging in 2017, which may suggest the object is less than 6 Jupiter masses.

<span class="mw-page-title-main">Satellite system (astronomy)</span> Set of gravitationally bound objects in orbit

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.

<span class="mw-page-title-main">Circumplanetary disk</span> Accumulation of matter around a planet

A circumplanetary disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a planet. They are reservoirs of material out of which moons may form. Such a disk can manifest itself in various ways.

Centaurus is a mission concept to flyby the centaurs 2060 Chiron and Schwassmann–Wachmann 1. It was submitted in response to the NASA Discovery program call for proposals in 2019 but ultimately was not among the four missions selected for further development by NASA in February 2020. If it had been selected, Centaurus would have been the first mission to attempt a flyby of a centaur.

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