Shepherd moon

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Prometheus (right) and Pandora (left) both orbit near Saturn's F ring, but only Prometheus is thought to act as a shepherd. PIA07712 - F ring animation.gif
Prometheus (right) and Pandora (left) both orbit near Saturn's F ring, but only Prometheus is thought to act as a shepherd.
Operation of a shepherd moon- particles are located in front or behind the Moon in its orbit, so these are either accelerated in the direction of the moon and thrown to the outside, or they are slowed on their path and pulled inwards. Hirtenmond.png
Operation of a shepherd moon– particles are located in front or behind the Moon in its orbit, so these are either accelerated in the direction of the moon and thrown to the outside, or they are slowed on their path and pulled inwards.

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.

Contents

Due to their gravitational influence, shepherd moons deflect ring particles from their original orbits due to proximity or through orbital resonances. This can carve gaps in the ring system, such as the Encke Gap maintained by Saturn's moon Pan, or lead to the confining of narrow ringlets, such as Saturn's F ring.

Discovery

The existence of shepherd moons was theorized in early 1979. [1] Observations of the rings of Uranus show that they are very thin and well defined, with sharp gaps between rings. To explain this, Goldreich and Tremaine suggested that two small satellites that were undetected at the time might be confining each ring. The first images of shepherd satellites were taken later that year by Voyager 1. [2]

Examples

Jupiter

Several of Jupiter's small innermost moons, namely Metis and Adrastea, are within Jupiter's ring system and are also within Jupiter's Roche limit. [3] It is possible that these rings are composed of material that is being pulled off these two bodies by Jupiter's tidal forces, possibly facilitated by impacts of ring material on their surfaces.

Saturn

The complex ring system of Saturn has several such satellites. These include Prometheus (F ring), [4] Daphnis (Keeler Gap), [5] Pan (Encke Gap), [6] Janus, and Epimetheus (both A ring). [7]

Uranus

Uranus also has shepherd moons on its ε ring, Cordelia and Ophelia. They are interior and exterior shepherds, respectively. [8] Both moons are well within Uranus's synchronous orbit radius, and their orbits are therefore slowly decaying due to tidal deceleration. [9]

Neptune

Neptune's rings are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but Voyager 2's images showed them to be complete rings with bright clumps. [10] It is thought that the gravitational influence of the shepherd moon Galatea and possibly other as-yet undiscovered shepherd moons are responsible for this clumpiness. [11]

Minor planets

Rings around some centaurs have been identified. Chariklo's rings are remarkably well-defined and are suspected to either be very young or kept in place by a shepherd moon similar in mass to the rings. [12] Chiron is also thought to have rings similar in form to those of Chariklo. [13]

Exoplanets

A major gap in the circumstellar disk or large ring system of the free-floating brown dwarf or rogue planet J1407b at about 61 million km (0.4 AU) from its center is considered to be indirect evidence of the existence of an exomoon (or exoplanet) with mass up to 0.8 Earth masses. [14] [15] [16]

See also

Related Research Articles

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In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationship is found between a pair of objects. The physical principle behind orbital resonance is similar in concept to pushing a child on a swing, whereby the orbit and the swing both have a natural frequency, and the body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be self-correcting and thus stable. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Neptune and Pluto. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance between bodies with similar orbital radii causes large planetary system bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.

<span class="mw-page-title-main">Ring system</span> Ring of cosmic dust orbiting an astronomical object

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.

<span class="mw-page-title-main">Umbriel (moon)</span> Moon of Uranus

Umbriel is the third-largest moon of Uranus. It was discovered on October 24, 1851, by William Lassell at the same time as neighboring moon Ariel. It was named after a character in Alexander Pope's 1712 poem The Rape of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a rocky core and an icy mantle. The surface is the darkest among Uranian moons, and appears to have been shaped primarily by impacts. However, the presence of canyons suggests early endogenic processes, and the moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface.

<span class="mw-page-title-main">Saturn</span> Sixth planet from the Sun

Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about nine times that of Earth. It has an eighth the average density of Earth, but is over 95 times more massive. Even though Saturn is almost as big as Jupiter, Saturn has less than a third the mass of Jupiter. Saturn orbits the Sun at a distance of 9.59 AU (1,434 million km), with an orbital period of 29.45 years.

<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">Pan (moon)</span> Moon of Saturn

Pan is the innermost named moon of Saturn. It is approximately 35 kilometres across and 23 km wide and orbits within the Encke Gap in Saturn's A Ring. Pan is a ring shepherd and is responsible for keeping the Encke Gap free of ring particles. It is sometimes described as having the appearance of a walnut, or ravioli.

<span class="mw-page-title-main">Janus (moon)</span> Moon of Saturn

Janus is an inner satellite of Saturn. It is also known as Saturn X. It is named after the mythological Janus. This natural satellite was first identified by Audouin Dollfus on December 15, 1966, although it had been unknowingly photographed earlier by Jean Texereau. Further observations led to the realization that Janus shares a unique orbital relationship with another moon, Epimetheus. The discovery of these two moons' peculiar co-orbital configuration was later confirmed by Voyager 1 in 1980.

<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">Moons of Uranus</span> Natural satellites of the planet Uranus

Uranus, the seventh planet of the Solar System, has 28 confirmed moons. Most of them are named after characters that appear in, or are mentioned in, the works of William Shakespeare and Alexander Pope. Uranus's moons are divided into three groups: thirteen inner moons, five major moons, and ten irregular moons. The inner and major moons all have prograde orbits and are cumulatively classified as regular moons. In contrast, the orbits of the irregular moons are distant, highly inclined, and mostly retrograde.

<span class="mw-page-title-main">Rings of Saturn</span>

The rings of Saturn are the most extensive and complex ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history, newer data from Cassini suggested they formed relatively late.

<span class="mw-page-title-main">Rings of Jupiter</span>

The rings of Jupiter are a system of faint planetary rings. The Jovian rings were the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. The main ring was discovered in 1979 by the Voyager 1 space probe and the system was more thoroughly investigated in the 1990s by the Galileo orbiter. The main ring has also been observed by the Hubble Space Telescope and from Earth for several years. Ground-based observation of the rings requires the largest available telescopes.

<span class="mw-page-title-main">Rings of Uranus</span>

The rings of Uranus are intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. William Herschel had also reported observing rings in 1789; modern astronomers are divided on whether he could have seen them, as they are very dark and faint.

<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.

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<span class="mw-page-title-main">Rings of Chariklo</span> Ring system around 10199 Chariklo

The rings of Chariklo are a set of two narrow rings around the minor planet 10199 Chariklo. Chariklo, with a diameter of about 250 kilometres (160 mi), is the second-smallest celestial object with confirmed rings and the fifth ringed celestial object discovered in the Solar System, after the gas giants and ice giants. Orbiting Chariklo is a bright ring system consisting of two narrow and dense bands, 6–7 km (4 mi) and 2–4 km (2 mi) wide, separated by a gap of 9 kilometres (6 mi). The rings orbit at distances of about 400 kilometres (250 mi) from the centre of Chariklo, a thousandth the distance between Earth and the Moon. The discovery was made by a team of astronomers using ten telescopes at various locations in Argentina, Brazil, Chile and Uruguay in South America during observation of a stellar occultation on 3 June 2013, and was announced on 26 March 2014.

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

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References

  1. Goldreich, Peter; Tremaine, Scott (1979). "Towards a theory for the Uranian rings" (PDF). Nature. 277 (5692): 97–99. Bibcode:1979Natur.277...97G. doi:10.1038/277097a0. S2CID   4232962.
  2. "Voyager 1".
  3. Faure, Gunter; Mensing, Teresa (2007). Introduction to Planetary Science: The Geological Perspective. Springer. ISBN   978-1-4020-5233-0.
  4. "On the masses and motions of mini-moons: Pandora's not a". www.planetary.org. Retrieved 2016-06-14.
  5. "NASA - Cassini Finds New Saturn Moon That Makes Waves". www.nasa.gov. Retrieved 2016-06-14.
  6. Showalter, Mark R. (1991-06-27). "Visual detection of 1981S13, Saturn's eighteenth satellite, and its role in the Encke gap". Nature. 351 (6329): 709–713. Bibcode:1991Natur.351..709S. doi:10.1038/351709a0. S2CID   4317496.
  7. Moutamid, Maryame El; Nicholson, Philip D.; French, Richard G.; Tiscareno, Matthew S.; Murray, Carl D.; Evans, Michael W.; French, Colleen McGhee; Hedman, Matthew M.; Burns, Joseph A. (2015-10-01). "How Janus' Orbital Swap Affects the Edge of Saturn's A Ring?". Icarus. 279: 125–140. arXiv: 1510.00434 . Bibcode:2016Icar..279..125E. doi:10.1016/j.icarus.2015.10.025. S2CID   51785280.
  8. Esposito, Larry W. (2002-01-01). "Planetary rings". Reports on Progress in Physics. 65 (12): 1741–1783. Bibcode:2002RPPh...65.1741E. doi:10.1088/0034-4885/65/12/201. ISSN   0034-4885. S2CID   250909885.
  9. Karkoschka, Erich (2001-05-01). "Voyager's Eleventh Discovery of a Satellite of Uranus and Photometry and the First Size Measurements of Nine Satellites". Icarus. 151 (1): 69–77. Bibcode:2001Icar..151...69K. doi:10.1006/icar.2001.6597.
  10. Miner, Ellis D.; Wessen, Randii R.; Cuzzi, Jeffrey N. (2007). "Present knowledge of the Neptune ring system". Planetary Ring System . Springer Praxis Books. ISBN   978-0-387-34177-4.
  11. Salo, Heikki; Hanninen, Jyrki (1998). "Neptune's Partial Rings: Action of Galatea on Self-Gravitating Arc Particles". Science. 282 (5391): 1102–1104. Bibcode:1998Sci...282.1102S. doi:10.1126/science.282.5391.1102. PMID   9804544.
  12. Braga-Ribas, F.; Sicardy, B.; Ortiz, J. L.; Snodgrass, C.; Roques, F.; Vieira-Martins, R.; Camargo, J. I. B.; Assafin, M.; Duffard, R. (April 2014). "A ring system detected around the Centaur (10199) Chariklo". Nature. 508 (7494): 72–75. arXiv: 1409.7259 . Bibcode:2014Natur.508...72B. doi:10.1038/nature13155. PMID   24670644. S2CID   4467484.
  13. Ortiz, J. L.; Duffard, R.; Pinilla-Alonso, N.; Alvarez-Candal, A.; Santos-Sanz, P.; Morales, N.; Fernández-Valenzuela, E.; Licandro, J.; Bagatin, A. Campo (2015). "Possible ring material around centaur (2060) Chiron". Astronomy & Astrophysics. 576: A18. arXiv: 1501.05911 . Bibcode:2015A&A...576A..18O. doi:10.1051/0004-6361/201424461. ISSN   0004-6361. S2CID   38950384.
  14. Kenworthy, Matthew A.; Mamajek, Eric E. (January 22, 2015). "Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons?". The Astrophysical Journal. 800 (2): 126. arXiv: 1501.05652 . Bibcode:2015ApJ...800..126K. doi:10.1088/0004-637X/800/2/126. S2CID   56118870.
  15. Sutton, P. J. (2019). "Mean motion resonances with nearby moons: an unlikely origin for the gaps observed in the ring around the exoplanet J1407b". Monthly Notices of the Royal Astronomical Society. 486 (2): 1681–1689. arXiv: 1902.09285 . Bibcode:2019MNRAS.486.1681S. doi: 10.1093/mnras/stz563 . S2CID   119546405.
  16. Kenworthy, M. A.; Klaassen, P. D.; et al. (January 2020). "ALMA and NACO observations towards the young exoring transit system J1407 (V1400 Cen)". Astronomy & Astrophysics . 633: A115. arXiv: 1912.03314 . Bibcode:2020A&A...633A.115K. doi:10.1051/0004-6361/201936141.

Further reading