Uranus Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...

, the seventh planet of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

, has 28 confirmed moons. The 27 with names are named after characters that appear in, or are mentioned in,

William Shakespeare William Shakespeare ( 23 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's nation ...

's plays and

Alexander Pope Alexander Pope (21 May 1688 Old Style and New Style dates, O.S. – 30 May 1744) was an English poet, translator, and satirist of the Age of Enlightenment, Enlightenment era who is considered one of the most prominent English poets of the early ...

's poem '' The Rape of the Lock''. 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. The inner moons are small dark bodies that share common properties and origins with Uranus's rings. The five major moons are ellipsoidal, indicating that they reached

hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...

at some point in their past (and may still be in equilibrium), and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces. The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, about one-twentieth the mass of the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

. The orbits of the regular moons are nearly

coplanar In geometry, a set of points in space are coplanar if there exists a geometric plane that contains them all. For example, three points are always coplanar, and if the points are distinct and non-collinear, the plane they determine is unique. How ...

with Uranus's equator, which is tilted 97.77° to its orbit. Uranus's irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at large distances from the planet.

William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...

discovered the first two moons, Titania and

Oberon Oberon () is a king of the fairy, fairies in Middle Ages, medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania ...

, in 1787. The other three ellipsoidal moons were discovered in 1851 by

William Lassell William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer.Ariel and

Umbriel 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''. Umb ...

) and in 1948 by

Gerard Kuiper Gerard Peter Kuiper ( ; born Gerrit Pieter Kuiper, ; 7 December 1905 – 23 December 1973) was a Dutch-American astronomer, planetary scientist, selenographer, author and professor. The Kuiper belt is named after him. Kuiper is consi ...

( Miranda). These five may be in

. The remaining moons were discovered after 1985, either during the '' Voyager 2'' flyby mission or with the aid of advanced

-based telescopes.

Discovery

The first two moons to be discovered were Titania and

, which were spotted by Sir William Herschel on January 11, 1787, six years after he had discovered the planet itself. Later, Herschel thought he had discovered up to six moons (see below) and perhaps even a ring. For nearly 50 years, Herschel's instrument was the only one with which the moons had been seen. In the 1840s, better instruments and a more favorable position of Uranus in the sky led to sporadic indications of satellites additional to Titania and Oberon. Eventually, the next two moons, Ariel and

, were discovered by

William Lassell William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer. The Roman numbering scheme of Uranus's moons was in a state of flux for a considerable time, and publications hesitated between Herschel's designations (where Titania and Oberon are Uranus II and IV) and William Lassell's (where they are sometimes I and II). With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck. In 1852, Herschel's son

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor and experimental photographer who invented the blueprint and did botanical work. ...

gave the four then-known moons their names. No other discoveries were made for almost another century. In 1948,

at the McDonald Observatory discovered the smallest and the last of the five large, spherical moons, Miranda. Decades later, the flyby of the ''

Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...

'' space probe in January 1986 led to the discovery of ten further inner moons. Another satellite, Perdita, was discovered in 1999 by Erich Karkoschka after studying old ''Voyager'' photographs. Uranus was the last giant planet without any known irregular moons until 1997, when astronomers using ground-based telescopes discovered

Sycorax Sycorax is an unseen character in William Shakespeare's play ''The Tempest'' (1611). She is a vicious and powerful witch and the mother of Caliban (character), Caliban, one of the few native inhabitants of the island on which Prospero, the he ...

and Caliban. From 1999 to 2003, astronomers continued searching for irregular moons of Uranus using more powerful ground-based telescopes, resulting in the discovery of seven more Uranian irregular moons. In addition, two small inner moons,

Cupid In classical mythology, Cupid ( , meaning "passionate desire") is the god of desire, erotic love, attraction and affection. He is often portrayed as the son of the love goddess Venus and the god of war Mars. He is also known as Amor (Latin: ...

and Mab, were discovered using the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

in 2003. No other discoveries were made until 2021 and 2023, when Scott Sheppard and colleagues discovered one more irregular moon of Uranus (and five more candidates waiting to be announced) using the Subaru Telescope at Mauna Kea, Hawaii.

Spurious moons

After Herschel discovered Titania and

on 11 January 1787, he subsequently believed that he had observed four other moons: two on 18 January and 9 February 1790, and two more on 28 February and 26 March 1794. It was thus believed for many decades thereafter that Uranus had a system of six satellites, though the four latter moons were never confirmed by any other astronomer. Lassell's observations of 1851, in which he discovered Ariel and

, however, failed to support Herschel's observations; Ariel and Umbriel, which Herschel certainly ought to have seen if he had seen any satellites besides Titania and Oberon, did not correspond to any of Herschel's four additional satellites in orbital characteristics. Herschel's four spurious satellites were thought to have sidereal periods of 5.89 days (interior to Titania), 10.96 days (between Titania and Oberon), 38.08 days, and 107.69 days (exterior to Oberon). It was therefore concluded that Herschel's four satellites were spurious, probably arising from the misidentification of faint stars in the vicinity of Uranus as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell.

Names

Although the first two Uranian moons were discovered in 1787, they were not named until 1852, a year after two more moons had been discovered. The responsibility for naming was taken by

, son of the discoverer of Uranus. Herschel, instead of assigning names from

Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...

, named the moons after magical spirits in

English literature English literature is literature written in the English language from the English-speaking world. The English language has developed over more than 1,400 years. The earliest forms of English, a set of Anglo-Frisian languages, Anglo-Frisian d ...

: the fairies Oberon and Titania from

's ''

A Midsummer Night's Dream ''A Midsummer Night's Dream'' is a Comedy (drama), comedy play written by William Shakespeare in about 1595 or 1596. The play is set in Athens, and consists of several subplots that revolve around the marriage of Theseus and Hippolyta. One s ...

'', and the

sylph A sylph (also called sylphid) is an air spirit stemming from the 16th-century works of Paracelsus, who describes sylphs as (invisible) beings of the air, his elementals of air. A significant number of subsequent literary and occult works have be ...

Ariel and

gnome A gnome () is a mythological creature and diminutive spirit in Renaissance magic and alchemy, introduced by Paracelsus in the 16th century and widely adopted by authors, including those of modern fantasy literature. They are typically depict ...

Umbriel from

's '' The Rape of the Lock'' (Ariel is also a sprite in Shakespeare's ''

The Tempest ''The Tempest'' is a Shakespeare's plays, play by William Shakespeare, probably written in 1610–1611, and thought to be one of the last plays that he wrote alone. After the first scene, which takes place on a ship at sea during a tempest, th ...

''). It is uncertain if John Herschel was the originator of the names, or if it was instead William Lassell (who discovered Ariel and Umbriel) who chose the names and asked Herschel for permission. Subsequent names, rather than continuing the airy spirits theme (only Puck and Mab continued the trend), have focused on Herschel's source material. In 1949, the fifth moon, Miranda, was named by its discoverer

after a thoroughly mortal character in Shakespeare's ''

''. The current

IAU The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

practice is to name moons after characters from Shakespeare's plays and ''The Rape of the Lock'' (although at present only Ariel, Umbriel, and Belinda have names drawn from the latter; all the rest are from Shakespeare). The outer retrograde moons are all named after characters from one play, ''The Tempest''; the sole known outer prograde moon,

Margaret Margaret is a feminine given name, which means "pearl". It is of Latin origin, via Ancient Greek and ultimately from Iranian languages, Old Iranian. It has been an English language, English name since the 11th century, and remained popular thro ...

, is named from ''

Much Ado About Nothing ''Much Ado About Nothing'' is a Shakespearean comedy, comedy by William Shakespeare thought to have been written in 1598 and 1599.See textual notes to ''Much Ado About Nothing'' in ''The Norton Shakespeare'' (W. W. Norton & Company, 1997 ) p. ...

''. Some

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

s, also named after the same Shakespearean characters, share names with moons of Uranus:

171 Ophelia 171 Ophelia is a large, dark Themistian asteroid that was discovered by French astronomer Alphonse Borrelly on 13 January 1877, and named after Ophelia in Shakespeare's ''Hamlet''. This asteroid is a member of the Themis family of asteroids t ...

, 218 Bianca, 593 Titania, 666 Desdemona, 763 Cupido, and 2758 Cordelia.

Characteristics and groups

The Uranian satellite system is the least massive among those of the

giant planet A giant planet, sometimes referred to as a jovian planet (''Jove'' being another name for the Roman god Jupiter (mythology), Jupiter), is a diverse type of planet much larger than Earth. Giant planets are usually primarily composed of low-boiling ...

s. Indeed, the combined mass of the five major satellites is less than half that of Triton (the seventh-largest moon in the Solar System) alone. The largest of the satellites, Titania, has a radius of 788.9 km, or less than half that of the

, but slightly more than that of Rhea, the second-largest moon of

Saturn 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 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

, making Titania the eighth-largest moon in the

. Uranus is about 10,000 times more massive than its moons.

Inner moons

As of , Uranus is known to have 13 inner moons, whose orbits all lie inside that of Miranda. The inner moons are classified into two groups based on similar orbital distances: these are the Portia group, which includes the six moons Bianca, Cressida,

Desdemona Desdemona () is a character in William Shakespeare's play ''Othello'' (c. 1601–1604). Shakespeare's Desdemona is a Venice, Italy, Venetian beauty who enrages and disappoints her father, a Venetian senator, when she elopes with Othello (char ...

Juliet Juliet Capulet () is the female protagonist in William Shakespeare's romantic tragedy ''Romeo and Juliet''. A 13-year-old girl, Juliet is the only daughter of the patriarch of the House of Capulet. She falls in love with the male protagonist Ro ...

, Portia, and Rosalind; and the Belinda group, which includes the three moons

, Belinda, and Perdita. All of the inner moons are intimately connected with the

rings of Uranus The rings of Uranus consists of 13 planetary rings. They 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 J ...

, which probably resulted from the fragmentation of one or several small inner moons. The two innermost moons, Cordelia and

Ophelia Ophelia () is a character in William Shakespeare's drama ''Hamlet'' (1599–1601). She is a young noblewoman of Denmark, the daughter of Polonius, sister of Laertes and potential wife of Prince Hamlet. Due to Hamlet's actions, Ophelia ultima ...

, are

shepherds A shepherd is a person who tends, herds, feeds, or guards flocks of sheep. Shepherding is one of the world's oldest occupations; it exists in many parts of the globe, and it is an important part of Pastoralism, pastoralist animal husbandry. ...

of Uranus's ε ring, whereas the small moon Mab is a source of Uranus's outermost μ ring. There may be two additional small (2–7 km in radius) undiscovered shepherd moons located about 100 km exterior to Uranus's α and β rings. At 162 km, Puck is the largest of the inner moons of Uranus and the only one imaged by ''Voyager 2'' in any detail. Puck and Mab are the two outermost inner satellites of Uranus. All inner moons are dark objects; their geometrical

albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

is less than 10%. They are composed of water ice contaminated with a dark material, probably radiation-processed organics. The inner moons constantly perturb each other, especially within the closely-packed Portia and Belinda groups. The system is chaotic and apparently unstable. Simulations show that the moons may perturb each other into crossing orbits, which may eventually result in collisions between the moons.

may collide with Cressida within the next million years, and

will likely collide with Belinda in the next 10 million years; Perdita and

may be involved in later collisions. Because of this, the rings and inner moons may be under constant flux, with moons colliding and re-accreting on short timescales.

Large moons

Uranus has five major moons: Miranda, Ariel,

, Titania, and

. They range in diameter from 472 km for Miranda to 1578 km for Titania. All these moons are relatively dark objects: their geometrical albedo varies between 30 and 50%, whereas their

Bond albedo The Bond albedo (also called spheric albedo, planetary albedo, and bolometric albedo), named after the American astronomer George Phillips Bond (1825–1865), who originally proposed it, is the fraction of power in the total electromagnetic radi ...

is between 10 and 23%. Umbriel is the darkest moon and Ariel the brightest. The masses of the moons range from 6.7 kg (Miranda) to 3.5 kg (Titania). For comparison, the

has a mass of 7.5 kg. The major moons of Uranus are thought to have formed in the

accretion disc An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...

, which existed around Uranus for some time after its formation or resulted from a large impact suffered by

early in its history. This view is supported by their large

thermal inertia Thermal inertia is a term commonly used to describe the observed delays in a body's temperature response during heat transfers. The phenomenon exists because of a body's ability to both store and transport heat relative to its environment. Sinc ...

, a surface property they share with

dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...

s like

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...

and

Haumea Haumea ( minor-planet designation: 136108 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 heade ...

. It differs strongly from the thermal behaviour of the Uranian irregular moons that is comparable to classical

trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has an orbital semi-major axis of 30.1 astronomical units (AU). ...

s. This suggests a separate origin. PIA25500-Uranus-MajorMoons-20230504

All major moons comprise approximately equal amounts rock and ice, except Miranda, which is made primarily of ice. The ice component may include

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

. Their surfaces are heavily cratered, though all of them (except Umbriel) show signs of endogenic resurfacing in the form of lineaments (canyons) and, in the case of Miranda, ovoid race-track like structures called coronae. Extensional processes associated with upwelling

diapir A diapir (; , ) is a type of intrusion in which a more mobile and ductilely deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–Taylor ...

s are likely responsible for the origin of the coronae. Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest. A past 3:1

orbital resonance 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 relation ...

between Miranda and Umbriel and a past 4:1 resonance between Ariel and Titania are thought to be responsible for the heating that caused substantial endogenic activity on Miranda and Ariel. One piece of evidence for such a past resonance is Miranda's unusually high orbital

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

(4.34°) for a body so close to the planet. The largest Uranian moons may be internally differentiated, with rocky cores at their centers surrounded by ice mantles. Titania and Oberon may harbor liquid water oceans at the core/mantle boundary. The major moons of Uranus are airless bodies. For instance, Titania was shown to possess no atmosphere at a pressure larger than 10–20 nanobar. The path of the Sun in the local sky over the course of a local day during Uranus's and its major moons' summer solstice is quite different from that seen on most other

worlds. The major moons have almost exactly the same rotational axial tilt as Uranus (their axes are parallel to that of Uranus). The Sun would appear to follow a circular path around Uranus's celestial pole in the sky, at the closest about 7 degrees from it, during the hemispheric summer. Near the equator, it would be seen nearly due north or due south (depending on the season). At latitudes higher than 7°, the Sun would trace a circular path about 15 degrees in diameter in the sky, and never set during the hemispheric summer, moving to a position over the celestial equator during the Uranian equinox, and then invisible below the horizon during the hemispheric winter.

Irregular moons

Uranus's irregular moons range in size from 120 to 200 km (

) to under 10 km ( S/2023 U 1). Due to the small number of known Uranian irregular moons, it is not yet clear which of them belong to groups with similar orbital characteristics. The only known group among Uranus's irregular moons is the Caliban group, which is clustered at orbital distances between and inclinations between 141°–144°. The Caliban group includes three retrograde moons, which are Caliban, S/2023 U 1, Stephano. The intermediate inclinations 60° < i < 140° are devoid of known moons due to the Kozai instability. In this instability region, solar perturbations at apoapse cause the moons to acquire large eccentricities that lead to collisions with inner satellites or ejection. The lifetime of moons in the instability region is from 10 million to a billion years.

is the only known irregular prograde moon of Uranus, and it has one of the most eccentric orbits of any moon in the Solar System.

List

The Uranian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a

spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface (mathematics), surface obtained by Surface of revolution, rotating an ellipse about one of its principal axes; in other words, an ellipsoid with t ...

are highlighted in light blue and bolded. The inner and major moons all have prograde orbits. Irregular moons with retrograde orbits are shown in dark grey. Margaret, the only known irregular moon of Uranus with a prograde orbit, is shown in light grey. The orbits and mean distances of the irregular moons are variable over short timescales due to frequent planetary and solar perturbations, therefore the listed orbital elements of all irregular moons are averaged over a 8,000-year

numerical integration In analysis, numerical integration comprises a broad family of algorithms for calculating the numerical value of a definite integral. The term numerical quadrature (often abbreviated to quadrature) is more or less a synonym for "numerical integr ...

by Brozović and Jacobson (2009). These may differ from osculating orbital elements provided by other sources. The orbital elements of major moons listed here are based on the

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

of 1 January 2000, while orbital elements of irregular satellites are based on the epoch of 1 January 2020.

Notes

References

External links

* Scott S. Sheppard
Uranus Moons

* *
Gazetteer of Planetary Nomenclature—Uranus (USGS)
* {{Authority control Lists of moons Solar System