The inner moons are small dark bodies that share common properties and origins withUranus's rings. The five major moons are ellipsoidal, indicating that they reachedhydrostatic equilibrium 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.[3] The largest of these five,Titania, is 1,578 km in diameter and theeighth-largest moon in the Solar System, about one-twentieth the mass of theEarth'sMoon. The orbits of the regular moons are nearlycoplanar 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.[4]
The first two moons to be discovered wereTitania andOberon, which were spotted bySir 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.[5] 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 andUmbriel, were discovered byWilliam Lassell in 1851.[6] 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).[7] With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck.[8] In 1852, Herschel's sonJohn Herschel gave the four then-known moons their names.[9]
No other discoveries were made for almost another century. In 1948,Gerard Kuiper at theMcDonald Observatory discovered the smallest and the last of the five large, spherical moons,Miranda.[9][10] Decades later, the flyby of theVoyager 2 space probe in January 1986 led to the discovery of ten further inner moons.[3] Another satellite,Perdita, was discovered in 1999[11] byErich Karkoschka after studying oldVoyager photographs.[12]
Uranus was the last giant planet without any knownirregular moons until 1997, when astronomers using ground-based telescopes discoveredSycorax andCaliban. 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.[4] In addition, two small inner moons,Cupid andMab, were discovered using theHubble Space Telescope in 2003.[13] No other discoveries were reported until 2024, whenScott Sheppard and colleagues announced the discovery of one irregular moon of Uranus (S/2023 U 1) from 2023 observations by theSubaru Telescope atMauna Kea, Hawaii.[14][15]
In August 2025, a team of astronomers led byMaryame El Moutamid at the Southwest Research Institute announced the discovery of a new inner moon of Uranus (S/2025 U 1) inJames Webb Space Telescope images from February 2025. It was not detected earlier byVoyager 2 and the Hubble Space Telescope because it was too small and faint.[1][16]
After Herschel discoveredTitania andOberon 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 discoveredAriel andUmbriel, 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 havesidereal 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).[17] 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.[18]
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 byJohn Herschel, son of the discoverer of Uranus. Herschel, instead of assigning names fromGreek mythology, named the moons after magical spirits inEnglish literature: the fairies Oberon and Titania fromWilliam Shakespeare'sA Midsummer Night's Dream, and thesylph Ariel andgnome Umbriel fromAlexander Pope'sThe Rape of the Lock (Ariel is also a sprite in Shakespeare'sThe Tempest).[19] 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.[20]
Subsequent names, rather than continuing the airy spirits theme (onlyPuck andMab continued the trend), have focused on Herschel's source material. In 1949, the fifth moon,Miranda, was named by its discovererGerard Kuiper after a thoroughly mortal character in Shakespeare'sThe Tempest.[9] The currentIAU practice is to name moons after characters from Shakespeare's plays andThe 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, is named fromMuch Ado About Nothing.[20]
The Uranian satellite system is the least massive among those of thegiant planets. Indeed, the combined mass of the five major satellites is less than half that ofTriton (the seventh-largest moon in the Solar System) alone.[a] The largest of the satellites,Titania, has a radius of 788.9 km,[22] or less than half that of theMoon, but slightly more than that ofRhea, the second-largest moon ofSaturn, making Titania theeighth-largest moon in theSolar System. Uranus is about 10,000 times more massive than its moons.[b]
Schematic of the Uranian moon–ring system, excludingS/2025 U 1, which was not known at the time of drawing
As of 2025, Uranus is known to have 14 inner moons, whose orbits all lie inside that ofMiranda.[13] Some of the inner moons are classified into two groups based on similar orbital distances: these are the Portia group, which includes the six moonsBianca,Cressida,Desdemona,Juliet,Portia, andRosalind; and the Belinda group, which includes the three moonsCupid,Belinda, andPerdita.[13][23]S/2025 U 1, which orbits betweenOphelia and Bianca, is not classified into either of these groups.[24] All of the inner moons are intimately connected with therings of Uranus, which probably resulted from the fragmentation of one or several small inner moons.[25] The two innermost moons,Cordelia and Ophelia, areshepherds of Uranus's ε ring, whereas the small moonMab is a source of Uranus's outermost μ ring.[13] There may be two additional small (2–7 km in radius) undiscovered shepherd moons located about 100 km exterior to Uranus'sα and β rings.[26]
At 162 km,Puck is the largest of the inner moons of Uranus and the only one imaged byVoyager 2 in any detail. Puck and Mab are the two outermost inner satellites of Uranus. All inner moons are dark objects; their geometricalalbedo is less than 10%.[27] They are composed of water ice contaminated with a dark material, probably radiation-processed organics.[28]
The inner moons constantlyperturb each other, especially within the closely-packed Portia and Belinda groups. The system ischaotic and apparently unstable.[29] Simulations show that the moons may perturb each other into crossing orbits, which may eventually result in collisions between the moons.[13]Desdemona may collide withCressida within the next million years,[30] andCupid will likely collide withBelinda in the next 10 million years;Perdita andJuliet may be involved in later collisions.[31] Because of this, the rings and inner moons may be under constant flux, with moons colliding and re-accreting on short timescales.[31]
Uranus and its six largest moons compared at their proper relative sizes and in the correct order. From left to right:Puck,Miranda,Ariel,Umbriel,Titania, andOberon
Uranus has fivemajor moons:Miranda,Ariel,Umbriel,Titania, andOberon. They range in diameter from 472 km for Miranda to 1578 km for Titania.[22] All these moons are relatively dark objects: their geometrical albedo varies between 30 and 50%, whereas theirBond albedo is between 10 and 23%.[27] Umbriel is the darkest moon and Ariel the brightest. The masses of the moons range from 6.7 × 1019 kg (Miranda) to 3.5 × 1021 kg (Titania). For comparison, theMoon has a mass of 7.5 × 1022 kg.[32] The major moons of Uranus are thought to have formed in theaccretion disc, which existed around Uranus for some time after its formation or resulted from a large impact suffered byUranus early in its history.[33][34] This view is supported by their largethermal inertia, a surface property they share withdwarf planets likePluto andHaumea.[35] It differs strongly from the thermal behaviour of the Uranian irregular moons that is comparable to classicaltrans-Neptunian objects.[36] This suggests a separate origin.
Moons (Ariel, Umbriel, Titania, Oberon, Miranda) Modeling (4 May 2023)
All major moons comprise approximately equal amounts rock and ice, except Miranda, which is made primarily of ice.[37] The ice component may includeammonia andcarbon dioxide.[38] Their surfaces are heavily cratered, though all of them (except Umbriel) show signs ofendogenicresurfacing in the form of lineaments (canyons) and, in the case of Miranda, ovoid race-track like structures calledcoronae.[3] Extensional processes associated with upwellingdiapirs are likely responsible for the origin of the coronae.[39] Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest.[3] A past 3:1orbital resonance 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.[40][41]One piece of evidence for such a past resonance is Miranda's unusually high orbitalinclination (4.34°) for a body so close to the planet.[42][43] The largest Uranian moons may be internally differentiated, with rockycores at their centers surrounded by icemantles.[37] Titania and Oberon may harbor liquid water oceans at the core/mantle boundary.[37] 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.[44]
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 otherSolar System worlds. The major moons have almost exactly the same rotational axial tilt as Uranus (their axes are parallel to that of Uranus).[3] 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,[c] 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 satellites of Jupiter (red), Saturn (green), Uranus (magenta) and Neptune (blue; including Triton at the top left), plotted by distance from their planet (semi-major axis) in the horizontal axis andorbital inclination in the vertical axis. The semi-major axis values are expressed as a fraction of the planet'sHill sphere's radius, while the inclination is expressed indegrees from theecliptic. The radius of the UranianHill sphere is approximately 73 million km.[4] The relative sizes of moons are indicated by the size of their symbols, and the Caliban group of Uranian moons is labeled. Data as of February 2024.
Uranus's irregular moons range in size from 120 to 200 km (Sycorax) to under 10 km (S/2023 U 1).[45] 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 6–7 million km (3.7–4.3 million mi) and inclinations between 141°–144°.[15] The Caliban group includes three retrograde moons, which areCaliban, S/2023 U 1, andStephano.[15]
The intermediate inclinations 60° < i < 140° are devoid of known moons due to theKozai instability.[4] In this instability region, solarperturbations 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.[4]Margaret 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.
Orbital diagram of theorbital inclination and orbital distances for Uranus's rings and moon system at various scales. Open the image for full resolution.
The Uranian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to havecollapsed into aspheroid are highlighted in light blue and bolded. The inner and major moons all haveprograde 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 solarperturbations, therefore the listed orbital elements of all irregular moons are averaged over a 10,000-yearnumerical integration by Sheppard et al. (2024). These may differ fromosculating orbital elements provided by other sources.[46] The orbital elements of major moons listed here are based on theepoch of 1 January 2000,[47] while orbital elements of irregular satellites are based on the epoch of 1 January 2020.[48]
^Label refers to theRoman numeral attributed to each moon in order of their discovery.[2]
^Diameters with multiple entries such as "60 × 40 × 34" reflect that the body is not a perfectspheroid and that each of its dimensions have been measured well enough. The diameters and dimensions of Miranda, Ariel, Umbriel, and Oberon were taken from Thomas, 1988.[22] The diameter of Titania is from Widemann, 2009.[44] The dimensions and radii of the inner moons are from Karkoschka, 2001,[12] except for Cupid, Mab, and S/2025 U 1. The radii of Cupid and Mab were taken from Showalter, 2006.[13] The dimensions of S/2025 U 1 are a calculated estimate. The radii of outer moons except Sycorax, Caliban, and S/2023 U 1 were taken from Sheppard's website.[45] The radii of Sycorax and Caliban are from Farkas-Takács et al., 2017.[36] The dimensions of S/2023 U 1 are a calculated estimate.
^Masses of Puck, Miranda, Ariel, Umbriel, Titania, and Oberon were taken from Jacobson & Park, 2025.[47] Masses of Cordelia, Ophelia, and Cressida were taken from French, 2024.[50] Masses of all other moons were calculated assuming a density of 1 g/cm3 and using given radii.
^abcdMean orbits of irregular satellites are taken from JPL Small System Dynamics,[48] while mean orbits of the five major moons and Puck are taken from Jacobson & Park (2025).[47]
^Negative orbital periods indicate aretrograde orbit around Uranus (opposite to the planet's orbit).
^For regular satellites, inclination measures the angle between the moon's orbital plane and the plane defined by Uranus's equator. For irregular satellites, inclination measures the angle between the moon's orbital plane and the ecliptic.
^"MPEC 2024-D113 : S/2023 U 1".Minor Planet Electronic Circular. Minor Planet Center. 23 February 2024.Archived from the original on 29 February 2024. Retrieved23 February 2024.
^abc"New Uranus and Neptune Moons".Earth & Planetary Laboratory. Carnegie Institution for Science. 23 February 2024.Archived from the original on 23 February 2024. Retrieved23 February 2024.
^Hughes, D. W. (1994). "The Historical Unravelling of the Diameters of the First Four Asteroids".R.A.S. Quarterly Journal.35 (3):334–344.Bibcode:1994QJRAS..35..331H.
^Green, Daniel W. E. (19 August 2025)."CBET 5593: S/2025 U 1".Central Bureau Electronic Telegram. Central Bureau for Astronomical Telegrams. Retrieved19 August 2025.
^Jacobson, R. A.; Campbell, J. K.; Taylor, A. H.; Synnott, S. P. (June 1992). "The masses of Uranus and its major satellites from Voyager tracking data and earth-based Uranian satellite data".The Astronomical Journal.103 (6):2068–2078.Bibcode:1992AJ....103.2068J.doi:10.1086/116211.
^Tittemore, William C.; Wisdom, Jack (June 1990). "Tidal evolution of the Uranian satellites: III. Evolution through the Miranda-Umbriel 3:1, Miranda-Ariel 5:3, and Ariel-Umbriel 2:1 mean-motion commensurabilities".Icarus.85 (2):394–443.Bibcode:1990Icar...85..394T.doi:10.1016/0019-1035(90)90125-S.hdl:1721.1/57632.
^abSheppard, Scott S."Moons of Uranus".Earth & Planets Laboratory. Carnegie Institution for Science.Archived from the original on 18 February 2024. Retrieved23 February 2024.
^ab"Planetary Satellite Mean Elements". Jet Propulsion Laboratory.Archived from the original on 9 October 2024. Retrieved9 October 2024. Note: Orbital elements of regular satellites are with respect to theLaplace plane, while orbital elements of irregular satellites are with respect to theecliptic. Inclinations greater than 90° are retrograde. Orbital periods of irregular satellites may not be consistent with their semi-major axes due to perturbations.
^French, Richard G.; Hedman, Matthew M.; Nicholson, Philip D.; Longaretti, Pierre-Yves; McGhee-French, Colleen A. (March 2024). "The Uranus system from occultation observations (1977–2006): Rings, pole direction, gravity field, and masses of Cressida, Cordelia, and Ophelia".Icarus.411 115957.arXiv:2401.04634.Bibcode:2024Icar..41115957F.doi:10.1016/j.icarus.2024.115957.
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