Moons of Uranus facts for kids

Uranus, the seventh planet of the Solar System, has 28 known 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.

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 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's Moon. The orbits of the regular moons are nearly coplanar 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 discovered the first two moons, Titania and Oberon, in 1787. The other three ellipsoidal moons were discovered in 1851 by William Lassell (Ariel and Umbriel) and in 1948 by Gerard Kuiper (Miranda). These five may be in hydrostatic equilibrium, and so would be considered dwarf planets if they were in direct orbit about the Sun. The remaining moons were discovered after 1985, either during the Voyager 2 flyby mission or with the aid of advanced Earth-based telescopes.

Discovery

The first two moons to be discovered were Titania and Oberon, 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 Umbriel, were discovered by William Lassell in 1851. 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 gave the four then-known moons their names.

No other discoveries were made for almost another century. In 1948, Gerard Kuiper 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 space probe in January 1986 led to the discovery of ten further inner moons. Another satellite, Perdita, was discovered in 1999 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 and Caliban. From 1999–2003, 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 and Mab, were discovered using the Hubble Space Telescope in 2003. No other discoveries were made until 2021 and 2023, when Scott Sheppard and colleagues discovered one more irregular moon of Uranus (and four more candidates waiting to be announced) using the Subaru Telescope at Mauna Kea, Hawaii.

Characteristics and groups

The Uranian satellite system is the least massive among those of the giant planets. 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 Moon, but slightly more than that of Rhea, the second-largest moon of Saturn, making Titania the eighth-largest moon in the Solar System. Uranus is about 10,000 times more massive than its moons.

Inner moons

Schematic of the Uranian moon–ring system

As of 2024, 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, Juliet, Portia, and Rosalind; and the Belinda group, which includes the three moons Cupid, Belinda, and Perdita. All of the inner moons are intimately connected with the rings of Uranus, which probably resulted from the fragmentation of one or several small inner moons. The two innermost moons, Cordelia and Ophelia, are shepherds 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 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. Desdemona may collide with Cressida within the next million years, and Cupid will likely collide with Belinda in the next 10 million years; Perdita and Juliet 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 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, and Oberon

Uranus has five major moons: Miranda, Ariel, Umbriel, Titania, and Oberon. 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 is between 10 and 23%. 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, the Moon has a mass of 7.5 × 10²² kg. The major moons of Uranus are thought to have formed in the accretion disc, which existed around Uranus for some time after its formation or resulted from a large impact suffered by Uranus early in its history. This view is supported by their large thermal inertia, a surface property they share with dwarf planets like Pluto and Haumea. It differs strongly from the thermal behaviour of the Uranian irregular moons that is comparable to classical trans-Neptunian objects. 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. The ice component may include ammonia and carbon dioxide. 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 diapirs 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 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 (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 Solar System 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 hermispheric 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

Irregular satellites of Jupiter (red), Saturn (green), Uranus (magenta) and Neptune (blue; including Triton), plotted by distance from their planet (semi-major axis) in the horizontal axis and orbital inclination in the vertical axis. The semi-major axis values are expressed as a fraction of the planet's Hill sphere's radius, while the inclination is expressed in degrees from the ecliptic. The radius of the Uranian Hill sphere is approximately 73 million km. 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 120–200 km (Sycorax) 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 6–7 million km (3.7–4.3 million mi) 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. 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.

List

Orbital diagram of the orbital inclination and orbital distances for Uranus's rings and moon system at various scales. Notable moons and rings are individually labeled. 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 have collapsed into a spheroid 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 by Brozović and Jacobson (2009). These may differ from osculating orbital elements provided by other sources. Otherwise, recently-discovered irregular moons without published proper elements are temporarily listed here with inaccurate osculating orbital elements that are italicized to distinguish them from other irregular moons with proper orbital elements. Their orbital elements are all based on the epoch of 1 January 2000 Terrestrial Time.

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Uranian moons

Label	Name	Pronunciation (key)	Image	Abs. magn.	Diameter (km)	Mass (× 1016 kg)	Semi-major axis (km)	Orbital period (d)	Inclination (°)	Eccentricity	Discovery year	Year announced	Discoverer	Group
06 VI	Cordelia			10.3	0040 40 ± 6 (50 × 36)	000004 ≈ 4.4	00049000 49770	0000.33 +0.33503	000.084 0.08479°	0.00026	1986	1986	Terrile (Voyager 2)	ε ring shepherd
07 VII	Ophelia			10.2	0043 43 ± 8 (54 × 38)	000005 ≈ 5.3	00053000 53790	0000.37 +0.37640	000.103 0.1036°	0.00992	1986	1986	Terrile (Voyager 2)	ε ring shepherd
08 VIII	Bianca			9.8	0051 51 ± 4 (64 × 46)	000009 ≈ 9.2	00059000 59170	0000.43 +0.43458	000.193 0.193°	0.00092	1986	1986	Smith (Voyager 2)	Portia
09 IX	Cressida			8.9	0080 80 ± 4 (92 × 74)	000034 ≈ 34	00061000 61780	0000.46 +0.46357	000.006 0.006°	0.00036	1986	1986	Synnott (Voyager 2)	Portia
10 X	Desdemona			9.3	0064 64 ± 8 (90 × 54)	000018 ≈ 18	00062000 62680	0000.47 +0.47365	000.111 0.11125°	0.00013	1986	1986	Synnott (Voyager 2)	Portia
11 XI	Juliet			8.5	0094 94 ± 8 (150 × 74)	000056 ≈ 56	00064000 64350	0000.49 +0.49307	000.065 0.065°	0.00066	1986	1986	Synnott (Voyager 2)	Portia
12 XII	Portia			7.7	0135 135 ± 8 (156 × 126)	000170 ≈ 170	00066000 66090	0000.51 +0.51320	000.059 0.059°	0.00005	1986	1986	Synnott (Voyager 2)	Portia
13 XIII	Rosalind			9.1	0072 72 ± 12	000025 ≈ 25	00069000 69940	0000.55 +0.55846	000.279 0.279°	0.00011	1986	1986	Synnott (Voyager 2)	Portia
27 XXVII	Cupid			12.6	0018 ≈ 18	000000.38 ≈ 0.38	00074000 74800	0000.61 +0.61800	000.100 0.100°	0.0013	2003	2003	Showalter and Lissauer	Belinda
14 XIV	Belinda			8.8	0090 90 ± 16 (128 × 64)	000049 ≈ 49	00075000 75260	0000.62 +0.62353	000.031 0.031°	0.00007	1986	1986	Synnott (Voyager 2)	Belinda
25 XXV	Perdita			11.0	0030 30 ± 6	000002 ≈ 1.8	00076000 76400	0000.63 +0.63800	000 0.0°	0.0012	1999	1999	Karkoschka (Voyager 2)	Belinda
15 XV	Puck			7.3	0162 162 ± 4	000290 ≈ 290	00086000 86010	0000.76 +0.76183	000.319 0.3192°	0.00012	1985	1986	Synnott (Voyager 2)
26 XXVI	Mab			12.1	0018 ≈ 18	000000.38 ≈ 0.38	00097000 97700	0000.92 +0.92300	000.133 0.1335°	0.0025	2003	2003	Showalter and Lissauer	μ ring source
05 V	Miranda♠			3.5	0470 471.6 ± 1.4 (481 × 468 × 466)	006600 6400±300	00129000 129390	0001.4 +1.41348	004 4.232°	0.0013	1948	1948	Kuiper
01 I	Ariel♠			1.0	1157 1157.8±1.2 (1162 × 1156 × 1155)	125000 125100±2100	00191000 191020	0002.5 +2.52038	000.260 0.260°	0.0012	1851	1851	Lassell
02 II	Umbriel♠			1.7	1169 1169.4±5.6	127000 127500±2800	00266000 266300	0004.1 +4.14418	000.205 0.205°	0.0039	1851	1851	Lassell
03 III	Titania♠			0.8	1577 1576.8±1.2	340000 340000±6100	00435000 435910	0008.7 +8.70587	000.340 0.340°	0.0011	1787	1787	Herschel
04 IV	Oberon♠			1.0	1522 1522.8±5.2	301000 307600±8700	00583000 583520	0013 +13.4632	000.058 0.058°	0.0014	1787	1787	Herschel
22 XXII	Francisco‡			12.4	0022 ≈ 22	000000.72 ≈ 0.72	04282900 4282900	0267 −267.09	147.250°	0.1324	2001	2003	Holman et al.
16 XVI	Caliban♦			9.1	0042 42+20 −12	000025 ≈ 25	07231100 7231100	0579 −579.73	141.529°	0.1812	1997	1997	Gladman et al.	Caliban
99	S/2023 U 1♦			13.7	0008 ≈ 8	000000.034 ≈ 0.034	7978000 7978000	680.76 −680.76	141.893°	0.1867	2023	2024	Sheppard et al.	Caliban
20 XX	Stephano♦			9.7	0032 ≈ 32	000002.2 ≈ 2.2	08007400 8007400	0677 −677.47	143.819°	0.2248	1999	1999	Gladman et al.	Caliban
21 XXI	Trinculo‡			12.7	0018 ≈ 18	000000.39 ≈ 0.39	08505200 8505200	0749 −749.40	166.971°	0.2194	2001	2002	Holman et al.
17 XVII	Sycorax‡			7.4	0157 157+23 −15	000230 ≈ 230	12179400 12179400	1288 −1288.38	159.420°	0.5219	1997	1997	Nicholson et al.
23 XXIII	Margaret†			12.7	0020 ≈ 20	000000.54 ≈ 0.54	14146700 14146700	1661 +1661.00	57.367°	0.6772	2003	2003	Sheppard and Jewitt
18 XVIII	Prospero‡			10.5	0050 ≈ 50	000008.5 ≈ 8.5	16276800 16276800	1978 −1978.37	151.830°	0.4445	1999	1999	Holman et al.
19 XIX	Setebos‡			10.7	0048 ≈ 48	000007.5 ≈ 7.5	17420400 17420400	2225 −2225.08	158.235°	0.5908	1999	1999	Kavelaars et al.
24 XXIV	Ferdinand‡			12.5	0012 ≈ 12	000000.54 ≈ 0.54	20430000 20430000	2790 −2790.03	169.793°	0.3993	2001	2003	Holman et al.

Sources: NASA/NSSDC, Sheppard, et al. 2005. For the outer irregular moons, the most accurate orbital data can be generated with the Minor Planet Center's Natural Satellites Ephemeris Service. The irregulars are significantly perturbed by the Sun.

See also

• List of natural satellites