Last update: October 4, 2025

Uranus' Moons: An Inclined Ballet

A unique lunar system in the solar system

Uranus has 27 confirmed natural satellites, all named after characters from the works of William Shakespeare (1564-1616) and Alexander Pope (1688-1744). This system is distinguished by its extreme 98° tilt, suggesting a catastrophic tilt early in the planet's history. The major moons, discovered between 1787 and 1948, show geologically active surfaces despite their small size.

Their composition reveals a mixture of water ice and rocky materials, with possible traces of subsurface oceans for some. The Voyager 2 mission (1986) remains the only one to have flown by this system, providing 60% of our current knowledge.

Unique orbital characteristics

Unlike other planetary systems where moons orbit in the equatorial plane, those of Uranus follow its axial tilt:

Average orbital inclination: 98° (almost perpendicular to Uranus' orbital plane)
Orbital periods ranging from 0.3 days (Cordelia) to 1,346 days (Ferdinand)
System divided into 3 groups: 13 inner moons, 5 major moons, 9 irregular outer moons

The major moons of Uranus

Properties of Uranus' natural satellites (2025 data)
Name	Diameter (km)	Semi-major axis (km)	Orbital period (days)	Group	Particularities
Titania	1,578	435,910	8.71	Major moons	Largest moon; 1,500 km canyons (Messina Chasma); possible subsurface ocean
Oberon	1,523	583,520	13.46	Major moons	Ancient surface with dark-floored craters; albedo of 0.24
Ariel	1,158	191,020	2.52	Major moons	Youngest surface among major moons; traces of cryovolcanism
Umbriel	1,170	266,000	4.14	Major moons	Wunda crater (131 km) with bright ring; darkest surface
Miranda	472	129,390	1.41	Major moons	20 km cliffs (Verona Rupes); chaotic surface suggesting a violent geological history
Puck	162	86,000	0.76	Inner moons	Discovered by Voyager 2 in 1985; spheroidal shape
Sycorax	150	12,179,000	1,288.3	Irregular moons	Retrograde orbit; reddish color (indication of organic materials)
Setebos	48	17,418,000	2,225.2	Irregular moons	Retrograde orbit; named after a god from The Tempest (Shakespeare)
Prospero	50	16,256,000	1,978.3	Irregular moons	Inclination of 152°; possible captured Kuiper Belt object
Stephano	32	8,004,000	677.4	Irregular moons	Prograde but highly eccentric orbit (e = 0.23)
Trinculo	18	8,504,000	749.2	Irregular moons	Discovered in 2001; chaotic orbit influenced by Sycorax
Francisco	22	4,276,000	266.6	Irregular moons	Retrograde orbit; unknown rotation period
Ferdinand	20	20,901,000	2,887.2	Irregular moons	Farthest moon; long-term unstable orbit
Caliban	72	7,231,000	579.7	Irregular moons	Red color (spectrum similar to trans-Neptunian objects)
Belinda	90	75,255	0.62	Inner moons	Gray surface; possible past collision with Cressida
Cressida	82	61,770	0.46	Inner moons	In 3:2 orbital resonance with Juliet; destined to collide with Desdemona
Juliet	94	64,360	0.49	Inner moons	Elongated shape; estimated density of 1.3 g/cm³
Rosalind	72	69,940	0.56	Inner moons	Cratered surface; named after a character from As You Like It
Portia	135	66,090	0.51	Inner moons	Second largest inner moon; possible fragment of a parent body
Desdemona	64	62,680	0.47	Inner moons	Orbits between Cressida and Juliet; risk of collision in 4-100 million years
Bianca	51	59,170	0.43	Inner moons	High albedo (0.35); heavily cratered surface
Cordelia	40	49,770	0.34	Inner moons	Closest moon to Uranus; shepherd of the ε ring
Ophelia	43	53,790	0.38	Inner moons	Outer shepherd of the ε ring; 4:3 resonance with Cordelia
Perdita	30	76,417	0.64	Inner moons	Discovered in 1999 on Voyager 2 images; chaotic orbit
Mab	25	97,736	0.92	Inner moons	Probable source of the μ ring; very low density (< 1 g/cm³)
Cupid	18	74,392	0.61	Inner moons	Discovered in 2003; perturbed by Belinda
Margaret	20	14,345,000	1,687.0	Irregular moons	Only prograde irregular moon; very eccentric orbit (e = 0.66)

Theories on their formation

Two hypotheses dominate to explain the origin of this atypical lunar system:

Giant impact model: A cataclysmic collision with an Earth-sized body could have tilted Uranus and formed a debris disk that accreted into moons (theory proposed by Julien Salmon and Ravit Helled in 2018).
Gravitational capture: The outer irregular moons could be captured trans-Neptunian objects, as suggested by simulations by Scott Tremaine (1990).

The low density of the major moons (< 1.7 g/cm³) indicates a high proportion of ice, consistent with formation in a volatile-rich disk. Their isotopic composition, measured by spectroscopy, shows similarities with Kuiper Belt comets.

Future exploration prospects

No mission is currently planned for Uranus, but several projects are under study:

Uranus Orbiter and Probe (NASA): Proposal for a launch in the 2030s, including an orbiter and an atmospheric probe. Objectives: study the magnetosphere and map the moons with a resolution < 100 m/pixel.
ODINUS (ESA): Concept for a joint mission with China to fly by the Uranian and Neptunian systems, focusing on potential subsurface oceans.

These missions could reveal whether moons like Ariel or Miranda harbor liquid oceans beneath their icy crusts, as suggested by the thermal model of Francis Nimmo (2020) based on VLA data.