Moons of Uranus

• 1. Introduction

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  The moons of Uranus carry names borrowed from English literature rather than Greek mythology, a deliberate choice that set them apart from every other moon in the Solar System. On the 11th of January 1787, William Herschel trained his telescope on Uranus and spotted two points of light circling the planet. He had discovered Titania and Oberon, named decades later after characters from William Shakespeare's A Midsummer Night's Dream. Today, 29 known moons orbit Uranus, grouped into three distinct families with very different histories, sizes, and fates. Some are quietly shepherding rings. Others may harbor liquid water beneath their surfaces. A few are on collision courses that could play out within the next ten million years. The story of how scientists found them, named them, and came to understand them stretches from the eighteenth century all the way to images captured by the James Webb Space Telescope in February 2025.

• 2. Centuries of Discovery

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  Herschel's 1787 discovery of Titania and Oberon left the Uranian moon count at two for nearly fifty years. His instrument was the only one capable of seeing them during that stretch. By the 1840s, better telescopes and a more favorable position of Uranus in the sky began hinting at additional satellites, but confirmation came only in 1851 when William Lassell identified two more: Ariel and Umbriel.

  There is a cautionary footnote to Herschel's legacy. After his initial find, he believed he had spotted four additional moons, claimed on dates including the 18th of January and the 9th of February 1790, and the 28th of February and the 26th of March 1794. Publications accepted a six-moon system for decades. When Lassell discovered Ariel and Umbriel in 1851, however, those moons bore no resemblance in orbital characteristics to Herschel's four extra candidates. The conclusion was that Herschel had mistaken faint background stars for satellites.

  In 1948, Gerard Kuiper at the McDonald Observatory discovered Miranda, the smallest and last of the five large spherical moons. Nearly four decades later, the Voyager 2 flyby in January 1986 changed the picture dramatically, revealing ten more inner moons in a single mission. Perdita was found in 1999 by Erich Karkoschka while studying old Voyager photographs. The first known irregular moons, Sycorax and Caliban, were identified by ground-based telescopes in 1997. Two small inner moons, Cupid and Mab, turned up in Hubble Space Telescope images in 2003. In 2024, Scott Sheppard and colleagues announced the detection of S/2023 U 1 using the Subaru Telescope at Mauna Kea. Then in August 2025, a team led by Maryame El Moutamid at the Southwest Research Institute found a new inner moon, designated Uranus XXVIII, in James Webb Space Telescope images from February 2025. It had been invisible to both Voyager 2 and Hubble because it was too small and faint.

• 3. Names from English Literature

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  John Herschel, son of the planet's discoverer, took on the task of naming the four moons known by 1852. He chose not to follow the tradition of Greek mythology that governed most of the Solar System. Instead, he reached for magical spirits in English literature: Oberon and Titania from Shakespeare's A Midsummer Night's Dream; the sylph Ariel and the gnome Umbriel from Alexander Pope's poem The Rape of the Lock. Ariel also appears as a sprite in Shakespeare's The Tempest. Whether John Herschel originated all four names or whether William Lassell, who discovered Ariel and Umbriel, proposed them and sought Herschel's approval remains uncertain.

  Gerard Kuiper named Miranda in 1949, selecting a thoroughly mortal character from The Tempest rather than one of the airy spirits. That shift toward Shakespeare's source material became standard practice. The International Astronomical Union now names Uranian moons after characters from Shakespeare's plays and The Rape of the Lock. Currently, only Ariel, Umbriel, and Belinda carry names drawn from Pope's poem; all the remaining moons come from Shakespeare. The outer retrograde irregular moons are all named after characters specifically from The Tempest, while Margaret, the only known outer prograde moon, takes her name from Much Ado About Nothing.

  The naming pattern created an unexpected overlap with the asteroid belt. Six asteroids share names with Uranian moons: 171 Ophelia, 218 Bianca, 593 Titania, 666 Desdemona, 763 Cupido, and 2758 Cordelia.

• 4. The Inner Moons and Their Rings

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  Fourteen inner moons orbit Uranus, all of them tucked inside the orbit of Miranda. Puck, at 162 km across, is the largest and the only one Voyager 2 photographed in any real detail. All inner moons are dark, with a geometric albedo under 10 percent. They are made of water ice laced with a dark material, most likely radiation-processed organic compounds.

  Two innermost moons, Cordelia and Ophelia, act as shepherds for Uranus's epsilon ring, their gravity confining the ring's edges. Mab, one of the two outermost inner moons, feeds material into Uranus's outermost mu ring. Scientists suspect there may be two additional undiscovered shepherd moons, each between 2 and 7 km in radius, orbiting roughly 100 km outside the alpha and beta rings.

  The inner moon system is not stable. The closely packed Portia group, which includes Bianca, Cressida, Desdemona, Juliet, Portia, and Rosalind, constantly perturbs its own members. Computer simulations suggest the moons may eventually nudge each other into crossing orbits and collide. Desdemona could strike Cressida within the next million years. Cupid is likely to hit Belinda within ten million years. Perdita and Juliet may follow in later collisions. The rings themselves are thought to be the product of earlier such collisions, fragments of inner moons that broke apart and re-accreted.

• 5. The Five Major Moons

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  Titania, Oberon, Umbriel, Ariel, and Miranda form the five major moons, ranging in diameter from 472 km for Miranda to 1,578 km for Titania. Titania is the eighth-largest moon in the Solar System and slightly wider than Rhea, the second-largest moon of Saturn. Yet the combined mass of all five major moons is less than half the mass of Neptune's moon Triton alone, making the Uranian satellite system the least massive among the giant planets.

  All five are thought to have reached hydrostatic equilibrium at some point, meaning gravity pulled them into roughly spherical shapes. They formed in an accretion disc around Uranus, either after the planet itself coalesced or following a large early impact. A property that supports this is their high thermal inertia, a surface characteristic they share with dwarf planets like Pluto and Haumea, quite different from the Uranian irregular moons, which behave thermally more like trans-Neptunian objects.

  Ariel has the youngest-looking surface with the fewest impact craters, while Umbriel appears oldest and darkest. Four of the five moons show signs of internally driven activity: lineaments (canyons) and, on Miranda, unusual oval structures called coronae, likely formed by extensional processes linked to upwelling diapirs. A past 3:1 orbital resonance between Miranda and Umbriel and a 4:1 resonance between Ariel and Titania are thought to have generated the heat that drove that activity. Miranda's notably high orbital inclination of 4.34 degrees, unusual for a body so close to its planet, stands as evidence that the resonance was real.

  Titania and Oberon may harbor liquid water oceans at the boundary between their rocky cores and ice mantles. Titania's atmosphere, if it exists at all, is below 10 to 20 nanobar of pressure. The Sun's path across the sky of these moons, during their summer solstice, traces a circle never setting for months, an experience entirely unlike anything on Earth.

• 6. The Irregular Moons

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  Beyond the regular moons lies a scattered population of irregular satellites, ranging from Sycorax at 120 to 200 km across down to S/2023 U 1 at under 10 km. Their orbits are distant, highly inclined, and mostly retrograde, distinguishing them sharply from the inner and major moons.

  The best-defined grouping is the Caliban group, three retrograde moons clustered at orbital distances with inclinations between 141 and 144 degrees: Caliban, Stephano, and S/2023 U 1. Uranus was the last giant planet without any known irregular moons until 1997, when Sycorax and Caliban were spotted by ground-based astronomers.

  A band of orbital inclinations between 60 and 140 degrees contains no known moons. This gap is caused by the Kozai instability, a mechanism by which the Sun's gravitational pull at the far point of an orbit drives the orbit to become highly eccentric. Moons that wander into this zone are sent on paths that lead to collisions with inner satellites or ejection from the system entirely. The survival time in this region runs from ten million to a billion years.

  Margaret stands as the only known irregular moon of Uranus with a prograde orbit, and it has one of the most eccentric orbits of any moon in the Solar System. Named after a character from Much Ado About Nothing, Margaret orbits at a mean distance of roughly 14,425,000 km from Uranus, with an eccentricity of 0.642.