Moons of Uranus

Uranus is home to a complex system of 29 known moons. These satellites vary greatly in size, composition, and orbital behavior. They are not a single uniform group. Instead, astronomers divide them into three distinct categories: fourteen inner moons, five major moons, and ten irregular moons. The inner and major moons are classified as regular moons because they follow prograde orbits. These orbits move in the same direction as the planet's rotation. In contrast, the irregular moons follow distant, highly inclined, and mostly retrograde orbits. This means they move in the opposite direction of the planet's rotation.

The fourteen inner moons exist in a chaotic and crowded environment. They are small, dark bodies with a geometrical albedo of less than 10%. This means they reflect very little light. They are composed of water ice mixed with dark material, likely radiation-processed organics. Some inner moons belong to specific groups, such as the Portia group and the Belinda group. These moons are so closely packed that they constantly perturb one another through gravity. This constant tugging makes the system unstable. Scientists believe these moons may eventually collide. For example, Desdemona may collide with Cressida within one million years.

These inner moons are also deeply connected to the ring system of Uranus. Some moons act as "shepherds" for the rings. Cordelia and Ophelia shepherd the epsilon ring. The small moon Mab is a source for the outermost mu ring. It is possible that the rings themselves came from the fragmentation of small inner moons. This process of colliding and re-accreting may keep the rings in a state of constant flux.

Even recently, new moons have been found in this region. In August 2025, a team led by Maryame El Moutamid identified a new inner moon, Uranus XXVIII, using James Webb Space Telescope images.

The five major moons are much larger and more complex. These are Miranda, Ariel, Umbriel, Titania, and Oberon. They are ellipsoidal in shape. This shape shows they reached hydrostatic equilibrium, meaning their own gravity pulled them into a rounded form. Most of these moons consist of roughly equal amounts of rock and ice. However, Miranda is made primarily of ice. Four of these moons show signs of endogenic processes. These are internal activities that reshape the surface. For instance, they show canyons or volcanic features.

Titania is the largest of the major moons, with a diameter of 1,578 km. It is the eighth-largest moon in the Solar System. Its mass is about one-twentieth the mass of Earth's Moon. The moons also show different levels of surface age. Ariel appears to have the youngest surface because it has the fewest impact craters. Umbriel has the oldest-looking surface. Scientists think past orbital resonances helped heat these moons. For example, a 3:1 resonance between Miranda and Umbriel may have caused internal heating. This heating likely created the unique structures on Miranda's surface, such as the coronae.

The history of discovering these moons is a long scientific journey. Sir William Herschel discovered the first two moons, Titania and Oberon, on January 11, 1787. For many years, his instruments were the only way to see them. In 1851, William Lassell discovered Ariel and Umbriel. Later, in 1948, Gerard Kuiper found Miranda at the McDonald Observatory. Many more were found during the Voyager 2 flyby in 1986. More recent discoveries have used the Hubble Space Telescope and the Subaru Telescope.

Even in 2024, new irregular moons like S/2023 U 1 were announced.

Naming these moons follows a unique literary tradition. Instead of using Greek mythology, they are named after characters from literature. Most names come from the plays of William Shakespeare. Some come from Alexander Pope's poem, The Rape of the Lock. For example, Oberon and Titania are characters from A Midsummer Night's Dream. The moons are part of a massive system that is much less massive than those of other giant planets. The combined mass of the five major moons is less than half the mass of Neptune's moon Triton. This makes the Uranian satellite system quite unique in our Solar System.