Moons of Uranus these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, and about one-twentieth the mass 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, and the other three ellipsoidal moons were discovered in 1851 by William Lassell (Ariel and Umbriel) and in 1948 by Gerard Kuiper (Miranda). These five have planetary mass,
Oberon (moon) be differentiated into a rocky core surrounded by an icy mantle. If this is the case, the radius of the core (480 km) is about 63% of the radius of the moon, and its mass is around 54% of the moon's mass—the proportions are dictated by the moon's composition. The pressure in the center of Oberon is about 0.5 GPa (5 kbar). The current state of the icy mantle is unclear. If the ice contains enough ammonia or other antifreeze, Oberon may possess a liquid ocean layer at the core–mantle boundary. The thickness of this ocean, if it exists, is