Uranus has 13 rings and 27 moons, but we could continue to find more! Major moons:
Miranda – The innermost and smallest of Uranus’s five major moons has a diameter of just 472km (290 miles).
Ariel – The fourth-largest moon, Ariel also has the third-greatest mass. Voyager 2 imaged about 35 per cent of its surface.
Umbriel – Umbriel has the darkest surface of the Uranian moons, and is one of the most heavily cratered.
Titania – Titania is the eighth-largest moon in the Solar System with a diameter of around 1,580km (980 miles).
Oberon – Oberon is the outermost of the five major moons and is partially outside of Uranus’s magnetosphere.
Discovering the Uranian ring system was a real surprise for astronomers. Since the planet is on its side, we have the opportunity to view the rings in a completely different way than we have for the rings of Jupiter and Saturn. Most of the rings were identified in 1977, with Voyager 2 and later the Hubble Telescope bringing the total to its current number of 13. Many of these rings are not quite in the plane of Uranus’s equator, and most of them are not exactly circular.
Unlike the rings of Jupiter, most of Uranus’s rings comprise mostly microscopic particles of water ice and an unknown material – not dust. And in comparison to Saturn’s rings, Uranus’s rings are very dark. The brightness seems to vary depending on the angle of the ring.
Usually the Uranian rings are classified into three different groups. The nine main ones are very narrow -6, 5, 4, Alpha, Beta, Eta, Gamma, Delta and Epsilon. The two dustier rings are in their own group: Zeta (1986U2R) and Lambda. The two outer rings are Nu and Mu. The brightest and densest ring of the whole system is Epsilon. It is also the most eccentric, with a thickness that varies from 19.7 kilometres (12 miles) to 96.4 kilometres (59 miles). All of the rings seem to be very young in comparison to the planet. This means that they must be replenished by things like collisions of larger particles, moons or meteorites. The Epsilon ring has shepherd satellites – two moons, Cordelia and Ophelia – that confine it and define its inner and outer edges. But how do the rings without shepherd satellites maintain their shape and not spread out? This could mean that there are more moons we haven’t seen.
All 27 of Uranus’s moons are named after characters from the writings of either Alexander Pope or William Shakespeare. The first ones to be discovered were Titania and Oberon, seen by William Herschel in 1787. The most recent discoveries, such as Cupid and Francisco, were made using the Hubble Space Telescope in 2003. Despite their number, the moons aren’t very massive – the mass of the five major moons combined is less than half of the mass of Triton, Neptune’s largest moon. These major moons are Miranda, Ariel, Umbriel, Titania and Oberon. The largest of these is Titania, with a radius of 789 kilometres (490 miles). All of the major moons have planetary mass and hydrostatic equilibrium – meaning that they have enough gravity to keep them spherical. If they were in orbit around the Sun instead of Uranus, they would probably be considered dwarf planets. Except for Miranda, which is primarily ice, the major moons seem to be equally rock and ice. The largest ones might even have differentiated interiors, with cores and mantles. They’re mostly heavily cratered from impacts with meteors and other objects. And, except for Umbriel, they all show signs of geological activity.
Inside Miranda’s orbit lie 13 inner moons, each of which are associated with one of the rings. We know very little about most of these moons -only Puck was imaged by Voyager 2. It’s the largest at 162 kilometres (100 miles) in diameter. These small moons are generally dark coloured and are highly perturbed by each other, with unstable orbits.
The nine irregular moons orbit very far away from Uranus compared to the other moons, and were probably captured by Uranus’s gravitational pull soon after the planet formed. They have eccentric orbits and all but one, Margaret, are retrograde (meaning they orbit in the opposite direction of the planet). The largest at about 150 kilometres (93 miles) in diameter is Sycorax. There are likely to be even more irregular moons that we haven’t yet discovered.
The orbits of the moons
2. Mab – Although photographed by Voyager, Mab’s existence was not confirmed until 2003. Its size is unconfirmed, but we know that it is highly perturbed by the orbits of neighbouring moons.
3. Puck – Puck is the largest inner moon of Uranus, and lies between the rings and Miranda, the first of the larger moons.
4. Cupid – Discovered in 2003, Cupid is the smallest of the inner moons at just 18 kilometres (11.2 miles) in diameter.
5. Portia – The second-largest inner moon, Portia heads a group of moons with similar orbits. It has a diameter of about 140 kilometres (87 miles).
6. Juliet, Desdemona and Cressida – Little is known about these small neighbouring moons, but their chaotic orbits may result in collisions within 100 million years.
7. Ophelia and Cordelia – These innermost known moons serve as shepherd satellites – defining the inner and outer edges of Uranus’s Epsilon ring.