Most of the outer planets have large swarms of satellites attending them. In many cases the satellites are arranged in regular orbits suggestive of miniature solar systems. Jupiter has four giant satellites, each comparable in mass to Earth's Moon, called the Galilean satellites. The internal densities of these satellites are now reasonably well known. The inner two Galilean satellites, Io and Europa, are largely rocky in composition. On the other hand, the outer two giant satellites, Ganymede and Callisto, are of a lower density, suggesting a much higher ice content. Closer to Jupiter than these Galilean satellites is a much smaller one, Amalthea. These five satellites lie in the plane of Jupiter's equator and have very nearly circular orbits. Because of this ordered arrangement, they are called the regular satellites. Three additional regular satellites, all very small, were discovered by Voyager spacecraft.
Orbiting far from these satellites are two swarms of so-called irregular satellites, each of them only a few kilometers in radius. Several of these bodies are so far known to exist, and there are indications of additional members. The satellites are called irregular because their orbits are inclined at substantial angles with respect to the plane of Jupiter's equator, and the orbits themselves are quite elliptical. Four of these satellites rotate in a direct (west to east) sense, but the others rotate in a retrograde (east to west) sense.
Saturn also has a system of regular satellites. One of these, Titan, is larger than the planet Mercury and is unique among the satellites in the solar system in having a substantial atmosphere. Four other satellites of Saturn have diameters of more than 1,000 km (600 mi), but the rest are much smaller. One of them, Phoebe, has a retrograde orbit. Studies of Voyager data have brought the total number above 20.
The five major satellites of Uranus are closely clustered in the plane of the Uranian equator, so that the plane of their orbits is also rotated 98 degrees to the plane of the ecliptic. The planet also has several smaller satellites.
The unusual system of Neptune contains one major satellite, Triton, whose mass is comparable to that of the Moon. The satellite moves in a circular but inclined retrograde orbit and has a very thin atmosphere. Neptune also has seven smaller, direct-rotating satellites.
A single moon of Pluto was discovered on June 22, 1978, and named Charon. Charon's diameter is 1,160 km (721 mi), which is about half the diameter of Pluto, making it the solar system's largest moon compared to its planet. If only because of this size relationship, Pluto and Charon can also be considered a double-planet system like the Earth and Moon.