Definition of a Moon

The planet Saturn and its moons and rings.
WireImage / Getty Images

Moons and rings are among the most fascinating objects in our solar system. Before the Space Race of the 1960s, astronomers knew that Earth, Mars, Jupiter, Saturn, Uranus, and Neptune had moons; at that time, only Saturn was known to have rings. With the advent of better telescopes and space-based probes that could fly to distant worlds, scientists began to discover many more moons and rings. Moons and rings are typically categorized as "natural satellites" that orbit other worlds.

Definition of a Moon

Pictures of the Moon - The Moon from Galileo's Perspective
NASA

For most people, the object that can be seen in the sky at night (and sometimes during the day) from Earth is the moon, but Earth's moon is just one of many moons in the solar system. It's not even the largest one. Jupiter's moon Ganymede has that honor. And in addition to the moons orbiting planets, nearly 300 asteroids are known to have moons of their own.

By convention, bodies orbiting other planets and asteroids are called "moons." Moons orbit bodies that already orbit the Sun. The technical term is "natural satellite", which differentiates them from the man-made satellites launched into space by space agencies. There are dozens of these natural satellites throughout the solar system. 

Different moons have different origin stories. For example, astronomers know that Earth's moon is made from leftovers of a huge collision between Earth and a Mars-sized object named Theia, which occurred early in solar system history. However, Mars's moons appear to be captured asteroids. 

What Moons Are Made Of

Jupiter, with its volcanic moon Io in the foreground
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Goddard Space Flight Center

Moon materials range from rocky material to icy bodies and mixtures of both. Earth's moon is made of rock (mostly volcanic). Mars's moons are the same material as rocky asteroids. Jupiter's moons are largely icy, but with rocky cores. The exception is Io, which is a completely rocky, highly volcanic world.

Saturn's moons are mostly ice with rocky cores. Its largest moon, Titan, is predominantly rocky with an icy surface. The moons of Uranus and Neptune are largely icy. Pluto's binary companion, Charon, is mostly rocky with an icy covering (as is Pluto). The exact makeup of its smaller moons, which were likely captured after a collision, is still being worked out by scientists.

Definition of a Ring

The Centaur minor planet with its ring system.
European Southern Observatory

Rings, another type of natural satellites, are collections of particles of rock and ice that orbit Jupiter, Saturn, Uranus, and Neptune. The rings of Jupiter were discovered by Voyager 1, and the rings of Uranus and Neptune were explored by Voyager 2.

At least one asteroid, named Chariklo, has a ring, too. Cariklo's ring was discovered through ground-based observations. Some planets, including Saturn, have moons orbiting within the ring systems. These moons are sometimes called "shepherd dogs" because they act to keep the ring particles in place.

The Characteristics of a Ring System

The New Horizons Long Range Reconnaissance Imager (LORRI) snapped this photo of Jupiter's ring system
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Ring systems can be extensive and well-populated, like Saturn's. Or, they can be diffuse and thin, like those at Jupiter, Uranus, Neptune, and Chariklo. The thickness of Saturn's rings is only a few kilometers, but the system extends from around 67,000 kilometers from Saturn's center to well over 13 million kilometers at their greatest extent. Saturn's rings are made mostly of water, ice, and dust. Jupiter's rings are composed of dusty dark material. They are thin and extend between 92,000 and 226,000 kilometers out from the planet's center.

The rings of Uranus and Neptune are also dark and tenuous​. They extend out tens of thousands of kilometers from their planets. Neptune has only five rings, and the distant asteroid Chariklo has only two narrow, densely populated bands of material surrounding it. Beyond these worlds, planetary scientists suspect that the asteroid 2060 Chiron has a pair of rings, and also one ring around ​the dwarf planet Haumea in the Kuiper Belt. Only time and observations will confirm their existence.

Comparing Moonlets and Ring Particles

ring particles
University of Colorado/public domain

There is no official definition of "moonlet" and "ring partipcle" by the International Astronomical Union (IAU). Planetary scientists have to use common sense to distinguish between these objects.

Ring particles, which are the building blocks of rings, are usually much smaller than moonlets. They're made of dust, pieces of rock, and ice, all formed in giant rings around their primary worlds. For example, Saturn has millions of ring particles, but only a few satellites that appear to be moonlets. Moonlets have enough gravitational pull to exert some influence on ring particles to keep them in line as they orbit the planet.

If a planet has no rings, then it naturally has no ring particles.

Moons and Rings in Other Solar Systems

moons and rings
NASA

Now that astronomers are finding planets around other stars—called exoplanets—it's highly likely that at least some will have moons, and maybe even rings. However, these exomoon and exo-ring systems may be difficult to find, as the planets themselves — let alone their potential moons and rings — are difficult to spot due to the glare of their stars. Until scientists design a technique to detect the rings and moons of distant planets, we will continue to wonder about the mystery of their existence.