Natural satellite

A natural satellite is an astronomical body that orbits a planet, a dwarf planet, or a small Solar System body. Most people simply call these objects moons, a name derived from Earth's own Moon. In our Solar System, there are 434 natural satellites with confirmed orbits. This total includes moons orbiting seven dwarf planets, such as Pluto, Haumea, Makemake, and Eris. As of February 2026, there are 630 other minor planets known to have their own natural satellites.

Historically, our understanding of these objects has changed significantly. For a long time, the Moon was actually considered a planet. This changed after Copernicus introduced his work in 1543. In 1610, Galileo Galilei discovered the moons of Jupiter. He initially called them planets, but later astronomers sought a different term. Johannes Kepler was the first to use the word "satellite" in 1610. He took the term from the Latin word "satelles," which means companion or attendant. This was because satellites accompany their primary planet through space.

Natural satellites are often categorized by how they move and where they came from. Regular satellites orbit close to their planet in circular, uninclined paths. These are thought to have formed from the same material as the planet itself. In contrast, irregular satellites follow distant, tilted, or eccentric orbits. These are likely asteroids that were captured by a planet's gravity. Some large moons are even more unique. The Moon and the moons of Pluto likely formed from massive collisions between protoplanetary objects.

Many moons experience a phenomenon called tidal locking. This occurs when tidal forces from a planet cause a satellite to lose energy. This process slows the satellite's rotation until the same side always faces the planet. Most regular moons in the Solar System are tidally locked. However, some exceptions exist, such as Saturn's moon Hyperion. Hyperion rotates chaotically due to the gravitational influence of the moon Titan. Other irregular satellites, like Jupiter's Himalia, rotate much faster because they are too far away to be locked.

The physical shape of a moon is often determined by its size and gravity. Larger moons tend to be rounded into spheres or ellipsoids through hydrostatic equilibrium. This is a state where gravity pulls the body into a balanced shape. Some moons, like Saturn's Mimas, are not yet fully relaxed into a sphere. Larger moons can also become ovoid, or egg-shaped, due to tidal forces. For example, Mimas has a major axis that is 9% greater than its polar axis.

Some natural satellites are incredibly active geologically. Io is the most volcanically active body in our entire Solar System. Other moons, such as Europa, Enceladus, and Triton, show signs of tectonic activity or cryovolcanism. This activity is often powered by tidal heating. Tidal heating happens when the gravitational pull of a planet creates internal heat. This heat can support subsurface oceans of liquid water. Scientists believe Europa, Ganymede, Callisto, and Titan may all have such oceans beneath their surfaces.

While most moons orbit planets, some unusual orbital patterns exist. Trojan moons, such as Telesto and Calypso, are small companions that stay at specific points in a moon's orbit. These are located sixty degrees ahead of and behind a larger body. There are also temporary satellites. These are asteroids that briefly orbit a planet before leaving. For example, 2006 VP1 was a temporary satellite of Earth for nine months. These complex systems show how much variety exists in the mechanics of our Solar System.