Natural satellite
From Halbeeg, the open encyclopedia · Af-Soomaali
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A natural satellite is a celestial body that orbits a planet, dwarf planet, or other object within a star system. These are bodies held in orbit by the gravitational pull of a larger object, which causes them to revolve continuously. Earth's moon is the most well-known example, and in Somali the term dayax is used for satellites generally without distinction.
Natural satellites vary greatly in size and shape. Some are large spherical bodies with internal heat and solid surfaces; others are small rocks without regular shapes, thought to be asteroids captured by gravity. The number of known natural satellites increases as telescopes and space probes discover new bodies.
Origin
Three common explanations account for the origin of natural satellites. The first is that they formed alongside the planet, from the same material and orbit. The second is capture: a larger body gravitationally traps a smaller, wandering object, which likely explains some small satellites of Mars and certain moons of Jupiter. The third is collision: a large body strikes a planet, and the debris coalesces to form a satellite—a model often applied to Earth's moon.
In the solar system
Mercury and Venus have no natural satellites. Earth has one. Mars has two small satellites, Phobos and Deimos. The gas giants—Jupiter, Saturn, Uranus, and Neptune—have extensive satellite systems. The largest among these include Ganymede, Titan, Callisto, Io, Europa, and Triton. Dwarf planets like Pluto also have satellites; Charon is Pluto's largest.
Rings and satellites
Large planets also have rings—bands of ice and rock debris, most prominent around Saturn. A relationship exists between rings and satellites: smaller bodies called shepherd satellites use their gravity to maintain ring structure, and collisions between satellites can contribute material to rings.
Scientific importance
Natural satellites are important subjects for scientific research, particularly those suspected to harbour liquid water beneath ice surfaces, such as Europa and Enceladus, and Titan, which has a dense atmosphere and complex chemistry. Additionally, a satellite orbiting a large body affects that body: tidal forces generate ocean tides and produce slight changes in a planet's rotation.