Regular moon
Encyclopedia
In astronomy, a regular moon is a natural satellite
following a relatively close and generally prograde orbit with little orbital inclination or eccentricity
. They are believed to have formed in orbit about their primary
, as opposed to irregular moons, which were captured.
There are at least 55 regular satellites of the eight planets: one at Earth, eight at Jupiter, 22 named regular moons at Saturn (not counting hundreds or thousands of moonlet
s), 18 known at Uranus, and 6 small regular moons at Neptune. (Large Triton appears to have been captured.) It is thought that Pluto's four moons and Haumea's two were formed in orbit about those dwarf planet
s out of debris created in giant collisions
.
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
following a relatively close and generally prograde orbit with little orbital inclination or eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
. They are believed to have formed in orbit about their primary
Primary (astronomy)
A primary is the main physical body of a gravitationally-bound, multi-object system. This body contributes most of the mass of that system and will generally be located near its center of mass....
, as opposed to irregular moons, which were captured.
There are at least 55 regular satellites of the eight planets: one at Earth, eight at Jupiter, 22 named regular moons at Saturn (not counting hundreds or thousands of moonlet
Moonlet
Moonlet is an informal term for a particularly small natural satellite. In astronomical literature, it has been used in at least two situations:...
s), 18 known at Uranus, and 6 small regular moons at Neptune. (Large Triton appears to have been captured.) It is thought that Pluto's four moons and Haumea's two were formed in orbit about those dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
s out of debris created in giant collisions
Collisional family
In astronomy, a collisional family is a group of objects that are thought to have a common origin in an impact . They have similar compositions, and most share similar orbital elements....
.