All types of moons
Pro Research Analysisby 
Searched over 200M research papers
Types of Moons in the Solar System
Regular Moons: Characteristics and Formation
Definition and Orbital Properties
Regular moons are typically larger satellites that orbit their host planets in nearly circular, prograde orbits close to the equatorial plane. These moons are believed to have formed through primordial accretion within the circum-planetary disc of their host planet, which explains their tight, stable orbits1 2.
Examples in the Solar System
Examples of regular moons include the Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), Saturn's Titan, and Neptune's Triton, although Triton is unique due to its retrograde orbit, suggesting it was captured rather than formed in situ1 2.
Irregular Moons: Characteristics and Capture
Definition and Orbital Properties
Irregular moons are generally smaller and have more eccentric and inclined orbits compared to regular moons. These moons are believed to have been captured by their host planets from heliocentric orbits, often through gravitational interactions or other forms of dissipation1 2.
Examples and Discoveries
Neptune's irregular moons, such as Nereid and the recently discovered five additional moons, exhibit a wide range of orbital characteristics, including both prograde and retrograde orbits. These moons are faint and small, with diameters ranging from 30 to 50 km, and their capture likely involved complex gravitational interactions1.
Unique Shapes of Saturn's Small Inner Moons
Formation through Merging Collisions
Saturn's small inner moons, such as Pan, Atlas, and Prometheus, have highly irregular shapes. These shapes are thought to result from merging collisions among similar-sized moonlets. The variety of shapes, from the 'ravioli-like' forms of Pan and Atlas to the elongated structure of Prometheus, can be explained by the dynamics of these collisions, which often involve multiple hit-and-run events before a final merger3.
Implications for Moon Formation
The peculiar shapes of these moons provide direct evidence of the processes at the final stages of their formation. This mechanism of merging collisions may also explain features observed on other moons, such as Iapetus' equatorial ridge and its oblate shape3.
Potential for Submoons
Stability and Conditions
While the giant planets have large moons, none of these moons have their own moons, or submoons. The dynamical stability of submoons is limited by tidal dissipation, which destabilizes their orbits around smaller moons or those too close to their host planet. However, a few large moons, such as Saturn's Titan and Iapetus, Jupiter's Callisto, and Earth's Moon, could potentially host long-lived submoons4.
Exomoon Candidates
The exomoon candidate Kepler-1625b-I, based on its mass and orbital separation, could theoretically host submoons. However, its large orbital inclination may pose challenges for the dynamical stability of such submoons4.
Conclusion
The moons of the Solar System exhibit a wide range of characteristics and origins. Regular moons are typically larger and have stable, circular orbits, while irregular moons are smaller with more eccentric and inclined orbits, often captured from heliocentric paths. The unique shapes of Saturn's small inner moons highlight the role of merging collisions in moon formation. Additionally, while submoons are theoretically possible, their existence is constrained by the dynamics of tidal dissipation and orbital stability.
Sources and full results
Most relevant research papers on this topic
Discovery of five irregular moons of Neptune
Five irregular moons with diameters of 30 to 50 km have been discovered, indicating that Neptune may have captured these moons from heliocentric orbits.
The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets
Saturn's small inner moons' diverse shapes are due to merging collisions among similar-sized moonlets, providing clues to their formation processes.
Can moons have moons?
Submoons can survive only around large, wide-separation orbits, with few exceptions, such as Saturn's moons and Earth's Moon.
The Moon Illusion, II
The moon illusion can be explained by egocentric or objective factors, with terrain providing superior distance cues.
Highly CitedEvolution of Saturn’s Mid-Sized Moons
Saturn's inner mid-sized moons, except for Mimas, formed early in the planet's history, with complex resonances shaping their geology and interiors.
The Geologic Evolution of the Moon
The moon's geologic evolution has been divided into four stages, with impact-based landforms and volcanic origins, and the moon now appears cold and rigid to depths of about 500 km.
Forming a Moon with an Earth-like Composition via a Giant Impact
A giant impact on early Earth could lead to a Moon with a composition similar to Earth's, potentially forming a moon from Earth's mantle.
Highly CitedOrigin of the Moon in a giant impact near the end of the Earth's formation
The Moon formed near the end of Earth's accumulation, with a smaller, more likely object than previously considered viable, and is iron-poor.
Highly Cited