Like planets, moons are a constant in the universe. There are many different types of moons that fall under various classifications.
- 1 Classes of Moons
- 2 Orbits
Classes of Moons
A major moon is a celestial body in orbit around a parent planet that is large enough to reach hydrostatic equilibrium and become a sphere under its own gravity.
Major moons are most typically formed from the remainder of the debris created from its parent planet's planetary accretion disk, though this is not always the case as they may be a captured satellite.
Dwarf moons are celestial bodies comparable to your typical asteroids in orbit around a planet. Dwarf moons can vary wildly from each other in terms of size, shape, and composition, however they all lack the required size to reach hydrostatic equilibirum.
Dwarf moons can either form along with its parent planet, or it can be captured by its parent planet's gravity, with the former being more common.
A dual-planet moon is a celestial body that is close in size with its parent planet, causing the two to have comparable gravity and making them orbit each other around a barycenter between the two. However to be classified as the: "Moon" of the system it needs to be smaller in diameter and hold less gravitational influence over the barycenter they orbit.
Due to their nature, most dual-planet moons form from around half of the debris created by the planetary accretion disk of its parent planet, though they could become gravitationally bound to it later on.
A captured moon is a celestial body that was once an independent body in orbit around its parent star that was captured by a planet's gravity. Captured moons are fairly common, as primordial star systems have chaotic orbits, with many celestial bodies being ejected from the system, or being captured.
Since captured moons don't form along with its parent planet's gravity, their orbit around the planet will slowly degrade and they will either escape their parent planet's gravity, or it will enter within the planet's Roche limit and will be torn apart, forming a planetary ring system. If the captured moon is similar in size to its parent planet, then if it is pushed closer to its parent planet, it will crash into it instead.
A subsatellite, or a moonmoon, is a celestial body that orbits a moon that is already in orbit around a planet. Subsatellites are fairly uncommon and typically don't last long, as it is fairly difficult for a moon's gravity to keep hold of a companion satellite without it being captured by its parent planet's more dominant gravity. A planetary ring around a moon can technically be considered as a subsatellite.
Subsatellites are usually captured later on and don't typically form as a subsatellite, however they may be created from ejecta from impacts on either the moon or its parent planet.
A quasi-satellite is a celestial object that orbits its parent star in a co-orbit with a planet already in orbit with the aforementioned star. A quasi-satellite will only remain in a co-orbit with a planet for a limited amount of time before being ejected by the planet's gravity.
Due to most star systems containing billions of asteroids, a planet will often have countless quasi-satellites throughout its existence, and sometimes a quasi-satellite will become a captured moon if its orbit crosses far enough in the planet's gravity. Quasi-satellites are usually small asteroids. They are sometimes planets, but this occurence would be exceedingly rare.
A Trojan is a celestial object that shares its orbital path with a planet around their shared parent star within the Lagrangian point between the planet and the star. A Trojan can also be a moon that shares its orbital path with another sister-moon while orbiting its parent planet. In some instances, two separate planets can exist in a Trojan system, but these are more uncommon then those created by moons, asteroids, and the like. Most planets that exist in a Trojan system will most likely either form a dual-planet system, or crash into each other.
Unlike a quasi-satellite, Trojan orbits are fairly stable, allowing a celestial object to keep hold of many Trojans for billions of years, provided that the celestial object has the required size and the system is not perturbed. Most trojans are asteroids, although some can be planets or dwarf planets.
A centaur is a celestial object that has its orbit around its parent star influenced by a planet's gravity in its respective star system. These planets are typically gas giants as they have large gravity. Centaurs will typically be asteroids and have unstable orbits as they cross within the orbits of one or more planets in its star system, so they may be ejected from the system, or will be captured by the planet.
Centaurs will also usually form in an asteroid belt like fashion as there will be multiple celestial objects influenced by a planet's gravity, and they in kind will become gravitationally bound to each other. A good example of a centaur is Chiron.
Planetary Rings are rings of debris in orbit around a celestial body. They are very common, with most gas giants having a ring system of some kind. Large planetary ring systems are typically created by a moon of a planet entering within its respective Roche limit, tearing it apart and causing the debris to become gravitationally bound to the planet in question. They can also be created by ejecta from asteroid impacts, or particles captured by the planet as it orbits its parent star.
While they can last for tens of millions of standard years, a planetary ring system will eventually fall apart and the debris may drift away, become dwarf moons of the planet, or crash into its parent planet, and the ejecta may form its own planetary ring system, beginning the cycle anew.
A Shepherd satellite is a moon in orbit around a planet in close proximity to a planetary ring. Its own gravitational field will influence the debris that makes up the ring it orbits within, confining the ring into a narrow band. They will typically form from gravitationally bound debris of the ring system coalescing, into a larger object, which will then orbit the planet in kind with the ring it was birthed from. A planet can have any number of Shepherd satellites.
Shepherd satellites will often dictate the appearance of a ring system, creating the various divides and grooves one may see, and they can either be an internal Shepard or an external Shepard. Internal Shepherds are ones that orbit within the ring system itself in a tight-knit bond and affect the debris within the inner layers of the ring system. While external Shepherds orbit outside the ring system and affects the debris of the outer layers of the ring system. Shepherd satellites usually look oblate and flat, and if this is the case, then it is due to dust particles collecting on the moon's equator to form a sort of ridge.
A companion satellite is a celestial body that orbits in between the barycenter created from two celestial bodies in comparable size. Companion satellites are very rare and typically only last for a few thousand standard years, as they will most likely be ejected from the dual-planet system thanks to the vast amounts of ever-changing gravitational forces created from the aforementioned system. A stable companion satellite system can last for several millions of standard years.
Companion Satellites are typically captured later on, much like subsatellites. Most typically, a companion satellite will be an asteroid or dwarf planet in the dual-planet's respective star system that was captured by its gravity.
A proseltellian satellite is a celestial body that constantly becomes captured by two or more planets in orbit around its parent star in a predicable pattern. Stable proseltellian satellites are extremely rare, as they will typically become captured moons of the planet they were traded to, and they will typically eject other moons that are in orbit around the planet they became gravitationally bounded to.
Proseltellian satellites are most typically in orbit around a dual-planet system, as both bodies will have comparably strong gravitational forces, allowing for the celestial body in question to have its orbit to be affected by both. In this case, the satellite will likely remain on this pseudo-stable orbit for a while.
An astra-moon is a major moon that is hot enough to produce light and heat in a similar way to that of a star. Astra-Moons are exceptionally rare, as the series of events needed to create one naturally are very precise. For a major moon to naturally become an astra-moon, it must have significant tidal forces inflicted upon it by its parent planet, requiring it to be extremely close to a large gas giant planet. As well, it needs to have a thick atmosphere to retain the heat generated by its parent planet, and its own magnetic field to transmit radiation to its sister-moons.
Astra-moons can provide heat, light, and radiation to its sister-moons in a similar vein to that of a star, hence the name: "astra." In some instances, an astra-moon can provide a moon with enough energy to create life on its respective surface.
An artificial satellite is an object created by an intelligent species in orbit around a celestial body, be them native or otherwise. These satellites can vary depending on the species or individual in question. They could be communication arrays, weather sensors, interstellar telescopes, orbital habitats, or weaponry. So long as they were created by an intelligent species, they are an artificial satellite.
As well, naturally occurring bodies can become artificial satellites if an intelligent species moves an independent body in the star system and put it in orbit around the celestial body desired. These will typically be planets or dwarf planets that they wish to terraform, putting it in orbit around their home world to make the process easier for the conditions they require, though they may do this for mere aesthetic purposes.
A prime example of an artificial satellite would be that of the Internal national Space Station constructed by the human species.
A regular orbit is a moon in orbit around its parent planet in a regular, repeating trajectory and will orbit along the same plane its parent planet does. As well, moons with regular orbits will share the same rotation to that of its parent planet. Regular orbits are the most stable orbit a moon can have, allowing it to be in orbit around its parent planet almost indefinitely.
Regular orbits are more indicative of moons that formed alongside their parent planet, though this is not absolute.
An irregular orbit is a moon that orbits around its parent planet in various different ways. These orbits can change the moons distance from its parent planet wildly, and can orbit along any plane or have an opposite rotation to that of its parent planet. Irregular orbits are the most unstable orbits and they may escape its parent planets gravity, approach to closely to it, or crash into other moons in orbit around its parent planet.
Most irregular orbit moons are captured satellites, though this isn't always the case as they may have had their orbit perturbed at some point. Irregular orbits are normally a combination of medium-high eccentricity or significant inclination and/or ascending node relative to its parent.
A polar orbit is a moon that orbits around its parent planet in a near perfect circle where it crosses above both the planet's north and south poles. These orbits, much like regular orbits, are very stable and will last for hundreds of billions of years.
Though irregular moons may pass over their parent planet's poles, polar orbit moons orbit above the poles in a predictable and constant pattern, making it fairly uncommon.
A changing orbit is a moon that orbits in a way that switches in a predictable pattern. For example: A moon may have a regular orbit throughout most of its lunar year, then for the final months it has its orbit perturbed by another moon, making it polar, then switching back at the beginning of the year.
Stable changing orbits are very uncommon as more often then not, the moon will be ejected by the second moon, requiring the two to be similar in size.
A horseshoe orbit is two moons in orbit around a planet that share a near identical orbit with one moon being faster then the other. As the slower approaches, its gravity slows down the faster, as the faster falls towards the plane of the parent planet, it speeds up the slower, pushing it farther away from the plane of the planet, switching positions in a repeating pattern.
Stable horseshoe orbits, while more common then stable changing orbits, are still fairly uncommon, requiring the right combination of orbital timing.