Satellite system (astronomy)
Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system.
The origin of many moons are investigated on a case-by-case basis, and the larger systems are thought to have formed through a combination of one or more processes.
Models of gas during the formation of planets coincide with a general rule for planet-to-satellite(s) mass ratio of 10,000:1[8] (a notable exception is Neptune).
[10] Ring systems are a type of circumplanetary disk that can be the result of satellites disintegrated near the Roche limit.
Objects in such a system may be part of a collisional family and this origin may be verified comparing their orbital elements and composition.
Similar models have been used to explain the creation of the Plutonian system as well as those of other Kuiper belt objects and asteroids.
[11] Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit.
[13][14] Models developed in 2018 explain the planet's unusual spin support an oblique collision with an object twice the size of Earth which likely to have re-coalesced to form the system's icy moons.
[18][19] Some scientists have put forward extended atmospheres around young planets as a mechanism for slowing the movement of a passing objects to aid in capture.
[20] A tell-tale sign of capture is a retrograde orbit, which can result from an object approaching the side of the planet which it is rotating towards.
In the case of the latter, however, virtually identical isotope ratios found in samples of the Earth and Moon cannot be explained easily by this theory.
The most notable examples are those around Saturn, but the other three gas giants (Jupiter, Uranus and Neptune) also have ring systems.
[28] Current theories suggest that some ring systems may form in repeating cycles, accreting into natural satellites that break up as soon as they reach the Roche limit.
Cassini's laws describe the motion of satellites within a system[30] with their precessions defined by the Laplace plane.
[35] A similar process is drawing Phobos closer to Mars, and it is predicted that in 50 million years it will either collide with the planet or break up into a planetary ring.
[36] Tidal acceleration, on the other hand, gradually moves the Moon away from Earth, such that it may eventually be released from its gravitational bounding and exit the system.
Simulations show that such interactions cause the orbits of the inner moons of the Uranus system to be chaotic and possibly unstable.
Nitrogen gas transfer between Pluto and Charon has also been modelled[41] and is expected to be observable by the New Horizons space probe.
However the geocentric model did not generally accommodate the possibility of celestial objects orbiting other observed planets, such as Venus or Mars.
190 BCE) made observations which may have included the phenomenon of tides,[42] which he supposedly theorized to be caused by the attraction to the Moon and by the revolution of the Earth around an Earth-Moon 'center of mass'.
As heliocentrism (the doctrine that the Sun is the centre of the universe) began to gain in popularity in the 16th century, the focus shifted to planets and the idea of systems of planetary satellites fell out of general favour.
Nicholas Copernicus published a model in which the Moon orbited around the Earth in the Dē revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres), in the year of his death, 1543.
It was not until the discovery of the Galilean moons in either 1609 or 1610 by Galileo, that the first definitive proof was found for celestial bodies orbiting planets.
When effects of eclipses as well as constraints from a satellite's orbital stability are included into this concept, one finds that — depending on a moon's orbital eccentricity — there is a minimum mass of roughly 0.2 solar masses for stars to host habitable moons within the stellar HZ.
