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Although the Moon is Earth's only natural satellite, there are a number of near-Earth objects (NEOs) with orbits that are in resonance with Earth. These have been called "second" moons of Earth or "minimoons". [2] [3] 469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth. [4]
Mars has two known moons, Phobos and Deimos ("fear" and "dread", after attendants of Ares, the Greek god of war, equivalent to the Roman Mars). Searches for more satellites have been unsuccessful, putting the maximum radius of any other satellites at 90 m (100 yd). [4] Jupiter has 95 moons with known orbits; 72 of them have received permanent ...
The rings are believed to have been present approximately 466 million years ago. [1] [7] [8] The Hirnantian glaciation may be a direct result of the rings shielding light from reaching the Earth, [9] and the rings may have existed for up to 40 million years.
An artist's impression of Rhea's rings. The density of the particles is exaggerated greatly to aid visibility. [1] Rhea, the second-largest moon of Saturn, may have a tenuous ring system consisting of three narrow, relatively dense bands within a particulate disk. This would be the first discovery of rings around a moon.
The orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0.4° and eccentricities less than 0.005. [16] The discovery of Nix and Hydra suggested that Pluto could have a ring system. Small-body impacts could eject debris off of the small moons which can form into a ring system.
The giant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five.
There are three ways that thicker planetary rings have been proposed to have formed: from material originating from the protoplanetary disk that was within the Roche limit of the planet and thus could not coalesce to form moons, from the debris of a moon that was disrupted by a large impact, or from the debris of a moon that was disrupted by tidal stresses when it passed within the planet's ...
Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, [40] similar to the prevailing theory for the origin of Earth's moon. The moons of Mars may have started with a huge collision with a protoplanet one third the mass of Mars that formed a ring around Mars.