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Nineteen moons are large enough to be round, and two, Titan and Triton, have substantial atmospheres The number of moons discovered in each year until November 2019. Mercury, the smallest and innermost planet, has no moons, or at least none that can be detected to a diameter of 1.6 km (1.0 mi). [2]
A "moon" was detected moving away from Mercury in 1974, and was eventually identified as a background star, 31 Crateris. 31 Crateris is a spectroscopic binary with a period of 2.9 days, and this may have been the source of the ultraviolet radiation detected in 1974. [6]
The sizes and masses of many of the moons of Jupiter and Saturn are fairly well known due to numerous ... Mercury: 2 439.7 ± 1.0 [15] 0.3829 ... The number of digits ...
Mercury is one of four terrestrial planets in the Solar System, which means it is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres (1,516.0 mi). [4] Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan.
The timeline of discovery of Solar System planets and their natural satellites charts the progress of the discovery of new bodies over history. Each object is listed in chronological order of its discovery (multiple dates occur when the moments of imaging, observation, and publication differ), identified through its various designations (including temporary and permanent schemes), and the ...
Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. 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 ...
For the round moons, this mostly matches the Roman numeral designations, with the exceptions of Iapetus and the Uranian system. This is because the Roman numeral designations originally reflected distance from the parent planet and were updated for each new discovery until 1851, but by 1892, the numbering system for the then-known satellites ...
The planetary-mass moons to scale, compared with Mercury, Venus, Earth, Mars, and Pluto. Sub-planetary Proteus and Nereid (about the same size as Mimas) have been included for comparison. Unimaged Dysnomia (intermediate in size between Tethys and Enceladus) is not shown; it is in any case probably not a solid body. [90]