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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 of Mercury was for a short time thought to exist. On March 27, 1974, two days before Mariner 10 made its flyby of Mercury, instruments began registering large amounts of ultraviolet radiation in the vicinity of Mercury that, according to one astronomer, "had no right to be there". [1]
The radius of Mercury's core is estimated to be 2,020 ± 30 km (1,255 ± 19 mi), based on interior models constrained to be consistent with a moment of inertia factor of 0.346 ± 0.014. [9] [33] Hence, Mercury's core occupies about 57% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten ...
Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth. In the Solar System , there are six planetary satellite systems containing 288 known natural satellites altogether.
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 ...
The large moons are generally not very inclined to their parent planets' equators, but Earth's Moon, Saturn's Iapetus, and Neptune's Triton are exceptions. Triton is unique among the large moons in that it orbits retrograde, i.e. in the direction opposite to its parent planet's rotation. [68]
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 ...
Several of the moons are quite large compared to their primaries: 90 Antiope, Mors–Somnus and Sila–Nunam (95%), Patroclus–Menoetius, Altjira and Lempo–Hiisi (90%, with Lempo–Paha at 50%). The largest known minor-planet moon in absolute size is Pluto's largest moon Charon , which itself has about half the diameter of Pluto.