By Jennifer Hayes
Updated Aug 30, 2022
In the past five decades the word “satellite” has become synonymous with the man‑made spacecraft that orbit our planet, yet it originally referred to any celestial body that orbits another. In the solar system, over 150 natural satellites—commonly called moons—circle the planets. Beyond Earth’s own Moon, we have observed moons around Mars, Jupiter, Saturn, Uranus and Neptune.
At just 227 million kilometers from Earth, Mars is the nearest planet with confirmed moons. Its two small satellites, Phobos (≈22 km diameter) and Deimos (≈12 km), were discovered by American astronomer Asaph Hall in 1877. Current research suggests they are captured asteroids that were captured by Mars’s gravity.
Jupiter’s gravitational pull has amassed more than 79 moons, making it the planet with the largest number of natural satellites. The four Galilean moons—Io, Europa, Ganymede, and Callisto—were first imaged by Galileo Galilei in 1610. Ganymede, at 5,268 km in diameter, is the biggest moon in the solar system; Callisto follows at 4,821 km.
Famous for its iconic rings, Saturn also boasts a rich collection of more than 50 named moons. The most prominent include Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe. Titan, discovered by Christiaan Huygens in 1655, is the second‑largest moon in the solar system with a diameter of 5,151 km.
Uranus hosts 27 known satellites, including the five large moons: Miranda, Ariel, Umbriel, Titania, and Oberon. William Herschel first observed Titania and Oberon in 1787. Ariel and Umbriel were identified by William Lassell in 1851, each about 1,100 km across. Miranda, discovered by Gerard Kuiper in 1948, has a diameter of roughly 512 km.
Neptune’s 13 moons include the trio of large satellites—Triton (2,710 km), Nereid (340 km), and Proteus (418 km). Triton, the largest, was first spotted by William Lassell in 1846. Nereid was named by Gerard Kuiper in 1949, and Proteus was discovered by Voyager 2 in 1989.
