With a total of 79 known moons — including four large moons known as the Galilean satellites — Jupiter almost qualifies as a solar system unto itself.

Most of the moons of Jupiter are small, with about 60 of the satellites being less than 6.2 miles in diameter. The number of moons changes fairly frequently, with the 79th moon being reported in July 2018. Most of the moons were discovered in the late 1970s and later as a result of several explorations by automated spacecraft, including NASA’s Voyager in 1979 and Galileo in 1995.

Not only is Jupiter the largest planet in the solar system, it is also the most massive at more than 300 times the mass of Earth. Its size plays a role in the number of moons orbiting Jupiter because there is a large area of gravitational stability around it to support many moons.

Jupiter also has the strongest magnetic field of any planet, so anything passing near it, such as an asteroid, is either destroyed by gravitational tides or captured into its orbit. Earth only has one moon because it lacks the strong gravitational field and mass necessary to hold another satellite in orbit.



The moons of Jupiter have orbital periods ranging from seven hours to almost three Earth years. Some of the orbits are nearly circular, while the moons farthest from Jupiter have more irregular orbits. The outer moons orbit in the opposite direction in which Jupiter spins, which is unusual and indicates the moons were asteroids that were sucked into Jupiter’s orbit after the initial system was formed.

This montage shows the best views of Jupiter's four large and diverse "Galilean" satellites as seen by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft during its flyby of Jupiter in late February 2007. The four moons are, from left to right: Io, Europa, Ganymede and Callisto. The images have been scaled to represent the true relative sizes of the four moons and are arranged in their order from Jupiter. (Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

Galilean moons

In January 1610, Italian astronomer Galileo Galilei discovered four of Jupiter’s moons — now called Io, Europa, Ganymede and Callisto. He originally referred to the individual moons numerically as I, II, III, and IV. The numerical system for naming the moons lasted for a few centuries until scientists determined that simply using numbers as a naming device would be confusing and impractical as more moons were discovered.

Galileo’s discovery was pivotal point in the history of astronomy as his observation revealed that not all celestial bodies revolved around the Earth. Until that time, Earth was thought to be the center of the universe.

Eight satellites — the four Galilean and four smaller moons — are closer to the planet and provide the dust that make up Jupiter's rings.

A translation of the key passages of Galileo Galilei's journal detailing his discovery of four moons orbiting Jupiter. Galileo made these sketches after discovering four moons orbiting Jupiter in January 1610. The moons, later named Io, Europa, Callisto and Ganymede, were the first discovered beyond Earth. (Image credit: NASA)

The closest of the Galilean moons to Jupiter is Io, the first moon to be discovered by Galileo. This satellite’s distinctive feature is its volcanoes, making it the only celestial body in the solar system besides Earth to have volcanic activity. This moon also has sulfur dioxide snowfields, leading to its characterization as a moon of fire and ice. Io has an iron or iron sulfide core and a brown silicate outer layer, which gives it a splotchy orange, yellow, black, red, and white appearance.

Moving outward from Jupiter is Europa. While slightly smaller than Earth’s moon, it is still one of the largest bodies in the solar system but the smallest of the Galilean satellites. Cracks and streaks crisscross the entire icy surface, which is marked with very few craters. Europa has a high degree of reflectivity, making it among the brightest moons in the solar system. At 20 to 180 million years old, the surface is fairly young. It is possible that an extensive ocean beneath the surface harbors life.

Ganymede is the third Galilean moon from Jupiter and the largest of the four. This low-density moon is about the size of Mercury but has about half the mass. Its outstanding characteristic is that it is the only moon to have its own magnetic field. The satellite’s iron core is topped off by a thick crust that is mostly ice. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain, which forms intricate patterns across Ganymede.

Callisto, the fourth and farthest of the Galilean moons from Jupiter, is the most heavily cratered object in the solar system. The moon’s landscape has essentially remained unchanged since its formation, which has garnered much interest among scientists. It is about the size of Mercury but very low in density. It is also experiences the least impact of Jupiter’s magnetic field as its orbit is the farthest from the planet and beyond Jupiter’s primary radiation belt.

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This article was updated on Oct. 1, 2018 by Space.com Senior Writer, Meghan Bartels. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.