There are 115 known moons of the planet Jupiter as of 9 April 2026. This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes. All together, Jupiter's moons form a satellite system, colloquially referred to as the Jovian system. The most massive of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto, all of which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers (or other sexual partners) or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 111 known moons and the rings together comprising just 0.003% of the total orbiting mass.
Of Jupiter's moons, eight are regular satellites with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiter's equatorial plane. The Galilean satellites are nearly spherical in shape due to their planetary mass, and are just massive enough that they would be considered planets if they were in direct orbit around the Sun. The other four regular satellites, known as the inner moons, are much smaller and closer to Jupiter; these serve as sources of the dust that makes up Jupiter's rings. The remainder of Jupiter's moons are outer irregular satellites whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities. The largest of these moons were likely asteroids that were captured from solar orbits by Jupiter before impacts with other small bodies shattered them into many kilometer-sized fragments, forming collisional families of moons sharing similar orbits. Jupiter is expected to have about 100 irregular moons larger than 1 km (0.6 mi) in diameter, plus around 500 more smaller retrograde moons down to diameters of 0.8 km (0.5 mi). Of the 107 known irregular moons of Jupiter, 58 of them have not yet been officially given names.
The physical and orbital characteristics of the moons vary widely. The four Galileans are all over 3,100 kilometres (1,900 mi) in diameter; the largest Galilean, Ganymede, is the ninth largest object in the Solar System, after the Sun and seven of the planets, Ganymede being larger than Mercury. All other Jovian moons are less than 250 kilometres (160 mi) in diameter, with most barely exceeding 5 kilometres (3.1 mi). Their orbital shapes range from nearly perfectly circular to highly eccentric and inclined, and many revolve in the direction opposite to Jupiter's rotation (retrograde motion).
Jupiter's regular satellites are thought to have formed from a circumplanetary disk, a ring of gravitated gas and solid debris analogous to a protoplanetary disk. They may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history.
Simulations suggest that, while the disk had a relatively high mass at any given moment, over time a substantial fraction (several tenths of a percent) of the mass of Jupiter captured from the solar nebula was passed through it. However, only 2% of the proto-disk mass of Jupiter is required to explain the existing satellites. Thus, several generations of Galilean-mass satellites may have been in Jupiter's early history. Each generation of moons might have spiraled into Jupiter, because of drag from the disk, with new moons then forming from the new debris captured from the solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned so that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance with each other, which still exists for Io, Europa, and Ganymede: they are in a 1:2:4 resonance. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io. Tidal dissipation in the Jovian system is still ongoing and Callisto will likely be captured into the resonance in about 1.5 billion years, creating a 1:2:4:8 chain.
The outer, irregular moons are thought to have originated from captured asteroids, whereas the proto-lunar disk was still massive enough to absorb much of their momentum and thus capture them into orbit. Many are believed to have been broken up by mechanical stresses during capture, or afterward by collisions with other small bodies, producing the moons we see today.
The formation of the Ananke and Carme groups was likely via a collision with a passing planetesimal, as opposed to a moon-moon impact. Due to their longer orbital periods and greater distances from Jupiter, collisions are rare among the retrograde satellites. Numerical integrations show the expected number of collisions in the past 4.5 billion years among all retrograde moons combined is around 1, which is probably not common enough to have produced the families. The impactors of the parent objects produced large craters, but were not large enough to catastrophically disrupt the progenitors, as the largest fragments still have 96% and 99% of the mass of the parent body, for the Ananke and Carme groups respectively. The impactors' diameters are required to be 0.53 km and 0.59 km; planetesimals of that size were very common early in the Solar System's formation. However, a moon-moon collisional origin is still plausible if the number of retrograde irregular moons around Jupiter was much more numerous in the past.
On the other hand, the Himalia group is unlikely to have formed from an impact with a passing planetesimal, it probably formed from a moon-moon collision instead. The parent body split into much more massive fragments proportionally; the largest fragment comprises only 78% of the mass of the parent body. Combined with the large assumed diameter of 150 km, this requires a more energetic impact, with an impactor of diameter ~13 km, so planetesimals large enough were probably too uncommon. Unlike the retrograde moons of Jupiter, prograde moons are expected to have been involved in numerous collisions. There is a high probability of collisions among the members of the prograde moons of Jupiter during the lifespan of the Solar System (e.g. on average 1.5 collisions between Himalia and Elara). In addition, the same simulations have shown fairly high probabilities of collisions between prograde and retrograde satellites (e.g. Pasiphae and Himalia have a 27% probability of collision within 4.5 gigayears). The fact that prograde moons are involved in collisions at a relatively common rate even in the present day suggests that the Himalia group could potentially have formed more recently, and likely did so from a moon-moon collision.
Chinese historian Xi Zezong claimed that the earliest record of a Jovian moon (Ganymede or Callisto) was a note by Chinese astronomer Gan De of an observation around 364 BC regarding a "reddish star". However, the first certain observations of Jupiter's satellites were those of Galileo Galilei in 1609. By January 1610, he had sighted the four massive Galilean moons with his 20× magnification telescope, and he published his results in March 1610.
Simon Marius had independently discovered the moons one day after Galileo, although he did not publish his book on the subject until 1614. Even so, the names Marius assigned are used today: Ganymede, Callisto, Io, and Europa. No additional satellites were discovered until E. E. Barnard observed Amalthea in 1892.
Photographic and spacecraft observations
With the aid of telescopic photography with photographic plates, further discoveries followed quickly over the course of the 20th century. Himalia was discovered in 1904, Elara in 1905, Pasiphae in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974.