Moons of Jupiter

Jupiter is surrounded by a massive collection of natural satellites known as the Jovian system. As of March 16, 2026, astronomers have identified 101 known moons orbiting this gas giant.

While there are many moons, the system is dominated by a few massive bodies. The four largest are called the Galilean moons: Io, Europa, Ganymede, and Callisto. These four moons are so large that they hold most of the system's mass. In fact, the other 97 moons and Jupiter's rings combined make up only 0.003% of the total orbiting mass.

The moons in the Jovian system are categorized by how they move and where they are located. Eight moons are classified as regular satellites. These moons have orbits that are nearly circular and stay close to Jupiter's equatorial plane. Among these are the four massive Galilean moons and four much smaller inner moons. The inner moons orbit very close to the planet. They act as sources of the dust that forms Jupiter's ring system.

The rest of the moons are known as irregular satellites. These moons orbit much farther away from Jupiter. Their paths are often highly eccentric, meaning they are oval-shaped rather than circular. They also have high inclinations, meaning their orbits are tilted relative to Jupiter's middle plane. Some of these moons even exhibit retrograde motion, which means they revolve in the opposite direction of Jupiter's rotation.

Scientists believe the regular satellites formed through a specific process involving a circumplanetary disk. This was a ring of gas and solid debris that gravitated around the young Jupiter. This disk is similar to the protoplanetary disks that form planets around stars. Researchers suggest that several generations of large moons may have existed in the past. In each generation, moons might have spiraled into Jupiter due to drag from the disk. New moons would then form from the debris captured from the solar nebula. The current Galilean moons are likely the fifth generation of such satellites.

The Galilean moons are currently locked in a complex gravitational relationship called orbital resonance. Io, Europa, and Ganymede exist in a 1:2:4 resonance. This means for every one orbit Ganymede makes, Europa makes two, and Io makes four. This resonance helps protect the moons from falling into the planet. Scientists believe Callisto will eventually be captured into this resonance in about 1.5 billion years. This would create a 1:2:4:8 chain of moons. This ongoing process is a result of tidal dissipation within the Jovian system.

The history of discovering these moons changed how we view our solar system. In 1610, Galileo Galilei used a telescope with 20x magnification to see the four Galilean moons. Simon Marius also discovered them independently just one day later. Although Galileo published first, we still use the names assigned by Marius today. These were the first objects ever found orbiting a body other than Earth or the Sun. For a long time, no new moons were found until E. E. Barnard discovered Amalthea in 1892.

Modern technology has allowed us to find many more small, faint moons. In the late 20th century, the Voyager space probes discovered three inner moons: Metis, Adrastea, and Thebe.

Since 2000, the use of digital charge-coupled device (CCD) cameras has revolutionized discovery. These cameras allow telescopes to see much fainter objects than older photographic plates. Astronomers like Scott Sheppard have used these tools to find dozens of irregular moons. Many of these moons are extremely small. For example, the moon Jupiter LII has a diameter of only 2 kilometers.

Some of these moons are so faint that they are easily lost if they are not tracked carefully.

The scale of these moons is truly remarkable. Ganymede is the largest of the Galilean moons and is actually larger than the planet Mercury. It is the ninth largest object in the entire solar system. Most other moons are much smaller, often with diameters of less than 10 kilometers. To understand this, a moon with a 250 km diameter has an area similar to the country of Belarus. A moon with a 5 km diameter is roughly the size of the island of Guernsey. These tiny rocks and massive worlds all work together in the complex Jovian system. New telescopes like the Vera C. Rubin Observatory will likely find even more moons in the coming years.