Moons of Jupiter

• 1. Introduction

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  Jupiter's moons number 115 at last count, and the tally is still climbing. That figure alone should give you pause. Earth has one moon. Mars has two. Jupiter has more than a hundred confirmed natural satellites, and astronomers believe hundreds more are waiting to be found. What are all these worlds, how did they get there, and why does Jupiter collect moons the way it does? Those are the questions that have driven centuries of sky-watching, from a cold Italian night in 1610 to survey teams hauling digital cameras up to mountaintops in Mauna Kea and the Chilean Andes. The answers touch on how the Solar System itself was assembled, and they are not finished yet.

• 2. The Galilean Four

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  In January 1610, Galileo Galilei aimed a telescope with 20-times magnification at Jupiter and picked out four points of light that moved night after night. He published the discovery in March of that year. Simon Marius made the same observation one day after Galileo but did not publish until 1614. The names we use today, Io, Europa, Ganymede, and Callisto, are Marius's, not Galileo's.

  Ganymede is the largest of the four, and it is the ninth largest object in the entire Solar System, outranking even the planet Mercury in diameter, though not in mass. Callisto nearly matches Mercury's diameter as well. Together the four moons contain approximately 99.997 percent of all the mass orbiting Jupiter. The remaining 111 known moons, plus Jupiter's rings, account for just 0.003 percent of the orbiting total.

  These moons are not just large; they are geologically varied. Io has active volcanoes, a fact confirmed when the Voyager probes visited in 1979. Europa has water ice on its surface, also confirmed in that same 1979 flyby. Later spacecraft found evidence suggesting liquid water beneath the surfaces of Europa, Ganymede, and Callisto, and Galileo's spacecraft detected a magnetic field around Ganymede during its orbital survey from 1995 to 2003.

  The innermost three of the Galilean moons are locked into a gravitational rhythm with one another. Io, Europa, and Ganymede orbit in a 1:2:4 resonance, meaning for every one orbit Ganymede completes, Europa completes two and Io completes four. Callisto sits outside this chain for now, but tidal forces are slowly drawing it in. Models suggest it will join the resonance in roughly 1.5 billion years, creating a 1:2:4:8 chain.

• 3. The Inner Ring-Keepers

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  Four smaller moons sit closer to Jupiter than the Galilean giants, orbiting in the innermost zone of the system. These are Metis, Adrastea, Amalthea, and Thebe, collectively called the inner satellites or the Amalthea group. Metis and Adrastea orbit so close to Jupiter that they complete a full circuit in less than a single Jovian day.

  Amalthea, discovered by E. E. Barnard in 1892, was the first Jovian moon found after the Galilean four, a gap of nearly three centuries. At 250 by 146 by 128 kilometers, it is an irregular chunk rather than a sphere. Evidence suggests it did not form where it now orbits; it may have migrated inward from farther out, or it may be a captured body from elsewhere in the Solar System.

  The practical role of these inner moons is maintenance. Metis and Adrastea help sustain Jupiter's main ring. Amalthea and Thebe each generate their own fainter outer rings. The moons shed dust through collisions and other processes, and that material feeds the ring system. Without them, Jupiter's rings would fade.

• 4. Captured Outcasts

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  Beyond the Galilean moons lies a much stranger population: the irregular satellites. These are small bodies on distant, tilted, elongated orbits, many of them traveling in the direction opposite to Jupiter's rotation. The current count of known irregular moons stands at 107, and 58 of them have not yet been officially named.

  Most astronomers believe these bodies were once asteroids traveling their own paths around the Sun until Jupiter's gravity snagged them. The planet was able to capture them because a proto-lunar disk still surrounded it at the time, and that disk absorbed enough of each incoming body's momentum to trap it into orbit. Many were then shattered by subsequent collisions, breaking into families of fragments that share similar orbits, colors, and compositions.

  The Carme group is one of the tightest families, confined within orbital inclinations between 164.3 and 164.9 degrees and thought to originate from a single D-type asteroid, possibly a Jupiter trojan. The Ananke group is more spread out, with members appearing mostly gray and believed to come from a separate captured asteroid. The Pasiphae group is the most dispersed of the three main retrograde families, with colors ranging from red to gray, which may reflect multiple collision events over time.

  One moon among the prograde irregulars stands out for an unusual reason. Valetudo, announced in July 2018, follows a prograde orbit that crosses the paths of retrograde moons. A future collision with one of those retrograde satellites is considered a real possibility.

• 5. Origins Deep in Time

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  The regular moons, including the Galilean four and the inner group, did not arrive the way the irregulars did. They formed in place, condensing out of a disk of gas and solid debris that surrounded Jupiter in the early Solar System, much as planets formed from the disk around the young Sun.

  Simulations of this process suggest something striking. The disk was probably massive enough at any given moment to form Galilean-scale moons repeatedly. Over time, a substantial fraction of the mass that Jupiter captured from the solar nebula passed through this disk, but only about 2 percent of that material was needed to build the moons we see today. The implication is that earlier generations of large moons may have formed, spiraled inward into Jupiter under the drag of the disk, and been swallowed. The current Galilean moons may represent a fifth generation of such satellites.

  By the time this current generation formed, the disk had thinned enough that it could no longer pull them all the way in. They settled into stable orbits, partly protected by the resonances they fell into with each other. Callisto formed the slowest of the four, with models suggesting its accretion in the low-density Jovian subnebula lasted up to 10 million years.

• 6. Four Centuries of Discovery

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  Chinese historian Xi Zezong noted that astronomer Gan De may have recorded a Jovian moon, likely Ganymede or Callisto, around 364 BC, describing a "reddish star" near Jupiter. That claim is contested. Certain observation begins with Galileo in 1609. No additional moons were confirmed until Barnard spotted Amalthea in 1892.

  Photographic plates accelerated the pace. Himalia was found in 1904, Elara and Pasiphae followed in the next few years, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974. By the time the Voyager probes reached Jupiter in 1979, thirteen moons were known. Voyager itself added three more inner moons that year: Metis, Adrastea, and Thebe.

  The next leap came with digital cameras. The Spacewatch survey found Callirrhoe in October 1999. Then in November 2000, Scott Sheppard, at the time a graduate student working with David Jewitt, used the 88-inch UH88 telescope at Mauna Kea Observatory with automated computer algorithms to find eleven new irregular moons in a single survey pass, including the previously lost moon Themisto, which had been detected in 1975 but was then misplaced for a quarter century due to insufficient tracking data.

  From 2001 onward, Sheppard and Jewitt continued their search using the 3.6-meter Canada-France-Hawaii Telescope, adding eleven moons in December 2001, one in October 2002, and nineteen in February 2003. A separate team led by Brett J. Gladman also used that telescope in 2003, finding four and co-discovering two others alongside Sheppard. By the end of these surveys in 2004, Jupiter's known moon count had grown from 17 to 63.

  Many of those small, faint discoveries were lost again because telescopes could not track them long enough to pin down their orbits. Beginning in 2009, a recovery team including Mike Alexandersen, Marina Brozovic, Brett Gladman, Robert Jacobson, and Christian Veillet set out to recapture them using the CFHT and the Palomar Observatory's 5.1-meter Hale Telescope. During that effort, in September 2010, two previously unknown moons turned up as a bonus. One of them, now designated Jupiter LII, has an apparent magnitude of 24, making it one of the faintest confirmed Jovian moons known.

  Sheppard, by then a faculty member at the Carnegie Institution for Science, continued the hunt using the institution's 6.5-meter Magellan Telescopes at Las Campanas Observatory. In 2016, while originally searching for distant trans-Neptunian objects, he noticed Jupiter in his field of view and detoured to scan for moons. Working with Chadwick Trujillo and David Tholen, he used the 4-meter Victor M. Blanco Telescope at Cerro Tololo and the 8.2-meter Subaru Telescope at Mauna Kea. His team reported two new moons in June 2017 and ten more in July 2018, pushing the known count to 79.

  From November 2021 to January 2023, Sheppard confirmed thirteen more, bringing the total to 92. Three additional moons were announced on the 22nd of February 2023. The Minor Planet Center added two more on the 30th of April 2025, then four on the 16th of March 2026 and fourteen more on the 9th of April 2026, reaching the current total of 115.

• 7. What Names Reveal

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  The four Galilean moons carry Marius's names: Io, Europa, Ganymede, and Callisto. But those names were largely ignored for three centuries. Astronomical literature preferred the system of Roman numerals, calling the moons Jupiter I, Jupiter II, and so on. The Galilean names only became standard in the mid-20th century.

  Amalthea broke the Roman-numeral pattern by a different route. When E. E. Barnard discovered it in 1892, the French astronomer Camille Flammarion proposed the name Amalthea as an unofficial but popular suggestion, and it stuck before any formal process existed.

  The International Astronomical Union formalized naming conventions in 1975, when its Task Group for Outer Solar System Nomenclature granted official names to satellites V through XIII and set rules for future discoveries. The tradition is to name Jupiter's moons after lovers, sexual partners, or other associates of the Roman god Jupiter or his Greek equivalent Zeus. Since 2004, descendants of Jupiter and Zeus have been added to the eligible pool. A naming code is built into the endings: names ending in "a" or "o" go to prograde irregular moons, while names ending in "e" mark retrograde ones. All moons from number XXXIV onward, starting with Euporie, carry the names of descendants, with one deliberate exception: number LIII, Dia, named after a lover of Jupiter.

  With moons now numbering in the dozens of kilometers or less, the IAU has drawn a practical limit: bodies with absolute magnitudes greater than 18, or diameters smaller than 1 kilometer, will not be officially named. Some of the most recently confirmed moons remain designated only by their provisional survey codes.

• 8. The Road Ahead

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  Nine spacecraft have visited Jupiter. Pioneer 10 arrived in 1973, followed by Pioneer 11 a year later. The Voyager probes came in 1979. The Galileo spacecraft entered Jovian orbit in 1995 and spent eight years making close passes of all four Galilean moons before the mission ended in 2003. The Cassini probe swung by in 2000 on its way to Saturn. New Horizons refined satellite orbital measurements during its 2007 flyby. Juno, which arrived in 2016, has been executing close flybys of the Galileans since a mission extension, visiting Ganymede in 2021, Europa and Io in 2022, and Io again in late 2023 and early 2024.

  The next leap in discovery will come from the ground. The Vera C. Rubin Observatory, with its 8.4-meter aperture and 3.5 square-degree field of view, is expected to probe Jupiter's irregular moons down to 1 kilometer in diameter at apparent magnitudes of 24.5, with the potential to increase the known population by up to tenfold. The Nancy Grace Roman Space Telescope will push further, reaching diameters of 0.3 kilometers at magnitude 27.7, with the potential to discover approximately 1,000 Jovian moons above that size threshold. Those numbers will finally reveal the true size distribution of Jupiter's irregular moon population and sharpen our picture of how impacts and captures shaped the outer Solar System from its earliest days.