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The planet Jupiter has 67 confirmed moons. This gives it the largest retinue of moons with "reasonably secure" orbits of any planet in the Solar System. The most massive of them, the four Galilean moons, were discovered in 1610 by Galileo Galilei and were the first objects found to orbit a body that was neither Earth nor the Sun. From the end of the 19th century, dozens of much smaller Jovian moons have been discovered and have received the names of lovers, conquests, or daughters of the Roman god Jupiter, or his Greek equivalent, Zeus. The Galilean moons are by far the largest and most massive objects in orbit around Jupiter, with the remaining 63 moons and the rings together comprising just 0.003 percent of the total orbiting mass.
Eight of Jupiter's moons 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 ellipsoidal in shape, due to having planetary mass, and so would be considered (dwarf) planets if they were in direct orbit about the Sun. The other four regular satellites 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 irregular satellites, whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities. These moons were probably captured by Jupiter from solar orbits. There are 17 recently discovered irregular satellites that have not yet been named.
The moons' physical and orbital characteristics 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). Orbital shapes range from nearly perfectly circular to highly eccentric and inclined, and many revolve in the direction opposite to Jupiter's spin (retrograde motion). Orbital periods range from seven hours (taking less time than Jupiter does to spin around its axis), to some three thousand times more (almost three Earth years).
Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting 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 low mass at any given moment, over time a substantial fraction (several tens of a percent) of the mass of Jupiter captured from the Solar nebula was processed through it. However, the disk mass of only 2% that of Jupiter is required to explain the existing satellites. Thus there may have been several generations of Galilean-mass satellites in Jupiter's early history. Each generation of moons would have spiraled into Jupiter, due to 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 out to the point 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 which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io.
The outer, irregular moons are thought to have originated from passing asteroids while the protolunar disk was still massive enough to absorb much of their momentum and thus capture them into orbit. Many broke up by the stresses of capture, or afterward by collisions with other small bodies, producing the families we see today.
The first claimed observation of one of Jupiter's moons is that of the Chinese astronomer Gan De around 364 BC. However, the first certain observations of Jupiter's satellites were those of Galileo Galilei in 1609. By March 1610, he had sighted the four massive Galilean moons with his 30x magnification telescope: Ganymede, Callisto, Io, and Europa. No additional satellites were discovered until E. E. Barnard observed Amalthea in 1892. With the aid of telescopic photography, further discoveries followed quickly over the course of the twentieth century. Himalia was discovered in 1904, Elara in 1905, Pasiphaë in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974. By the time Voyager space probes reached Jupiter around 1979, 13 moons had been discovered; while Themisto was observed in 1975, but due to insufficient initial observation data, it was lost until 2000. The Voyager missions discovered an additional three inner moons in 1979: Metis, Adrastea, and Thebe.
For two decades no additional moons were discovered; but between October 1999 and February 2003, researchers using sensitive ground-based detectors found and later named another 34 moons, most of which were discovered by a team led by Scott S. Sheppard and David C. Jewitt. These are tiny moons, in long, eccentric, generally retrograde orbits, and average of 3 km (1.9 mi) in diameter, with the largest being just 9 km (5.6 mi) across. All of these moons are thought to be captured asteroidal or perhaps cometary bodies, possibly fragmented into several pieces, but very little is actually known about them. A number of 17 additional moons have been discovered but not yet named since 2003, bringing the total number of known moons of Jupiter to 67. As of 2013, this is the most of any planet in the Solar System, but additional undiscovered, tiny moons may exist.
The Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto) were named by Simon Marius soon after their discovery in 1610. However, these names fell out of favor until the 20th century: the astronomical literature instead simply referred to "Jupiter I", "Jupiter II", etc., or "the first satellite of Jupiter", "Jupiter's second satellite", and so on. The names Io, Europa, Ganymede, and Callisto became popular in the 20th century, while the rest of the moons, usually numbered in Roman numerals V (5) through XII (12), remained unnamed. By a popular though unofficial convention, Jupiter V, discovered in 1892, was given the name Amalthea, first used by the French astronomer Camille Flammarion.
The other moons, in the majority of astronomical literature, were simply labeled by their Roman numeral (i.e. Jupiter IX) until the 1970s. In 1975, the International Astronomical Union's (IAU) Task Group for Outer Solar System Nomenclature granted names to satellites V–XIII, and provided for a formal naming process for future satellites to be discovered. The practice was to name newly discovered moons of Jupiter after lovers and favorites of the god Jupiter (Zeus), and since 2004, after their descendants also. All of Jupiter's satellites from XXXIV (Euporie) are named after daughters of Jupiter or Zeus.
Some asteroids share the same names as moons of Jupiter: 9 Metis, 38 Leda, 52 Europa, 85 Io, 113 Amalthea, 239 Adrastea. Two more asteroids previously shared the names of Jovian moons until spelling differences were made permanent by the IAU: Ganymede and asteroid 1036 Ganymed; and Callisto and asteroid 204 Kallisto.
These have prograde and nearly circular orbits of low inclination and are split into two groups:
The irregular satellites are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit (semi-major axis, inclination, eccentricity) and composition; it is believed that these are at least partially collisional families that were created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter's gravitational field. These families bear the names of their largest members. The identification of satellite families is tentative, but the following are typically listed:
The moons of Jupiter are listed below by orbital period. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in bold. These are the four Galilean moons, which are comparable in size to Earth's Moon. The four inner moons are much smaller. The irregular captured moons are shaded light gray when prograde and dark gray when retrograde.
|1||XVI||Metis||ˈmiːtɨs||60×40×34||~3.6||127,690||+7h 4m 29s||0.06°||0.000 02||1979||Synnott|
|2||XV||Adrastea||ˌædrəˈstiːə||20×16×14||~0.2||128,690||+7h 9m 30s||0.03°||0.0015||1979||Jewitt|
|3||V||Amalthea||ˌæməlˈθiːə||167 ± 4.0 km|
|208||181,366||+11h 57m 23s||0.374°||0.0032||1892||Barnard||Inner|
|4||XIV||Thebe||ˈθiːbiː||116×98×84||~43||221,889||+16h 11m 17s||1.076°||0.0175||1979||Synnott|
|9||XVIII||Themisto||θɨˈmɪstoʊ||8||0.069||7,393,216||+129.87||45.762°||0.2115||1975/2000||Kowal & Roemer/|
Sheppard et al.
|14||—||S/2000 J 11||4||0.009 0||12,570,424||+287.93||27.584°||0.2058||2001||Sheppard et al.||Himalia?|
|15||XLVI||Carpo||ˈkɑrpoʊ||3||0.004 5||17,144,873||+458.62||56.001°||0.2735||2003||Sheppard et al.||Carpo|
|16||—||S/2003 J 12||1||0.000 15||17,739,539||−482.69||142.680°||0.4449||2003||Sheppard et al.||?|
|17||XXXIV||Euporie||juːˈpɒrɨ.iː||2||0.001 5||19,088,434||−538.78||144.694°||0.0960||2002||Sheppard et al.||Ananke|
|18||—||S/2003 J 3||2||0.001 5||19,621,780||−561.52||146.363°||0.2507||2003||Sheppard et al.||Ananke|
|19||—||S/2003 J 18||2||0.001 5||19,812,577||−569.73||147.401°||0.1569||2003||Gladman et al.||Ananke|
|20||—||S/2011 J 1||1||–||20,155,290||−582.22||162.8°||0.2963||2011||Sheppard et al.||?|
|21||—||S/2010 J 2||1||20,307,150||−588.36||150.4°||0.307||2010||Veillet||Ananke?|
|22||XLII||Thelxinoe||θɛlkˈsɪnɵʊiː||2||0.001 5||20,453,753||−597.61||151.292°||0.2684||2003||Sheppard et al.||Ananke|
|23||XXXIII||Euanthe||juːˈænθiː||3||0.004 5||20,464,854||−598.09||143.409°||0.2000||2002||Sheppard et al.||Ananke|
|24||XLV||Helike||ˈhɛlɨkiː||4||0.009 0||20,540,266||−601.40||154.586°||0.1374||2003||Sheppard et al.||Ananke|
|25||XXXV||Orthosie||ɔrˈθɒsɨ.iː||2||0.001 5||20,567,971||−602.62||142.366°||0.2433||2002||Sheppard et al.||Ananke|
|26||XXIV||Iocaste||ˌaɪ.ɵˈkæstiː||5||0.019||20,722,566||−609.43||147.248°||0.2874||2001||Sheppard et al.||Ananke|
|27||—||S/2003 J 16||2||0.001 5||20,743,779||−610.36||150.769°||0.3184||2003||Gladman et al.||Ananke|
|28||XXVII||Praxidike||prækˈsɪdɨkiː||7||0.043||20,823,948||−613.90||144.205°||0.1840||2001||Sheppard et al.||Ananke|
|29||XXII||Harpalyke||hɑrˈpælɨkiː||4||0.012||21,063,814||−624.54||147.223°||0.2440||2001||Sheppard et al.||Ananke|
|30||XL||Mneme||ˈniːmiː||2||0.001 5||21,129,786||−627.48||149.732°||0.3169||2003||Gladman et al.||Ananke|
|31||XXX||Hermippe||hərˈmɪpiː||4||0.009 0||21,182,086||−629.81||151.242°||0.2290||2002||Sheppard et al.||Ananke?|
|32||XXIX||Thyone||θaɪˈoʊniː||4||0.009 0||21,405,570||−639.80||147.276°||0.2525||2002||Sheppard et al.||Ananke|
|34||L||Herse||ˈhɜrsiː||2||0.001 5||22,134,306||−672.75||162.490°||0.2379||2003||Gladman et al.||Carme|
|35||XXXI||Aitne||ˈaɪtniː||3||0.004 5||22,285,161||−679.64||165.562°||0.3927||2002||Sheppard et al.||Carme|
|36||XXXVII||Kale||ˈkeɪliː||2||0.001 5||22,409,207||−685.32||165.378°||0.2011||2002||Sheppard et al.||Carme|
|37||XX||Taygete||teɪˈɪdʒɨtiː||5||0.016||22,438,648||−686.67||164.890°||0.3678||2001||Sheppard et al.||Carme|
|38||—||S/2003 J 19||2||0.001 5||22,709,061||−699.12||164.727°||0.1961||2003||Gladman et al.||Carme|
|39||XXI||Chaldene||kælˈdiːniː||4||0.007 5||22,713,444||−699.33||167.070°||0.2916||2001||Sheppard et al.||Carme|
|40||—||S/2003 J 15||2||0.001 5||22,720,999||−699.68||141.812°||0.0932||2003||Sheppard et al.||Ananke?|
|41||—||S/2003 J 10||2||0.001 5||22,730,813||−700.13||163.813°||0.3438||2003||Sheppard et al.||Carme?|
|42||—||S/2003 J 23||2||0.001 5||22,739,654||−700.54||148.849°||0.3930||2004||Sheppard et al.||Pasiphaë|
|43||XXV||Erinome||ɨˈrɪnɵmiː||3||0.004 5||22,986,266||−711.96||163.737°||0.2552||2001||Sheppard et al.||Carme|
|44||XLI||Aoede||eɪˈiːdiː||4||0.009 0||23,044,175||−714.66||160.482°||0.6011||2003||Sheppard et al.||Pasiphaë|
|45||XLIV||Kallichore||kəˈlɪkɵriː||2||0.001 5||23,111,823||−717.81||164.605°||0.2041||2003||Sheppard et al.||Carme?|
|46||XXIII||Kalyke||ˈkælɨkiː||5||0.019||23,180,773||−721.02||165.505°||0.2139||2001||Sheppard et al.||Carme|
|49||XXXII||Eurydome||jʊˈrɪdəmiː||3||0.004 5||23,230,858||−723.36||149.324°||0.3769||2002||Sheppard et al.||Pasiphaë?|
|50||—||S/2011 J 2||1||–||23,329,710||−725.06||151.8°||0.3867||2011||Sheppard et al.||Pasiphaë?|
|51||XXXVIII||Pasithee||pəˈsɪθɨ.iː||2||0.001 5||23,307,318||−726.93||165.759°||0.3288||2002||Sheppard et al.||Carme|
|52||—||S/2010 J 1||2||23,314,335||−722.83||163.2°||0.320||2010||Jacobson et al.||Pasiphaë?|
|53||XLIX||Kore||ˈkɔəriː||2||0.001 5||23,345,093||−776.02||137.371°||0.1951||2003||Sheppard et al.||Pasiphaë|
|54||XLVIII||Cyllene||sɨˈliːniː||2||0.001 5||23,396,269||−731.10||140.148°||0.4115||2003||Sheppard et al.||Pasiphaë|
|55||XLVII||Eukelade||juːˈkɛlədiː||4||0.009 0||23,483,694||−735.20||163.996°||0.2828||2003||Sheppard et al.||Carme|
|56||—||S/2003 J 4||2||0.001 5||23,570,790||−739.29||147.175°||0.3003||2003||Sheppard et al.||Pasiphaë|
|58||XXXIX||Hegemone||hɨˈdʒɛməniː||3||0.004 5||23,702,511||−745.50||152.506°||0.4077||2003||Sheppard et al.||Pasiphaë|
|59||XLIII||Arche||ˈɑrkiː||3||0.004 5||23,717,051||−746.19||164.587°||0.1492||2002||Sheppard et al.||Carme|
|60||XXVI||Isonoe||aɪˈsɒnɵʊiː||4||0.007 5||23,800,647||−750.13||165.127°||0.1775||2001||Sheppard et al.||Carme|
|61||—||S/2003 J 9||1||0.000 15||23,857,808||−752.84||164.980°||0.2761||2003||Sheppard et al.||Carme|
|62||—||S/2003 J 5||4||0.009 0||23,973,926||−758.34||165.549°||0.3070||2003||Sheppard et al.||Carme|
|64||XXXVI||Sponde||ˈspɒndiː||2||0.001 5||24,252,627||−771.60||154.372°||0.4431||2002||Sheppard et al.||Pasiphaë|
|65||XXVIII||Autonoe||ɔːˈtɒnɵʊiː||4||0.009 0||24,264,445||−772.17||151.058°||0.3690||2002||Sheppard et al.||Pasiphaë|
|66||XIX||Megaclite||ˌmɛɡəˈklaɪtiː||5||0.021||24,687,239||−792.44||150.398°||0.3077||2001||Sheppard et al.||Pasiphaë|
|67||—||S/2003 J 2||2||0.001 5||30,290,846||−1077.02||153.521°||0.1882||2003||Sheppard et al.||?|
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