# Galilean moons <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Io in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Io&embed=1" data-title="Io in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Io`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: press l to label the four moons, then set the speed to 1 day/s and count two laps of Io for each of Europa's and four for each of Ganymede's, with Callisto lagging far behind; drag to an edge-on view and watch the moons pass in front of and behind Jupiter, the events that astronomers once timed to find longitude.* The **Galilean moons** are the four largest moons of [[Jupiter]]: [[Io_(moon)|Io]], [[Europa_(moon)|Europa]], [[Ganymede_(moon)|Ganymede]] and [[Callisto_(moon)|Callisto]], in order outward from the planet.[^jpl-satphys] Galileo Galilei saw them in January 1610 and within days understood that they circle Jupiter; they were the first bodies found to orbit a planet other than [[Earth]], and the discovery became a turning point in the case for a Sun-centred planetary system.[^vanhelden1989][^galileo-project] Simon Marius observed them independently, and the names in use today are the ones he published in 1614 on [[Johannes_Kepler|Johannes Kepler]]'s suggestion.[^pasachoff2015] All four are large enough to be round and are among the largest objects in the [[PORTAL_Solar_System|Solar System]]; Ganymede, 5,268 km across, is the largest moon of any planet and wider than Mercury.[^showman1999][^jpl-satphys] Their densities fall with distance from Jupiter, from Io's rocky 3.5 g/cm³ to Callisto's half-ice 1.8 g/cm³, and the inner three are locked in a 1:2:4 orbital [[Resonance|resonance]] that keeps their orbits eccentric and heats their interiors through [[Tide|tides]].[^nasa-litho][^showman1999] They are bright enough to see with binoculars, and nearly bright enough for the unaided eye.[^dutton1976] The explorer at the top of this page opens in Jupiter's own frame on Io, 421,700 km from the planet with a period of 1.77 days, and shows the other three moons on their true orbits; the moons' starting phases are ILLUSTRATIVE, but their distances and periods follow JPL values.[^jpl-satphys] ## History ### Pre-discovery Chinese records credit the astronomer Gan De with seeing, in 365 BC, a small reddish star beside [[Jupiter]], which the historian Xi Zezong interpreted as a naked-eye sighting of a moon, perhaps [[Ganymede_(moon)|Ganymede]].[^brecher1981][^xi1981] If so it would precede Galileo by some two thousand years, but the reported red colour is hard to reconcile with a moon too faint for colour to be seen by eye.[^huang1997] ### Discovery Galileo's improved telescope, magnifying about 20 times, showed him on 7 January 1610 three small "stars" in a line with Jupiter; they were in fact Ganymede, Callisto and the combined light of Io and Europa.[^vanhelden1974][^galileo-nuncius] The next night they had shifted. He saw all four together for the first time on 13 January, and by 15 January concluded that they were bodies travelling around Jupiter.[^galileo-nuncius][^nasa-ganymede] He announced the discovery in *Sidereus Nuncius* in March 1610.[^vanhelden1989] The result mattered beyond Jupiter. It showed that the telescope revealed objects invisible to the eye, and a planet carrying its own moons contradicted the Ptolemaic picture in which everything circled [[Earth]]; *Sidereus Nuncius* did not argue for Copernicus explicitly, but Galileo accepted the Copernican system.[^vanhelden1989][^galileo-project] Simon Marius later reported observations from late 1609, with a first written record dated 29 December 1609. Because he used the Julian calendar, that date corresponds to 8 January 1610 in Galileo's Gregorian calendar, one day after Galileo's first sighting.[^solarviews-disc][^pasachoff2015] ### Names Galileo, seeking the patronage of his former pupil Cosimo II de' Medici, Grand Duke of Tuscany, first called the moons *Cosmica Sidera* and then, on the advice of Cosimo's secretary, the Medicean Stars, honouring the four Medici brothers.[^vanhelden1989] Other proposals followed, including names by Giovanni Battista Hodierna, who published the first tables of the moons' positions in 1656.[^bruxelles1879] The names that lasted are Marius's: Io, Europa, Ganymede and Callisto, figures from the stories of Zeus, published in his *Mundus Jovialis* of 1614 after a suggestion from Kepler.[^vanhelden1994][^pasachoff2015] Galileo refused them and numbered the moons I to IV outward from Jupiter, a scheme still used alongside the names; the numbers were standard until the mid-20th century, when Marius's names came into general use.[^marazzini2005] ### Determination of longitude A navigator can find latitude from the sky, but longitude requires knowing the time at a reference meridian: each hour of difference corresponds to 15° of longitude, because Earth turns 360° in 24 hours (derived). Galileo proposed using the moons of [[Jupiter]] as a celestial clock, since their eclipses and transits can be predicted in advance and observed from anywhere Jupiter is visible.[^howse1980] He applied in 1616 for the Spanish prize for a solution, worth 6,000 ducats and a pension, and years later for a Dutch prize.[^danson2006] At sea the method failed, because the moons were too hard to observe through a telescope on a moving deck, despite devices such as Galileo's celatone headgear.[^rmg2014] On land it worked, and Giovanni Domenico Cassini and Jean Picard used it to survey France.[^howse1997] ## Comparative structure The four moons form a sequence. Their mean densities, 3.53, 3.01, 1.94 and 1.83 g/cm³ from Io to Callisto, show that rock and [[Iron|iron]] dominate close to [[Jupiter]] and ice becomes a large fraction farther out.[^nasa-litho] Gravity data from the Galileo spacecraft indicate that the inner three are differentiated, with dense cores beneath lighter layers, while Callisto is only partly separated into rock and ice.[^anderson1996][^anderson2001] The surfaces follow the same trend. [[Io_(moon)|Io]] has no [[Water|water]] ice but sulfur-rich plains and active volcanoes; [[Europa_(moon)|Europa]] has a young, lightly cratered ice crust broken by cracks and ridges; Ganymede mixes old, dark, cratered terrain with younger grooved terrain; [[Callisto_(moon)|Callisto]] is ancient and saturated with craters.[^showman1999] The pattern is explained by tidal heating. Jupiter's tidal pull falls off steeply with distance, as the inverse cube, and the resonance keeps the inner three moons on slightly eccentric orbits so that the tide flexes them continuously.[^openstax][^musotto2002] The heating is strongest at Io, which is volcanically active, significant at Europa, which likely has an ocean under a thin crust, and weakest at Callisto, which is outside the resonance.[^showman1999][^musotto2002] Radiation varies even more steeply. Jupiter's trapped particles deliver an estimated 3,600 rem, or 36 sieverts, per day at Io's surface and 540 rem (5.4 Sv) at Europa, against 8 rem at Ganymede and 0.01 rem at Callisto (SI values derived).[^ringwald2000] ## Members | Moon | Diameter (km) | Density (g/cm³) | Semi-major axis (km) | Period (days) | Eccentricity | |---|---|---|---|---|---| | Io | 3,643 | 3.53 | 421,800 | 1.769 | 0.0041 | | Europa | 3,122 | 3.01 | 671,100 | 3.551 | 0.0094 | | Ganymede | 5,268 | 1.94 | 1,070,400 | 7.155 | 0.0011 | | Callisto | 4,821 | 1.83 | 1,882,700 | 16.689 | 0.0074 | Values from NASA and JPL satellite parameters.[^nasa-litho][^jpl-satphys][^showman1999] ### Io Io, Jupiter I, is slightly larger than Earth's [[Moon]] and is the most volcanically active body in the Solar System, with more than 400 active volcanoes.[^lopes2004] It has more than 100 mountains, some higher than Mount Everest, and is made mainly of silicate rock around an iron or iron-sulfide core.[^schenk2001][^anderson1996] Its thin atmosphere is mostly [[Sulfur|sulfur]] dioxide.[^mcewen1998] ### Europa Europa, Jupiter II, is the smallest of the four, 3,122 km across. Beneath its smooth, bright ice lies a layer of water about 100 km thick, and magnetometer data from Galileo point to a salty liquid ocean under the ice, kept warm by tidal flexing.[^schenk2004][^zimmer2000] That ocean makes Europa a prime target in the search for life, though no evidence of life has been found.[^phillips2006] Its surface has few craters and is crossed by long cracks shaped by tidal stress; reported water plumes have been questioned by a 2026 analysis of Hubble ultraviolet data.[^greenberg1998][^roth2026] ### Ganymede Ganymede, Jupiter III, is the largest moon in the Solar System, bigger than Mercury but with less than half its mass.[^showman1999] It is the only moon known to generate its own magnetic field, probably through [[Convection|convection]] in a liquid iron core.[^kivelson2002] A salty ocean is thought to lie some 200 km below the surface, between layers of ice, and a thin oxygen atmosphere has been detected by its ultraviolet airglow.[^jpl-ganymede2000][^hall1998] ### Callisto Callisto, Jupiter IV, is 4,821 km across and lies outside the resonance, so it receives little tidal heating.[^musotto2002] Its surface is one of the most heavily cratered in the Solar System. It has a tenuous carbon dioxide atmosphere, and magnetometer data suggest a subsurface ocean within about 300 km of the surface.[^carlson1999][^zimmer2000] Its distance from Jupiter's radiation belts has made it a candidate site for a future crewed base.[^trautman2003] ## Origin and evolution The regular satellites of [[Jupiter]] are thought to have formed in a circumplanetary disk, a disk of gas and dust around the young planet much like the [[Protoplanetary_disk|protoplanetary disk]] around the [[Sun]].[^canup2009][^alibert2005] In the model of Canup and Ward, gas and solids flowed through this disk from the surrounding solar nebula for as long as Jupiter was still growing. Satellites formed in the disk, grew to about the Galilean moons' size, and were then dragged inward by the gas until they spiralled into Jupiter, to be replaced by new ones.[^canup2009] Several generations may have come and gone; the present four could be the last, perhaps the fifth, formed as the inflow waned and the disk thinned.[^chown2009][^canup2009] The Laplace resonance is the survivor of that phase. As the moons migrated, [[Ganymede_(moon)|Ganymede]], the most massive, moved inward fastest, and the inner three were captured into the chain that still links them: Io completes four orbits for each two of Europa and each one of Ganymede.[^canup2009][^musotto2002] With periods of 1.769, 3.551 and 7.155 days the ratios are 2.007 and 2.015 (derived), and the resonance forces the small eccentricities in the table above that keep the tides working.[^nasa-litho] Tidal evolution continues. Energy dissipated in Jupiter and the moons slowly expands their orbits, and Lari and colleagues calculate that [[Callisto_(moon)|Callisto]] will probably be captured into the chain within about 1.5 billion years, making a 1:2:4:8 sequence.[^lari2020] ## Visibility The Galilean moons shine at apparent magnitudes between 4.6 and 5.6 when Jupiter is at opposition, bright enough for the naked eye on a dark night if they stood alone, and about one magnitude fainter near conjunction.[^jpl-satphys] The difficulty is [[Jupiter]]'s glare. At the mean opposition distance of 628.8 million km, [[Io_(moon)|Io]] never strays more than about 2.3 arcminutes from the planet, Europa 3.7, Ganymede 5.9 and Callisto 10.3 (derived from the semi-major axes).[^nasa-fs][^nasa-litho] Those separations are near the limit of human visual acuity, so Ganymede and Callisto, near greatest elongation, are the likeliest to be glimpsed without optics.[^dutton1976] Any binoculars show all four, and over a few nights their changing positions trace out the orbits Galileo first plotted. The moons also offer regular events. Because Jupiter's axis is tilted only 3° and the moons orbit close to its equator, from [[Earth]] they are regularly seen passing in front of the planet (transits) and behind it or into its shadow (occultations and eclipses), the same events whose predicted timings were once proposed as a clock for finding longitude.[^nasa-fs][^howse1980] ## Gallery ### Orbit animations The explorer at the top of this page serves as this article's orbit animation: in [[Jupiter]]'s frame it shows the four moons circling at their true relative distances and periods, with Io's 1.77-day lap, Europa's 3.55 days and Ganymede's 7.15 days making the 1:2:4 resonance visible as a rhythm. Callisto's 16.7-day orbit, outside the chain, drifts in and out of step.[^jpl-satphys] ### Latest flyby NASA's Juno spacecraft, orbiting Jupiter since 2016, has made the most recent close passes of three of the moons: [[Ganymede_(moon)|Ganymede]] on 7 June 2021 at about 1,050 km, the closest approach since Galileo in 2000; Europa on 29 September 2022; and Io on 30 December 2023 and 3 February 2024, each time within about 1,500 km.[^nasa-juno] ESA's JUICE, launched in April 2023, is due to reach Jupiter in July 2031, and NASA's Europa Clipper, launched in October 2024, in 2030.[^nasa-juno][^nasa-clipper] ## See also - [[Jupiter]] - [[Io_(moon)]] · [[Europa_(moon)]] · [[Ganymede_(moon)]] · [[Callisto_(moon)]] - [[Moons_of_Mars]] · [[Titan_(moon)]] · [[Triton_(moon)]] - [[Discovery_and_exploration_of_the_Solar_System]] ## Notes Derived values: angular separation = semi-major axis / Earth–Jupiter distance at mean opposition (628.81 × 10⁶ km, NASA Jupiter fact sheet), converted with 1 rad = 3,437.75′; period ratios 3.551/1.769 = 2.007 and 7.155/3.551 = 2.015; 1 rem = 0.01 Sv. The explorer's local frame uses JPL mean distances and periods; initial phases are illustrative. ## References [^jpl-satphys]: JPL Solar System Dynamics. "Planetary satellite physical parameters" and "planetary satellite mean elements". https://ssd.jpl.nasa.gov/sats/phys_par/ (fetched 2026-09-18). [^vanhelden1989]: Galilei, G.; Van Helden, A. (trans.) (1989). *Sidereus Nuncius, or The Sidereal Messenger*. University of Chicago Press, pp. 14–16. ISBN 978-0-226-27902-2. [^galileo-project]: The Galileo Project (1995). "Satellites of Jupiter". Rice University. https://galileo.rice.edu/sci/observations/jupiter_satellites.html [^pasachoff2015]: Pasachoff, J. M. (2015). "Simon Marius's Mundus Iovialis: 400th anniversary in Galileo's shadow". *Journal for the History of Astronomy* 46: 218–234. https://doi.org/10.1177/0021828615585493 [^showman1999]: Showman, A. P.; Malhotra, R. (1999). "The Galilean satellites". *Science* 286: 77–84. https://doi.org/10.1126/science.286.5437.77 [^nasa-litho]: NASA. "Galilean moons of Jupiter" (lithograph). https://www.nasa.gov/wp-content/uploads/2009/12/moons_of_jupiter_lithograph.pdf [^dutton1976]: Dutton, D. (1976). "Naked-eye observations of Jupiter's moons". *Sky & Telescope*, December 1976, pp. 482–484. http://www.denisdutton.com/jupiter_moons.htm [^brecher1981]: Brecher, K. (1981). "Ancient astronomy in modern China". *Bulletin of the American Astronomical Society* 13: 793. Bibcode 1981BAAS...13..793B. [^xi1981]: Xi, Z.-Z. (1981). "The sighting of Jupiter's satellite by Gan De 2000 years before Galileo". *Chinese Astronomy and Astrophysics* 5: 242–243. https://doi.org/10.1016/0275-1062(81)90039-4 [^huang1997]: Huang, Y.-L. (1997). "Gan De". In Selin, H. (ed.), *Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures*. Springer, p. 342. ISBN 978-0-7923-4066-9. [^vanhelden1974]: Van Helden, A. (1974). "The telescope in the seventeenth century". *Isis* 65: 38–58. https://doi.org/10.1086/351216 [^galileo-nuncius]: Galilei, G. (1610). *Sidereus Nuncius*, trans. E. Carlos. University of Oklahoma History of Science Collections. http://hsci.cas.ou.edu/images/barker/5990/Sidereus-Nuncius-whole.pdf [^nasa-ganymede]: NASA Solar System Exploration. "Ganymede: in depth". https://solarsystem.nasa.gov/moons/jupiter-moons/ganymede/in-depth/ [^solarviews-disc]: Hamilton, C. J. "The discovery of the Galilean satellites". *Views of the Solar System*. https://solarviews.com/eng/galdisc.htm [^bruxelles1879]: *Annuaire de l'Observatoire royal de Bruxelles* (1879). Académie royale des sciences, des lettres et des beaux-arts de Belgique, p. 263. https://books.google.com/books?id=eF4LAQAAIAAJ&pg=PA263 [^vanhelden1994]: Van Helden, A. (1994). "Naming the satellites of Jupiter and Saturn". *The Newsletter of the Historical Astronomy Division of the American Astronomical Society* 32. https://had.aas.org/sites/had.aas.org/files/HADN32.pdf [^marazzini2005]: Marazzini, C. (2005). "The names of the satellites of Jupiter: from Galileo to Simon Marius". *Lettere Italiane* 57: 391–407. [^howse1980]: Howse, D. (1980). *Greenwich Time and the Discovery of the Longitude*. Oxford University Press, p. 12. ISBN 978-0-19-215948-9. [^danson2006]: Danson, E. (2006). *Weighing the World: The Quest to Measure the Earth*. Oxford University Press. ISBN 978-0-19-518169-2. [^rmg2014]: Royal Museums Greenwich (16 October 2014). "Solving longitude: Jupiter's moons". https://www.rmg.co.uk/stories/blog/solving-longitude-jupiters-moons [^howse1997]: Howse, D. (1997). *Greenwich Time and the Longitude*. Philip Wilson, pp. 26, 31. [^anderson1996]: Anderson, J. D.; Sjogren, W. L.; Schubert, G. (1996). "Galileo gravity results and the internal structure of Io". *Science* 272: 709–712. https://doi.org/10.1126/science.272.5262.709 [^anderson2001]: Anderson, J. D.; Jacobson, R. A.; McElrath, T. P.; et al. (2001). "Shape, mean radius, gravity field and interior structure of Callisto". *Icarus* 153: 157–161. https://doi.org/10.1006/icar.2001.6664 [^openstax]: Ling, S. J.; Sanny, J.; Moebs, W. (2016). *University Physics Volume 1*, ch. 13 "Gravitation" (tidal forces). OpenStax. https://open.umn.edu/opentextbooks/textbooks/university-physics-volume-1 [^musotto2002]: Musotto, S.; Varadi, F.; Moore, W. B.; Schubert, G. (2002). "Numerical simulations of the orbits of the Galilean satellites". *Icarus* 159: 500–504. https://doi.org/10.1006/icar.2002.6939 [^ringwald2000]: Ringwald, F. A. (29 February 2000). "SPS 1020 (Introduction to Space Sciences): Jupiter". California State University, Fresno. https://zimmer.csufresno.edu/~fringwal/w08a.jup.txt [^lopes2004]: Lopes, R. M. C.; Kamp, L. W.; Smythe, W. D.; et al. (2004). "Lava lakes on Io: observations of Io's volcanic activity from Galileo NIMS during the 2001 fly-bys". *Icarus* 169: 140–174. https://doi.org/10.1016/j.icarus.2003.11.013 [^schenk2001]: Schenk, P.; Hargitai, H.; Wilson, R.; McEwen, A.; Thomas, P. (2001). "The mountains of Io: global and geological perspectives from Voyager and Galileo". *Journal of Geophysical Research: Planets* 106: 33201–33222. https://doi.org/10.1029/2000JE001408 [^mcewen1998]: McEwen, A. S.; Keszthelyi, L.; Spencer, J. R.; et al. (1998). "High-temperature silicate volcanism on Jupiter's moon Io". *Science* 281: 87–90. https://doi.org/10.1126/science.281.5373.87 [^schenk2004]: Schenk, P. M.; Chapman, C. R.; Zahnle, K.; Moore, J. M. (2004). "Ages and interiors: the cratering record of the Galilean satellites". In Bagenal, F.; Dowling, T. E.; McKinnon, W. B. (eds.), *Jupiter: The Planet, Satellites and Magnetosphere*. Cambridge University Press. https://lasp.colorado.edu/mop/files/2015/08/jupiter_ch18-1.pdf [^zimmer2000]: Zimmer, C.; Khurana, K. K.; Kivelson, M. G. (2000). "Subsurface oceans on Europa and Callisto: constraints from Galileo magnetometer observations". *Icarus* 147: 329–347. https://doi.org/10.1006/icar.2000.6456 [^phillips2006]: Phillips, C. (28 September 2006). "Time for Europa". *Space.com*. https://www.space.com/2954-time-europa.html [^greenberg1998]: Greenberg, R.; Geissler, P.; Hoppa, G.; et al. (1998). "Tectonic processes on Europa: tidal stresses, mechanical response, and visible features". *Icarus* 135: 64–78. https://doi.org/10.1006/icar.1998.5986 [^roth2026]: Roth, L.; Retherford, K. D.; Saur, J.; et al. (2026). "Europa's Lyman-α emissions from HST/STIS observations". *Astronomy & Astrophysics* 709: A59. https://doi.org/10.1051/0004-6361/202659406 [^kivelson2002]: Kivelson, M. G.; Khurana, K. K.; Volwerk, M. (2002). "The permanent and inductive magnetic moments of Ganymede". *Icarus* 157: 507–522. https://doi.org/10.1006/icar.2002.6834 [^jpl-ganymede2000]: NASA/JPL (16 December 2000). "Solar System's largest moon likely has a hidden ocean". https://www.jpl.nasa.gov/releases/2000/aguganymederoundup.html [^hall1998]: Hall, D. T.; Feldman, P. D.; McGrath, M. A.; Strobel, D. F. (1998). "The far-ultraviolet oxygen airglow of Europa and Ganymede". *The Astrophysical Journal* 499: 475–481. https://doi.org/10.1086/305604 [^carlson1999]: Carlson, R. W. (1999). "A tenuous carbon dioxide atmosphere on Jupiter's moon Callisto". *Science* 283: 820–821. https://doi.org/10.1126/science.283.5403.820 [^trautman2003]: Trautman, P.; Bethke, K. (2003). "Revolutionary concepts for human outer planet exploration (HOPE)". NASA. https://www.nasa-academy.org/soffen/travelgrant/bethke.pdf [^canup2009]: Canup, R. M.; Ward, W. R. (2009). "Origin of Europa and the Galilean satellites". In Pappalardo, R. T.; McKinnon, W. B.; Khurana, K. (eds.), *Europa*. University of Arizona Press, pp. 59–83. ISBN 978-0-8165-2844-8. [^alibert2005]: Alibert, Y.; Mousis, O.; Benz, W. (2005). "Modeling the Jovian subnebula I. Thermodynamic conditions and migration of proto-satellites". *Astronomy & Astrophysics* 439: 1205–1213. https://doi.org/10.1051/0004-6361:20052841 [^chown2009]: Chown, M. (7 March 2009). "Cannibalistic Jupiter ate its early moons". *New Scientist*. https://www.newscientist.com/article/mg20126984.300-cannibalistic-jupiter-ate-its-early-moons.html [^lari2020]: Lari, G.; Saillenfest, M.; Fenucci, M. (2020). "Long-term evolution of the Galilean satellites: the capture of Callisto into resonance". *Astronomy & Astrophysics* 639: A40. https://doi.org/10.1051/0004-6361/202037445 [^nasa-fs]: NASA NSSDCA. "Jupiter fact sheet" (mean opposition distance, obliquity). https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html (fetched 2026-09-18). [^nasa-juno]: NASA Science. "Juno" (mission page: moon flybys and future missions). https://science.nasa.gov/mission/juno/ [^nasa-clipper]: NASA Science. "Europa Clipper" (mission page). https://science.nasa.gov/mission/europa-clipper/ ## External links - NASA Science: Jupiter's moons. https://science.nasa.gov/jupiter/jupiter-moons/ - JPL Solar System Dynamics: planetary satellites. https://ssd.jpl.nasa.gov/sats/ - ESA: JUICE mission. https://www.esa.int/Science_Exploration/Space_Science/Juice - NASA Science: Europa Clipper. https://science.nasa.gov/mission/europa-clipper/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Galilean_moons) : [Wikitube](https://en.wikitube.io/wiki/Galilean_moons) · pinned revision [1375331363](https://en.wikipedia.org/w/index.php?oldid=1375331363) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-056 · explorer state `?obj=Io`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->