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A subsatellite, also known as a submoon or informally a moonmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet.

It is inferred from the empirical study of natural satellites in the Solar System that subsatellites may be rare, albeit possible, elements of planetary systems. In the Solar System, the giant planets have large collections of natural satellites. The majority of detected exoplanets are giant planets; at least one, Kepler-1625b, may have a very large exomoon, named Kepler-1625b I, which could theoretically host a subsatellite. Nonetheless, aside from human-launched satellites in temporary lunar orbit, no subsatellite is known in the Solar System or beyond. In most cases, the tidal effects of the planet would make such a system unstable.

Terminology

Terms used in scientific literature for subsatellites include "submoons" and "moon-moons". Colloquial terms that have been suggested include moonitos, moonettes, and moooons.

Possible natural instances

Rhea

There is a possible detection of a ring system around Saturn's natural satellite Rhea that led to calculations that indicated that satellites orbiting Rhea would have stable orbits. The rings suspected were thought to be narrow, a phenomenon normally associated with shepherd moons; however, targeted images taken by the Cassini spacecraft failed to detect any subsatellites or rings associated with Rhea, at least no particles larger than a few millimeters, making the chance of a ring system around Rhea slim.

Iapetus

It has also been proposed that Saturn's satellite Iapetus possessed a subsatellite in the past; this is one of several hypotheses that have been put forward to account for its unusual equatorial ridge. An ancient giant impact on Iapetus could have produced a subsatellite; as Saturn despun Iapetus, the subsatellite's orbit would then decay until it crossed Iapetus' Roche limit, forming a transient ring which then impacted Iapetus to form a ridge. Such a scenario could have happened on the other giant-planet satellites as well, but only for Iapetus and perhaps Oberon would the resulting ridge have formed after the Late Heavy Bombardment and thus survived to the present day.

Irregular moons of Saturn

Light-curve analysis suggests that Saturn's irregular satellite Kiviuq is extremely prolate, and is likely a contact binary or even a binary moon. Other candidates among the Saturnian irregulars include Bestla, Erriapus, and Bebhionn.

Artificial subsatellites

Historical

Many spacecraft have orbited the Moon, including crewed craft of the Apollo program. In 1988, the Soviet Union unsuccessfully attempted to put two robotic probes on quasi-orbits around the Martian moon Phobos. As of 2024, no spacecraft has successfully orbited any natural satellite other than the Moon.

Current

Launched June 18, 2009, the Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic missions to the Moon. Its detailed mapping program is identifying safe landing sites, locating potential resources on the Moon, characterizing the radiation environment, and demonstrating new technologies.

CAPSTONE is a project launched in June 2022. Composed of a 12-unit collection of CubeSats which spent a few months in transit to the Moon to arrive in November 2022. It will spend about 9 months in the Moon's near rectilinear halo orbit. CAPSTONE is intended to test and verify the viability of the planned NRHO of planned future Lunar Gateway and its communication efficiency.

Future planned artificial moon satellites

The interplanetary spacecraft JUICE launched in 2023 will enter an orbit around Ganymede in 2032, becoming the first spacecraft to orbit a moon other than Earth's.

Additionally, the multi-agency supported Lunar Gateway human-rated space station is due to begin construction in 2024 in a near-rectilinear halo orbit (NRHO), primarily in support of the later stage NASA Artemis program missions to the Moon. Lunar Gateway will also potentially support future missions to Mars and outlying asteroids.

In fiction

• In Planet 51, the titular planet is orbited by 2 moons with Saturn-like ring systems.
• Dumb Martian, a short story by John Wyndham, mostly takes place on Jupiter IV/II, a sub-moon of Callisto (see The Seeds of Time).
• In Star Wars, the moon Ajan Kloss has two moons of its own.

See also

• Binary asteroid
• Circumplanetary disk – Accumulation of matter around a planet
• Minor-planet moon
• Moons of Jupiter
• Moons of Neptune
• Moons of Saturn
• Moons of Uranus
• Satellite system (astronomy)

References

1. ^ Forgan, Duncan (4 October 2018). "The habitable zone for Earthlike exomoons orbiting Kepler-1625b". arXiv:1810.02712v1 .
2. ^ Chou, Felcia; Villard, Ray; Hawkes, Alison (3 October 2018). Brown, Katherine (ed.). "Astronomers Find First Evidence of Possible Moon Outside Our Solar System". Solar System and Beyond (Press release). NASA. Retrieved 11 October 2018.
3. "Where is Earth's submoon?". phys.org. Retrieved 2020-10-13.
4. Drake, Nadia (3 October 2018). "Weird giant may be the first known alien moon - Evidence is mounting that a world the size of Neptune could be orbiting a giant planet far, far away". National Geographic Society. Archived from the original on October 3, 2018. Retrieved 11 October 2018.
5. "Hubble finds compelling evidence for a moon outside the Solar System". Hubble Space Telescope. 3 October 2018. Retrieved 11 October 2018.
6. Daley, Jason (11 October 2018). "If a Moon Has a Moon, Is Its Moon Called a Moonmoon? - A new study suggests it's possible some moons could have moons and the internet wants to give them a name—but scientists have yet to actually find one". Smithsonian. Retrieved 10 September 2020.
7. Jones GH, Roussos E, Krupp N, et al. (7 March 2008). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science. 319 (5868): 1380–1384. Bibcode:2008Sci...319.1380J. doi:10.1126/science.1151524. PMID 18323452. S2CID 206509814. Retrieved 12 October 2018.
8. Hecht, Jeff (6 March 2008). "Saturn satellite reveals first moon rings". New Scientist. Retrieved 12 October 2018.
9. Tiscareno, Matthew S.; Burns, Joseph A.; Cuzzi, Jeffrey N.; Hedman, Matthew M. (July 2010). "Cassini imaging search rules out rings around Rhea". Geophysical Research Letters. 37 (14): L14205. arXiv:1008.1764. Bibcode:2010GeoRL..3714205T. doi:10.1029/2010GL043663. S2CID 59458559. Archived from the original on 2010-08-10.
10. Fitzpatrick, Tony (13 December 2010). "How Iapetus, Saturn's outermost moon, got its ridge". Washington University in St. Louis. Retrieved 12 October 2018.
11. Dombard, Andrew J.; Cheng, Andrew F.; McKinnon, William B.; Kay, Jonathan P. (2012). "Delayed formation of the equatorial ridge on Iapetus from a subsatellite created in a giant impact". Journal of Geophysical Research: Planets. 117 (E3). Bibcode:2012JGRE..117.3002D. doi:10.1029/2011JE004010.
12. Denk, T.; Mottola, S. (2019). Cassini Observations of Saturn's Irregular Moons (PDF). 50th Lunar and Planetary Science Conference. Lunar and Planetary Institute.
13. Denk, T.; Mottola, S.; Bottke, W. F.; Hamilton, D. P. (2018). "The Irregular Satellites of Saturn". Enceladus and the Icy Moons of Saturn (PDF). Vol. 322. University of Arizona Press. pp. 409–434. Bibcode:2018eims.book..409D. doi:10.2458/azu_uapress_9780816537075-ch020. ISBN 9780816537488.
14. Edwin V. Bell II (11 April 2016). "Phobos Project Information". NASA Space Science Data Coordinated Archive. NASA. Retrieved 2018-10-15.
15. Figliozzi, Gianine (May 20, 2022). "CAPSTONE Spacecraft Launch Targeted No Earlier Than June 6". NASA. Retrieved May 25, 2022.
16. Hidalgo, Pablo (20 December 2019). Star Wars: The Rise of Skywalker: The Visual Dictionary. DK Publishing. ISBN 9781465479037.

External links

• Media related to Extrasolar moons at Wikimedia Commons
• Shadow Moons: The Unknown Sub-Worlds that Might Harbor Life
• Likely First Photo of Planet Beyond the Solar System
• Working Group on Extrasolar Planets – Definition of a "Planet" Position statement on the definition of a planet. (IAU)
• The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project

v t e Natural satellites of the Solar System
v t e Moons of terrestrial planets and asteroids Earth Mars Asteroid moons The Moon Phobos Deimos Binaries List
v t e Moons of Jupiter Listed in increasing approximate distance from Jupiter Inner moons Metis Adrastea Amalthea Thebe Galilean moons Io Europa Ganymede Callisto Themisto Themisto Himalia group (9) Leda Ersa S/2018 J 2 Himalia Pandia Lysithea Elara S/2011 J 3 Dia Carpo group (2) S/2018 J 4 Carpo Valetudo Valetudo Ananke group (26) Euporie Jupiter LV Eupheme S/2021 J 3 Jupiter LII Jupiter LIV Mneme Euanthe S/2003 J 16 Harpalyke Orthosie S/2022 J 3 Helike S/2021 J 2 Praxidike Jupiter LXIV S/2021 J 1 S/2003 J 12 Jupiter LXVIII Thelxinoe Thyone S/2003 J 2 Ananke Iocaste Hermippe Jupiter LXX Carme group (30) S/2022 J 1 S/2022 J 2 S/2016 J 3 Pasithee Jupiter LXIX S/2021 J 6 S/2003 J 24 Chaldene Jupiter LXIII Isonoe S/2021 J 4 Kallichore Erinome Kale Eirene Aitne Eukelade Arche Taygete Jupiter LXXII Carme Herse Jupiter LXI Jupiter LI S/2003 J 9 Jupiter LXVI Kalyke S/2018 J 3 S/2021 J 5 S/2003 J 10 Pasiphae group (18) Philophrosyne Eurydome Jupiter LVI S/2003 J 4 S/2016 J 4 Jupiter LXVII Hegemone Pasiphae Sponde Megaclite Cyllene Sinope Jupiter LIX Aoede Autonoe Callirrhoe S/2003 J 23 Kore See also Rings of Jupiter Jupiter's moons in fiction Category
v t e Moons of Saturn Listed in approximate increasing distance from Saturn Ring moonlets S/2009 S 1 Aegaeon Ring shepherds Pan Daphnis Prometheus Epimetheus Janus Other inner moons Atlas Pandora Alkyonides Methone Anthe Pallene Large moons (with trojans) Mimas Enceladus Tethys Telesto Calypso Dione Helene Polydeuces Rhea Titan Hyperion Iapetus Inuit group (13) Kiviuq subgroup S/2019 S 1 Kiviuq S/2005 S 4 S/2020 S 1 Ijiraq Paaliaq subgroup Paaliaq Siarnaq subgroup S/2004 S 31 Tarqeq S/2019 S 14 Siarnaq S/2020 S 3 S/2019 S 6 S/2020 S 5 Gallic group (7) Albiorix Bebhionn S/2007 S 8 Saturn LX Erriapus Tarvos S/2020 S 4 Norse group (100) Phoebe subgroup Phoebe S/2006 S 20 S/2006 S 9 Skathi S/2007 S 5 S/2007 S 7 S/2007 S 2 S/2004 S 37 S/2004 S 47 S/2004 S 40 S/2019 S 2 S/2019 S 3 S/2020 S 7 Skoll S/2020 S 2 S/2019 S 4 S/2004 S 41 S/2004 S 42 Hyrrokkin Greip S/2004 S 13 S/2007 S 6 Mundilfari S/2006 S 1 S/2004 S 43 S/2006 S 10 S/2019 S 5 Gridr Bergelmir Jarnsaxa Narvi Suttungr S/2004 S 44 S/2007 S 3 S/2004 S 45 Hati S/2004 S 17 S/2006 S 11 S/2004 S 12 Eggther S/2006 S 13 S/2007 S 9 S/2019 S 7 S/2019 S 8 Farbauti Thrymr Bestla S/2019 S 9 S/2004 S 46 Angrboda S/2019 S 11 Aegir S/2019 S 10 Beli S/2019 S 12 Gerd S/2019 S 13 S/2006 S 14 Gunnlod S/2019 S 15 S/2020 S 6 S/2004 S 7 S/2006 S 3 S/2005 S 5 Skrymir S/2006 S 16 S/2006 S 15 S/2004 S 28 S/2020 S 8 Alvaldi Kari S/2004 S 48 Geirrod Fenrir S/2004 S 50 S/2006 S 17 S/2004 S 49 S/2019 S 17 Surtur S/2006 S 18 Loge Ymir S/2019 S 19 S/2019 S 18 S/2004 S 21 S/2004 S 39 S/2019 S 16 S/2004 S 53 S/2004 S 36 Thiazzi S/2019 S 20 S/2006 S 19 Saturn LXIV Fornjot S/2004 S 51 S/2020 S 10 S/2020 S 9 Saturn LVIII S/2019 S 21 S/2004 S 52 Outlier prograde irregular moons S/2006 S 12 S/2004 S 24 Rings of Saturn Cassini–Huygens Chiron Chrysalis Themis S/2004 S 3 S/2004 S 4 S/2004 S 6 In fiction
v t e Moons of Uranus Listed in approximately increasing distance from Uranus Inner Rings of Uranus Cordelia Ophelia Bianca Cressida Desdemona Juliet Portia Rosalind Cupid Belinda Perdita Puck Mab Major (spheroid) Miranda Ariel Umbriel Titania Oberon Outer (irregular) Prograde Margaret Retrograde Francisco Caliban Stephano S/2023 U 1 Trinculo Sycorax Prospero Setebos Ferdinand Geological features Arielian Kachina Chasmata Yangoor Mirandian Verona Rupes Oberonian Hamlet Mommur Chasma Puckian Titanian Gertrude Messina Chasmata Ursula Rousillon Rupes Umbrielian Wunda Vuver Skynd
v t e Moons of Neptune Listed in approximately increasing distance from Neptune Regular (inner) Naiad Thalassa Despina Galatea Larissa Hippocamp Proteus Irregular Triton Triton atmosphere climate geology Prograde Nereid Sao S/2002 N 5 Laomedeia Retrograde Halimede Psamathe Neso S/2021 N 1 See also Neptune trojans Rings of Neptune Category Portal
v t e Moons of likely dwarf planets Pluto Haumea Eris Makemake Gonggong Charon Styx Nix Kerberos Hydra ring Namaka Hiʻiaka Dysnomia S/2015 (136472) 1 Xiangliu Quaoar Orcus Salacia Varda 2013 FY27 rings Weywot Vanth Actaea Ilmarë 2013 FY27 I
v t e Natural satellites of the Solar System Planetary satellites of Earth Mars Jupiter Saturn Uranus Neptune Dwarf planet satellites of Orcus Pluto Haumea Quaoar Makemake Gonggong Eris Minor-planet moons Near-Earth Florence Didymos Dimorphos Moshup Squannit 1994 CC 2001 SN263 Main belt Kalliope Linus Euphrosyne Daphne Peneius Eugenia Petit-Prince Sylvia Romulus Remus Minerva Aegis Gorgoneion Camilla Elektra Kleopatra Alexhelios Cleoselene Ida Dactyl Roxane Olympias Pulcova Balam Dinkinesh (Selam) Jupiter trojans Patroclus Menoetius Hektor Skamandrios Eurybates Queta TNOs Lempo Hiisi Paha 2002 UX25 Sila–Nunam Salacia Actaea Varda Ilmarë Gǃkúnǁʼhòmdímà Gǃòʼé ǃHú 2013 FY27 Ranked by size Ganymede largest: 5268 km / 0.413 Earths Titan Callisto Io Moon Europa Triton Titania Rhea Oberon Iapetus Charon Umbriel Ariel Dione Tethys Dysnomia Enceladus Miranda Vanth Proteus Mimas Ilmarë Nereid Hiʻiaka Actaea Hyperion Phoebe ... Discovery timeline Inner moons Irregular moons List Planetary-mass moons Naming Subsatellite Regular moons Trojan moons
Discovery timeline Inner moons Irregular moons List Naming Subsatellite Regular moons Trojan moons

v t e Exoplanets
Planet Definition IAU Planetary science
Main topics	Exoplanet Exoplanet orbital and physical parameters Methods of detecting exoplanets Planetary system Planet-hosting stars
Sizes and types	Terrestrial Carbon planet Coreless planet Desert planet Dwarf planet Hycean planet Ice planet Iron planet (Super-Mercury) Lava planet Ocean world Mega-Earth Sub-Earth Super-Earth Gaseous Eccentric Jupiter Mini-Neptune (Gas dwarf) Helium planet Hot Jupiter Hot Neptune Gas giant Ice giant Super-Jupiter Super-Neptune Super-puff Ultra-hot Jupiter Ultra-hot Neptune Other types Blanet Brown dwarf Chthonian planet Circumbinary planet Circumtriple planet Disrupted planet Double planet Ecumenopolis Eyeball planet Giant planet Mesoplanet Planemo Planet/Brown dwarf boundary Planetesimal Protoplanet Pulsar planet Sub-brown dwarf Sub-Neptune Toroidal planet Ultra-cool dwarf Ultra-short period planet (USP)
Formation and evolution	Accretion Accretion disk Asteroid belt Circumplanetary disk Circumstellar disc Circumstellar envelope Cosmic dust Debris disk Detached object Disrupted planet Excretion disk Exozodiacal dust Extraterrestrial materials Extraterrestrial sample curation Giant-impact hypothesis Gravitational collapse Hills cloud Internal structure Interplanetary dust cloud Interplanetary medium Interplanetary space Interstellar cloud Interstellar dust Interstellar medium Interstellar space Kuiper belt List of interstellar and circumstellar molecules Merging stars Molecular cloud Nebular hypothesis Oort cloud Outer space Planetary migration Planetary system Planetesimal Planet formation Protoplanetary disk Ring system Rubble pile Sample-return mission Scattered disc Star formation
Systems	Exocomet Interstellar Exomoon Tidally detached Rogue planet Orbits Retrograde Trojan Mean-motion resonances Titius–Bode law
Host stars	A B Binary star Brown dwarfs F/Yellow-white dwarfs G/Yellow dwarfs Herbig Ae/Be K/Orange dwarfs M/Red dwarfs Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giants
Detection	Astrometry Direct imaging list Microlensing list Polarimetry Timing list Radial velocity list Transit method list Transit-timing variation
Habitability	Astrobiology Astrooceanography Circumstellar habitable zone Earth analog Extraterrestrial liquid water Galactic habitable zone Habitability of binary star systems Habitability of F-type main-sequence star systems Habitability of K-type main-sequence star systems Habitability of natural satellites Habitability of neutron star systems Habitability of red dwarf systems Habitability of yellow dwarf systems Habitable zone for complex life List of potentially habitable exoplanets Tholin Superhabitable planet
Catalogues	Nearby Habitable Systems Exoplanet Data Explorer Extrasolar Planets Encyclopaedia NASA Exoplanet Archive NASA Star and Exoplanet Database Open Exoplanet Catalogue
Lists	Exoplanetary systems Host stars Multiplanetary systems Stars with proto-planetary discs Exoplanets Discoveries Extremes Firsts Nearest Largest Heaviest Terrestrial candidates Kepler 1–500 501–1000 1001–1500 1501–2000 K2 Potentially habitable Proper names Discovered exoplanets by year before 2000 2000–2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024
Other	Carl Sagan Institute Exoplanet naming convention Exoplanet phase curves Exoplanetary Circumstellar Environments and Disk Explorer Extragalactic planet Extrasolar planets in fiction Geodynamics of terrestrial exoplanets Neptunian desert Nexus for Exoplanet System Science Planets in globular clusters Small planet radius gap Sudarsky's gas giant classification
Discoveries of exoplanets Search projects

• Astronomy
• Stars
• Spaceflight
• Outer space
• Solar System

• Exoplanetology
• Hypothetical moons