List of exomoon candidates explained

, there have been no positive confirmations of satellites of extra-solar planets (exomoons); however, some evidence in favour of their existence has been produced.

Timeline

• — It has been surmised that J1407b, a possibly planetary-mass object that eclipsed the star V1400 Centauri (aka. J1407) in 2007, may have a few moons based on gaps observed in its circumstellar disk or ring system.^[1] Later studies have since found that J1407b is most likely a free-floating sub-brown dwarf or rogue planet, possibly less than 6 Jupiter masses.
• — The confirmed hot Jupiter planet WASP-12b may also possess a moon.^[2]
• , — A candidate exomoon of a free-floating planet MOA-2011-BLG-262L, was announced, but due to degeneracies in the modelling of the microlensing event, the observations can also be explained as a Neptune-mass planet orbiting a low-mass red dwarf, a scenario the authors consider to be more likely.^{[3] [4] [5]} In 2024 the latter scenario was confirmed.
• — researchers using the Hubble Space Telescope published observations of the candidate exomoon Kepler-1625b I, which suggest that the host planet is likely several Jupiter masses, while the exomoon may have a mass and radius similar to Neptune. The study concluded that the exomoon hypothesis is the simplest and best explanation for the available observations, though warned that it is difficult to assign a precise probability to its existence and nature.^{[6] [7]}
  • — reanalysis concluded that the data was fit better by a planet-only model. According to this study, the discrepancy was an artifact of the data reduction, and Kepler-1625b I likely does not exist.^[8]
• — A paper by Chris Fox and Paul Wiegert examined the Kepler dataset for indications of exomoons solely from transit timing variations. Eight candidate signals were found that were consistent with an exomoon, however the signals could also be explained by the presence of another planet. Fox and Wiegert's conclusion was more and higher quality transit timing data would be required to establish whether these are truly moons or not.^[9] David Kipping re-derived the timings of six of the eight targets (based on a pre-peer review version) and evaluated the TTV evidence as uncompelling. The same study finds that Kepler-1625b I remains an exomoon candidate.^[10]
• — astronomers reported an habitable-zone 1.7 exomoon candidate transiting one of the components in the planetary-mass binary 2MASS J1119-1137AB.^[11]
• — an exomoon candidate was reported around the planet Kepler-1708b, and because it is orbiting a planet at approximately 1.6 AU from a star that is slightly more luminous than the Sun, it too could be within the habitable zone.^[12] However, this candidate is based on limited observations (only two transits) and some consider the data to be non-convincing.^[13]
• — another exomoon candidate was reported around the planet Kepler-1513b (KOI-3678.01). Unlike the previous giant exomoon candidates of Kepler-1625 and Kepler-1708, this exomoon would be terrestrial-mass, ranging from 0.76 Lunar masses to 0.34 Earth masses depending on the planet's mass and moon's orbital period.^[14]
  • — a follow-up study by the same team found that the observed TTVs are caused by a second planet in the system, and not by a moon.^[15]
• — The exomoon candidate around Kepler-1625b was again challenged, along with the Kepler-1708b candidate. This study argues that the statistical significance of these exomoon candidates is lower than previously claimed (with false positive probabilities of 10.9% and 1.6%, respectively) and that true giant exomoons would have stronger evidence. Evidence for exomoon transits may be caused by stellar activity in the Kepler light curves.^[16] Kipping's team published a response arguing that these exomoon candidates remain possible.^[17]
• — New measurements with the Very Large Telescope of the star WASP-49 gave more evidences favoring the presence of a possible volcanically active-moon around the hot Jupiter WASP-49b.^[18]

Table

Host star of the host planet(s)	Planet designation	Planet mass	Planet semimajor axis (AU)	Exomoon semimajor axis	Exomoon mass	Notes
N/A	J1407b		N/A	0.396–0.421 AU	<0.8	One possible exomoon residing in a 4 million km-wide gap in J1407b's circumplanetary disk. Other ring gaps in J1407b's disk may also contain exomoons.
N/A	2MASS J1119-1137A or B	3.7	3.6 ± 0.9 separation from each other	0.004 - 0.009 AU	0.5 - 1	Found using the transit method. A habitable-zone exomoon candidate transiting a directly imaged free-floating planet or isolated planetary-mass object.
N/A	2MASS J2117-2940	7	N/A	0.005 AU	~0.5	Candidate exomoon transit detected in Spitzer observations of 2MASS J21171431-2940034.^[19]
		10.6	330	10 AU	318	Candidate Jupiter-mass satellite from direct imaging. If confirmed, it could also be considered a planet orbiting a brown dwarf.^[20]
		1.13	0.031	0.0087 AU	?	Found by studying periodic increases and decreases in light given off from HD 189733 b. Outside of planet's Hill sphere.^[21]
				<0.00112 AU	~ 0.015	Exo-Io candidate; The sodium and potassium data^[22] ^[23] at HD 189733b is consistent with evaporating exomoons and/or their corresponding gas torus.^[24]
		1.00	0.320	0.222 RHill	0.300	Possible exomoon from transit timing variations, since deemed unlikely.
		7.59	0.298	0.278 RHill	0.499	Possible exomoon from transit timing variations, since deemed unlikely.
		38.02	0.308	0.289 RHill	2.931	Possible exomoon from transit timing variations.
		14.13	0.376	0.208 RHill	1.636	Possible exomoon from transit timing variations.
		19.95	0.534	0.235 RHill	1.551	Possible exomoon from transit timing variations, since deemed unlikely.
		24.55	0.2691	0.295 RHill	6.057	Possible exomoon from transit timing variations, since deemed unlikely.
		14.13	0.405	0.208 RHill	1.586	Possible exomoon from transit timing variations, since deemed unlikely.
		^[25]	0.98	0.022 AU	19.0	Possible Neptune-sized exomoon or double planet, indicated by transit observations.^[26]
			1.64	0.005 AU (11.7 RP)		Possible Neptune-sized exomoon or double planet, indicated by transit observations.
		9.33	0.47	0.217 RHill	0.817	Possible exomoon from transit timing variations, since deemed unlikely.
		9.85	N/A	0.24 AU	33.7	Found by microlensing; however it is unknown if the system is a super-Neptune-mass planet orbiting a free-floating planet, or a binary brown dwarf system.^[27]
WASP-12	WASP-12b^[28]	1.465	0.0232		(radius)^[29]
				6 RP		Found by studying periodic increases and decreases in light given off from WASP-12b. Outside of planet's Hill sphere.
		0.37	0.0379	< 1.74 RP	~ 0.015	Exo-Io candidate; The sodium exosphere around WASP-49b could be due to a volcanically active Io-like exomoon.^[30]
		0.92	0.033	1.125 RP	~ 0.015	Exo-Io candidate; Sodium detected via absorption spectroscopy around WASP-76b^[31] is consistent with an extrasolar toroidal atmosphere^[32] generated by an evaporating exomoon.
		1.184	0.02544	~ 1.9 RP	~ 0.015	Exo-Io candidate; The sodium detected via absorption spectroscopy around WASP-121b^[33] is consistent with an extrasolar gas torus possibly fueled by a hidden exo-Io.

Notes and References

1. Web site: Saturn-like ring system eclipses Sun-like star . 9 March 2018 . live . https://web.archive.org/web/20160919150711/https://www.sciencedaily.com/releases/2012/01/120109115830.htm . 19 September 2016 . "Mamajek thinks his team could be either observing the late stages of planet formation if the transiting object is a star or brown dwarf, or possibly moon formation if the transiting object is a giant planet".
2. http://www.ria.ru/science/20120206/558647431.html Российские астрономы впервые открыли луну возле экзопланеты
3. Bennett, D.P. . etal . A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge . The Astrophysical Journal. 1312.3951 . 2014ApJ...785..155B . 10.1088/0004-637X/785/2/155 . 785 . 2 . 155. 2014 . 118327512 .
4. Web site: Clavin . Whitney . Faraway Moon or Faint Star? Possible Exomoon Found . 10 April 2014 . . 10 April 2014 . live . https://web.archive.org/web/20140412095852/http://www.jpl.nasa.gov/news/news.php?release=2014-109 . 12 April 2014 .
5. Web site: First exomoon glimpsed – 1800 light years from Earth. New Scientist. 20 December 2013. live. https://web.archive.org/web/20131220020356/http://www.newscientist.com/article/dn24773-first-exomoon-glimpsed--1800-light-years-from-earth.html. 20 December 2013.
6. HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I . Teachey . Alex . Kipping . David M. . Schmitt . Allan R. . 1 . The Astronomical Journal . 155 . 1 . 36 . 2017 . 1707.08563 . 2018AJ....155...36T . 10.3847/1538-3881/aa93f2 . 118911978 . free .
7. Evidence for a large exomoon orbiting Kepler-1625b . Teachey . Alex . Kipping . David M. . Science Advances . 4 . 10 . eaav1784 . 4 October 2018 . 10.1126/sciadv.aav1784 . 30306135 . 6170104 . 2018SciA....4.1784T . 1810.02362 .
8. No Evidence for Lunar Transit in New Analysis of HST Observations of the Kepler-1625 System. Laura Kreidberg. Rodrigo Luger. Megan Bedell. The Astrophysical Journal . 24 April 2019. 877 . 2 . 10.3847/2041-8213/ab20c8. 1904.10618. 129945202 . free . 2019ApJ...877L..15K .
9. Fox . Chris . Wiegert . Paul . 2006.12997 . Exomoon Candidates from Transit Timing Variations: Eight Kepler systems with TTVs explainable by photometrically unseen exomoons . Monthly Notices of the Royal Astronomical Society . 23 November 2020 . 501 . 2 . 2378–2393 . 10.1093/mnras/staa3743 . free . 2021MNRAS.501.2378F . 219980961 .
10. Kipping . David . 2008.03613 . An Independent Analysis of the Six Recently Claimed Exomoon Candidates . The Astrophysical Journal . 8 August 2020. 900 . 2 . L44 . 10.3847/2041-8213/abafa9 . 2020ApJ...900L..44K . 225253170 . free .
11. Limbach. Mary Anne. Vos. Johanna M.. Winn. Joshua N.. Heller. Rene. Mason. Jeffrey C.. Schneider. Adam C.. Dai. Fei. 2021-08-18. On the Detection of Exomoons Transiting Isolated Planetary-mass Objects. The Astrophysical Journal Letters. 918. 2. L25. 10.3847/2041-8213/ac1e2d. 2108.08323. 2021ApJ...918L..25L. 237213523. en . free .
12. Kipping . David . Bryson . Steve . etal . 13 January 2022 . An exomoon survey of 70 cool giant exoplanets and the new candidate Kepler-1708 b-i . . 6. 3. 367–380. 10.1038/s41550-021-01539-1. 35399159 . 8938273 . 2201.04643 . 2022NatAs...6..367K.
13. Web site: Astronomers may have found a huge moon around a Jupiter-like exoplanet . New Scientist . 28 January 2022.
14. Kipping . David . Yahalomi . Daniel A. . January 2023 . A search for transit timing variations within the exomoon corridor using Kepler data . . 518 . 3 . 3482–3493 . 10.1093/mnras/stac3360 . free . 2211.06210 . 2023MNRAS.518.3482K.
15. Yahalomi . Daniel A. . Kipping . David . etal . January 2024 . Not So Fast Kepler-1513: A Perturbing Planetary Interloper in the Exomoon Corridor . . 527 . 1 . 620–639 . 10.1093/mnras/stad3070 . free . 2310.03802 . 2024MNRAS.527..620Y.
16. Heller . René . Hippke . Michael . December 2023 . Large exomoons unlikely around Kepler-1625 b and Kepler-1708 b . . 8. 2. 193–206. 10.1038/s41550-023-02148-w . 2312.03786 . 2024NatAs...8..193H.
17. Kipping . David . Teachey . Alex . January 2024 . A Reply to: Large Exomoons unlikely around Kepler-1625 b and Kepler-1708 b . . 2401.10333.
18. Oza . Apurva V. . Seidel . Julia V. . Hoeijmakers . H. Jens . Unni . Athira . Kesseli . Aurora Y. . Schmidt . Carl A. . Sivarani . Thirupathi . Bello-Arufe . Aaron . Gebek . Andrea . Meyer zu Westram . Moritz . Sousa . Sérgio G. . Lopes . Rosaly M. C. . Hu . Renyu . de Kleer . Katherine . Fisher . Chloe . 2024-10-01 . Redshifted Sodium Transient near Exoplanet Transit . The Astrophysical Journal Letters . 973 . 2 . L53 . 10.3847/2041-8213/ad6b29 . free . 2409.19844 . 2024ApJ...973L..53O . 2041-8205.
19. Limbach . Mary Anne . Occurrence Rates of Exosatellites Orbiting 3-30M$_$ Hosts from 44 Spitzer Light Curves . 2024-05-13 . 2405.08116 . Vos . Johanna M. . Vanderburg . Andrew . Dai . Fei. astro-ph.EP .
20. Lazzoni . C. . etal . 20 July 2020 . The search for disks or planetary objects around directly imaged companions: A candidate around DH Tau B . Astronomy & Astrophysics . 641 . A131 . 10.1051/0004-6361/201937290 . 2007.10097. 2020A&A...641A.131L . 220647289 .
21. Ben-Jaffel . Lotfi . Ballester . Gilda . 1404.1084 . Transit of Exomoon Plasma Tori: New Diagnosis . The Astrophysical Journal . 3 April 2014 . 785 . 2 . L30 . 10.1088/2041-8205/785/2/L30 . 2014ApJ...785L..30B . 119282630 .
22. Wyttenbach . A. . Ehrenreich . D. . Lovis . C. . Udry . S. . Pepe . F. . Spectrally resolved detection of sodium in the atmosphere of HD 189733b with the HARPS spectrograph . Astronomy & Astrophysics . 5 May 2015 . 577 . A62 . 10.1051/0004-6361/201525729 . 1503.05581 . 2015A&A...577A..62W . 54935174 .
23. Keles . Engin . Mallonn . Matthias . von Essen . Carolina . Carroll . Thorsten . Alexoudi . Xanthippi . Pino . Lorenzo . Ilyin . Ilya . Poppenhager . Katja . Kitzmann . Daniel . Nascimbeni . Valerio . Turner . Jake D . Strassmeier . Klaus G . The potassium absorption on HD189733b and HD209458b . Monthly Notices of the Royal Astronomical Society: Letters . October 2019 . 489 . 1 . L37-L41 . 10.1093/mnrasl/slz123. free . 1909.04884 . 2019MNRAS.489L..37K . 202134796 .
24. Gebek . Andrea . Oza . Apurva . Alkaline exospheres of exoplanet systems: evaporative transmission spectra . Monthly Notices of the Royal Astronomical Society . 29 July 2020 . 497 . 4 . 5271–5291 . 10.1093/mnras/staa2193 . free . 2005.02536 . 2020MNRAS.497.5271G . 218516741 . 8 December 2020.
25. Timmermann . Anina . etal . 2001.10867 . Radial velocity constraints on the long-period transiting planet Kepler-1625 b with CARMENES . Astronomy & Astrophysics . 29 January 2020 . 635 . A59 . 10.1051/0004-6361/201937325 . 2020A&A...635A..59T . 210942758 .
26. News: Drake . Nadia . Nadia Drake . 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. . https://web.archive.org/web/20181003234439/https://www.nationalgeographic.com/science/2018/10/news-first-exomoon-nasa-kepler-planets-facts-space/ . dead . 3 October 2018 . 3 October 2018 . . 4 October 2018 .
27. Miyazaki . S. . etal . 1804.00830 . MOA-2015-BLG-337: A Planetary System with a Low-mass Brown Dwarf/Planetary Boundary Host, or a Brown Dwarf Binary . The Astronomical Journal . 24 July 2018 . 156 . 3 . 136 . 10.3847/1538-3881/aad5ee . 2018AJ....156..136M . 58928147 . free .
28. Encyclopedia: WASP-12 b. 1 February 2015. live. https://web.archive.org/web/20150201211333/http://exoplanet.eu/catalog/wasp-12_b/. Extrasolar Planets Encyclopaedia. 1 February 2015.
29. http://www.ria.ru/science/20120206/558647431.html Российские астрономы впервые открыли луну возле экзопланеты
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33. Hoeijmakers . H.J. . Seidel . J.V. . Pino . L. . Kitzmann . D. . Sindel . J.P. . Ehrenreich . D. . Oza . A.V. . Bourrier . V. . Allart . R. . Gebek . A. . Lovis . C. . Yurchenko . S.N. . Astudillo-Defru . N. . Bayliss . D. . Cegla . H. . Lavie . B. . Lendl . M. . Melo . C. . Murgas . F. . Nascimbeni . V. . Pepe . F. . Segransan . D. . Udry . S. . Wyttenbach . A. . Heng . K. . Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) - IV. A spectral inventory of atoms and molecules in the high-resolution transmission spectrum of WASP-121 b . Astronomy & Astrophysics . 18 September 2020 . 641 . A123 . 10.1051/0004-6361/202038365. 2006.11308 . 2020A&A...641A.123H . 219966241 .