> ## Content Index > Fetch the complete content index at: https://bullquill.com/llms.txt > Use this file to discover other available public pages before exploring further. # First exomoon candidates explained: why none is confirmed - URL: https://bullquill.com/exomoons-explained-first-candidates/ - Published: 2026-10-26T12:18:39.000Z - Updated: 2026-10-26T12:25:21.000Z - Description: No moon outside the Solar System has been confirmed. The strongest claim, an exosatellite found with ESO's Very Large Telescope and published in Nature, still needs independent follow-up, while earlier Kepler candidates have been seriously disputed. - Author: Rowan Ashby - Tags: Science, Explainer, Exomoon, European Southern Observatory, Very Large Telescope, CD-35 2722, Nature, #lang-en, #evergreen, #tr-exomoons-explained-first-candidates ## Key takeaways - No exomoon has been confirmed yet. Astronomers have a short list of candidates, and each one still needs independent follow-up. - The strongest new claim is an "exosatellite" at least as massive as Jupiter. It orbits a brown dwarf that in turn orbits the star CD-35 2722, and it was found with ESO's Very Large Telescope and published in *Nature*. - Earlier candidates around Kepler-1625b and Kepler-1708b have been seriously disputed. HD 206893 and WASP-49 b offer tantalising hints but no firm detection. - Part of the problem is vocabulary: there is no agreed definition of an exomoon, so even a real detection can be argued over. - Next-generation instruments, above all ESO's 39-metre Extremely Large Telescope, are expected to find smaller and less ambiguous moons. Has anyone found the first exomoon? Not yet. As of October 2026, the best candidate is a Jupiter-mass companion circling a brown dwarf in the CD-35 2722 system, and even its discoverers call it "the first plausible detection", not a confirmed one. The result went back into circulation this week, when ScienceDaily republished the European Southern Observatory (ESO) release on 24 October under a "first moon beyond our Solar System" headline. The paper also appears in the print edition of *Nature* (vol. 655, issue 8124). ESO originally announced the finding on 22 July 2026\. This explainer sets out what an exomoon is, how astronomers hunt for them, which candidates exist and why none has crossed the line to "confirmed". ## What is an exomoon? An exomoon, or exosatellite, is a natural satellite that orbits something other than the Sun's planets, usually a planet around another star. The Solar System has hundreds of moons. Beyond it, more than 6,000 exoplanets have been confirmed (the NASA Exoplanet Archive passed that mark in 2025), but not a single moon around any of them is accepted by the whole field. The reason is scale. Moons are much smaller and lighter than the planets they orbit, and those planets are already hard to see next to their bright host stars. A moon's signal is usually a small effect layered on top of an already small effect. Definitions add a second layer of difficulty. The International Astronomical Union (IAU) has a working definition of an exoplanet: an object below roughly 13 Jupiter masses (the deuterium-fusion limit) that orbits a star, a brown dwarf or a stellar remnant, with a mass ratio to its host below about 1 in 25\. Under that wording, a planetary-mass object orbiting a brown dwarf can count as a planet. The IAU has no matching definition for an exomoon. So when the host is itself a "failed star", calling the companion a moon is a judgement call, as the authors of the newest study acknowledge. ## How astronomers search for exomoons Nobody has photographed an exomoon directly. Every claim so far is indirect, inferred from the effect a moon would have on its planet or on light passing through the system. Four main approaches are in use. ### 1\. Transit timing and transit dips When a planet crosses in front of its star, it dims the starlight on a regular schedule. A moon tugs the planet back and forth, so the crossings can arrive slightly early or late (transit timing variations). A large enough moon may also add its own small dip just before or after the planet's. This is how the Kepler-1625b and Kepler-1708b candidates were found. The weakness is that stellar activity, instrument quirks and modelling choices can mimic such tiny signals. ### 2\. Radial velocity A spectrograph measures how an object moves towards and away from us by tracking shifts in its light. A companion makes its host wobble on a regular cycle. Radial velocity is the method that found the first exoplanet around a Sun-like star in 1995\. In the CD-35 2722 study, it was applied to the brown dwarf rather than the star, using the CRIRES+ infrared spectrograph on the Very Large Telescope (VLT) in Chile. ### 3\. Astrometry Instead of measuring speed, astrometry measures position. If a directly imaged companion traces a small extra loop in the sky as it orbits, an unseen moon may be pulling on it. ESO's VLT Interferometer, which combines the four 8-metre VLT telescopes through the GRAVITY instrument, is precise enough to try this. It produced the HD 206893 candidate. ### 4\. Gas and dust signatures A volcanically active moon, like Jupiter's Io, can spray material into space. Astronomers look for clouds of gas, such as sodium, that behave as if they come from a separate orbiting source rather than from the planet. This is the basis of the WASP-49 b claim. ## The CD-35 2722 "exosatellite" in detail CD-35 2722 is a young red dwarf about 21 parsecs (roughly 68 light-years) from Earth. In 2011, the Gemini NICI planet-finding campaign led by Zahed Wahhaj imaged a brown dwarf companion to it, estimated at 31 ± 8 Jupiter masses, at a projected separation of about 67 astronomical units. That team placed the system in the young AB Doradus group, at an age of roughly 100 million years. According to ESO, the new team, led by Kevin Hoy (an ESO student in Chile, also at Universidad Diego Portales and the Millennium Nucleus of Young Exoplanets and their Moons, YEMS), monitored the brown dwarf with CRIRES+. They found a periodic wobble consistent with an object of at least one Jupiter mass orbiting it. The host star has about half the Sun's mass, and ESO describes the brown dwarf as more than 30 Jupiter masses. The paper is titled "Planetary-mass exosatellite detected around the substellar companion of a star" ([DOI 10.1038/s41586-026-10751-w](https://doi.org/10.1038/s41586-026-10751-w?ref=bullquill.com)). Hoy called the system "super weird" compared with our own. YEMS director Alice Zurlo described it as "the first plausible detection of an exosatellite," according to the [ESO release](https://www.eso.org/public/news/eso2610/?ref=bullquill.com). Why "exosatellite" rather than "moon"? The object is massive enough to be a planet, and its host is a brown dwarf, not a planet. On the reported masses, the companion appears to sit inside the IAU's planet criteria for objects orbiting brown dwarfs. Yet it is also the third body in a star–brown dwarf–companion hierarchy, which is why the team found the word "moon" tempting. ESO itself frames the result conditionally: if confirmed, it would be the first moon-like object found beyond the Solar System. ## Exomoon candidates compared | Candidate | Host object | Method | Estimated moon mass | Status (Oct 2026) | | ----------------------- | ---------------------------------------------------------------------------- | ------------------------------------------- | ----------------------- | ------------------------------------------------------------------------ | | CD-35 2722 exosatellite | Brown dwarf (>30 Jupiter masses) orbiting a red dwarf, \~68 light-years away | Radial velocity (VLT/CRIRES+) | At least 1 Jupiter mass | Candidate; published in *Nature* 2026, awaiting independent confirmation | | HD 206893 B candidate | Brown dwarf companion (\~20 Jupiter masses) \~40 parsecs away | Astrometry (VLTI/GRAVITY) | \~0.5 Jupiter mass | Candidate; authors say a definitive claim is premature | | Kepler-1625b-i | Giant planet (several Jupiter masses) | Transit timing + extra dip (Kepler, Hubble) | Roughly Neptune-like | Disputed; 2023 reanalysis calls a large moon unlikely | | Kepler-1708b-i | Jupiter-sized planet | Transit dips (Kepler) | Not well constrained | Disputed; 2023 reanalysis found much weaker evidence | | WASP-49 b "exo-Io" | Saturn-sized gas giant \~635 light-years away | Sodium cloud behaviour (VLT) | Not measured | Indirect hint; alternative explanations remain | Sources: ESO (eso2610), Nature, Paris Observatory/Astronomy & Astrophysics, Science Advances, Nature Astronomy, NASA JPL. Data as of 26 October 2026. ## Why earlier "first exomoon" claims fell short **Kepler-1625b.** In October 2018, Alex Teachey and David Kipping reported in *Science Advances* evidence for a Neptune-like moon around Kepler-1625b. It rested largely on a single Hubble transit that showed a timing offset and a second, shallower dip. In December 2023, a team including René Heller and Michael Hippke argued in *Nature Astronomy* that the moon signal comes almost entirely from that one Hubble transit and can be explained as a fitting artefact. They concluded that "neither Kepler-1625 b nor Kepler-1708 b are likely to be orbited by a large exomoon," according to their [paper](https://arxiv.org/abs/2312.03786?ref=bullquill.com). **Kepler-1708b.** This candidate came from a 2022 survey of 70 cool giant planets in Kepler data by Kipping, Teachey and colleagues. The same 2023 reanalysis found much weaker statistical support and suggested that stellar activity could distort the apparent transits. **HD 206893.** In January 2026, a team led by Quentin Kral of Paris Observatory reported in *Astronomy & Astrophysics* a roughly nine-month wobble in the position of the brown dwarf HD 206893 B. A moon of about half a Jupiter mass at around 0.22 astronomical units would explain it. The team said a definitive detection claim would be premature, and co-author Sylvestre Lacour stressed that "the level of confidence required is extremely high" for objects this difficult to detect. **WASP-49 b.** In October 2024, NASA's Jet Propulsion Laboratory reported that a sodium cloud near this gas giant moves in ways the planet's atmosphere cannot easily explain. A model with a moon on an eight-hour orbit matched the cloud's behaviour. NASA said longer observations are needed, and other explanations, such as a ring of ionised gas, have not been ruled out. ## What would count as confirmation? Astronomers generally want three things before treating a moon as real. First, the signal must repeat on the predicted schedule. Second, it should be seen by a different team or instrument, and ideally by a different method. Third, simpler explanations must be ruled out: stellar activity, instrument drift, an extra planet or a modelling choice. The CD-35 2722 and HD 206893 teams both say more observing time is needed. For HD 206893, the team has already requested follow-up to check whether the wobble recurs. ESO expects its Extremely Large Telescope, with a 39-metre main mirror, to push detection down to smaller moons than current telescopes can reach. That would move the field beyond the giant, borderline objects that dominate today's list. ## What it means For readers searching "first exomoon", the honest answer is that 2026 has produced the most convincing candidates so far, not a confirmed discovery. The CD-35 2722 result matters for two reasons. It shows that radial-velocity spectroscopy on faint, cool companions is now sensitive enough to find their satellites. It also forces astronomers to decide what to call a planet-mass body orbiting a brown dwarf. Whether or not "exomoon" sticks, these systems are a new testing ground for how planets and moons form. As ESO puts it, satellites in other systems could reveal how varied planet formation and evolution can be. For more on moons in our own neighbourhood, see our explainer on [JAXA's MMX mission to Mars' moon Phobos](https://bullquill.com/jaxa-mmx-phobos-sample-mission-explained/). For another recent precision measurement in astronomy, read [how astronomers measured the Big Bang's helium to 0.5%](https://bullquill.com/big-bang-helium-measurement-explained/). Ongoing coverage is collected on our [Exomoon](https://bullquill.com/tag/exomoon/) and [European Southern Observatory](https://bullquill.com/tag/european-southern-observatory/) topic pages. ## Frequently asked questions ### Has the first exomoon been found? Not definitively. Several candidates exist, including a Jupiter-mass exosatellite around a brown dwarf in the CD-35 2722 system reported in Nature in 2026, but none has been independently confirmed. ### What is an exomoon? An exomoon is a natural satellite orbiting a planet (or planet-like body) outside the Solar System. There is no official IAU definition of an exomoon yet, which is why some researchers prefer the term exosatellite. ### How do astronomers detect exomoons? Indirectly: through changes in a planet's transit timing, extra dips in starlight, radial-velocity wobbles, tiny shifts in a companion's position measured by astrometry, or gas clouds that behave as if they come from a separate orbiting body. ### What happened to the Kepler-1625b exomoon? A Neptune-like moon was proposed in 2018 from Hubble and Kepler data, but a 2023 reanalysis in Nature Astronomy concluded that a large moon around Kepler-1625b is unlikely and that the signal may be a fitting artefact.