Planetary science: New observations raise possibility of first exomoon detection (Nature)
23 July 2026
Observations of what may be the first known three-tiered hierarchical system — a satellite orbiting a brown dwarf, which in turn orbits a star — are presented in Nature this week. It remains unclear whether the satellite is an exoplanet or an exomoon, as conclusive classification systems for exomoons are lacking. However, the discovery of this object may mark a step towards the conclusive identification of exomoons in planetary systems, which have evaded confident detection to date.
In systems with three levels of hierarchy — a star with an orbiting companion (such as a planet or brown dwarf) and a third object orbiting the companion — the third object can be called an exosatellite. Exosatellites orbiting an exoplanet can be described as exomoons. However, there is uncertainty regarding whether a satellite orbiting a brown dwarf could also be described as an exomoon. More than 6,000 exoplanets have been discovered, but no exomoon has ever been definitively identified.
Kevin Hoy and colleagues present evidence of an exosatellite orbiting a brown dwarf companion called CD-35 2722 B. They use a method called radial velocity analysis — the same technique that was used to discover the first exoplanet around a Sun-like star — to monitor this brown dwarf between October 2023 and February 2026. The analysis reveals the signal of what appears to be at least one orbiting satellite, although models for two satellites are highly unstable. The authors modelled the properties of this system, which indicates an object with a mass just under that of Jupiter (around 0.9 Jupiter masses) orbiting the brown dwarf over a period of around 170 days.
The object’s position as the last component in a three-tiered hierarchical system might be seen to support a moon-like classification but, as the object is planet-sized and orbiting a brown dwarf rather than a planet, it is probably not apt to call it an exomoon, the authors note. The work demonstrates that radial velocity analysis could allow the conclusive detection of an exomoon in the future, but a clear classification system for exosatellites will be needed, the authors conclude.
- Article
- Published: 22 July 2026
Hoy, K., Zurlo, A., Peña R, P.A. et al. Planetary-mass exosatellite detected around the substellar companion of a star. Nature 655, 865–869 (2026). https://doi.org/10.1038/s41586-026-10751-w
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https://www.nature.com/articles/d41586-026-02299-6
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