The first candidate for an exosatellite outside our solar system has been discovered.

This celestial body orbits a brown dwarf, which in turn orbits a star. This does not fit well into the conventional "star-planet-moon" classification system, and it appears to be the first system ever captured in a "star-brown dwarf-satellite" configuration.

Artist's impression of the CD-35 2722 system, which features a moon-like object. The CD-35 2722 system consists of a central star (top left) with 40% of the Sun's mass, orbited by a brown dwarf (CD-35 2722 B, right) with 37 times the mass of Jupiter, which has a 5,000-year orbital period. The research team announced the discovery of an exosatellite candidate, roughly 0.9 times the size of Jupiter, orbiting this brown dwarf with a period of 171 days. [Courtesy of ESO/M. Kornmesser]

An international research team led by Dr. Kevin Hoy of Diego Portales University in Chile and the European Southern Observatory (ESO) announced on the 23rd in the scientific journal Nature that they had found strong evidence of a Jupiter-class object orbiting the brown dwarf "CD-35 2722 B," located 87 light-years from Earth, using the ESO's Very Large Telescope (VLT).

The research team noted that while over 6,000 exoplanets have been discovered to date, there have only been candidates for exosatellites without any confirmed cases. They stated that if the existence of this object is verified, it could be the first confirmed detection of an exosatellite.

The CD-35 2722 system is a young, 10-million-year-old stellar system located in the southern constellation of Antlia. At its center is a red dwarf with 40% of the Sun's mass, and the brown dwarf CD-35 2722 B orbits this star every 5,000 years at a distance 67 times greater than that between the Earth and the Sun.

A brown dwarf is a celestial body with a mass between that of a planet and a star; it is more massive than a planet but lacks sufficient mass to sustain hydrogen fusion, preventing it from being classified as a typical star.

For this study, the team conducted 23 precision observations of the brown dwarf between October 2023 and February 2026 using the VLT's high-resolution infrared spectrograph (CRIRES+). They analyzed the subtle wobbles of the brown dwarf by applying the radial velocity technique, commonly used in the search for exoplanets.

The radial velocity technique measures the subtle shifts in the light spectrum of a central body caused by the Doppler effect as the gravitational pull of an orbiting body induces a slight wobble. In this study, the team applied this technique to a brown dwarf rather than a star.

Comparing the Doppler shifts from the infrared spectra obtained in each observation, the team detected a signal indicating that the brown dwarf oscillates back and forth with a regular period of approximately 171 days.

The research team verified this through various orbital models and concluded that the 171-day wobble is best explained by a single exosatellite with at least 0.9 times the mass of Jupiter (approximately 286 times the mass of Earth) orbiting the brown dwarf.

Furthermore, the team examined models with one or two satellites and found that a two-satellite system would be unstable, with one being ejected within a few hundred years, whereas a single-satellite model could remain stable for a long period.

However, the team explained that while this discovery is strong evidence suggesting the existence of an exosatellite, confirmation through additional observations is necessary, and it remains unclear whether this object can be strictly classified as an "exomoon."

The difficulty lies in the fact that because the CD-35 2722 system consists of a star-brown dwarf-exosatellite configuration, and the discovered object—while planetary in mass—orbits a brown dwarf rather than a planet, it is difficult to apply the solar system's "Sun-planet-moon" classification directly.

The research team expressed hope that as the upcoming ESO Extremely Large Telescope (ELT) enables the detection of even smaller exosatellites, similar systems may be discovered, necessitating the establishment of a clear classification system for exosatellites.

Dr. Hoy stated, "The CD-35 2722 system is difficult to define using solar system terminology like planet or moon. However, since the discovered object is the third member of this system, I would like to call it a moon, even though it is entirely different from the moons in our solar system."

◆ Source: Nature, Kevin Hoy et al., 'Planetary-mass exosatellite detected around the substellar companion of a star', https://www.nature.com/articles/s41586-026-10751-w