Jupiter-mass object blurs line between planet and moon

July 22, 2026 at 08:00 PM1 min

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Astronomers have detected a Jupiter-mass object orbiting a brown dwarf in the CD-35 2722 system, the first such 'exosatellite' found. The discovery, reported July 22 in Nature, challenges existing definitions of planet and moon. The object was identified using radial velocity measurements from the Very Large Telescope in Chile.

The Discovery

Kevin Hoy of the University Diego Portales and colleagues observed the brown dwarf CD-35 2722 B from October 2023 to February 2026 using the Very Large Telescope. Periodic shifts in the brown dwarf's light revealed a massive companion tugging it, a technique used to find hundreds of exoplanets since the 1990s. This is the first time it has identified an object orbiting a body that itself orbits a star, which the team calls an 'exosatellite'.

Definitional Debate

The International Astronomical Union's 2018 definition classifies a planet-mass object orbiting a brown dwarf as an exoplanet. Astronomer David Kipping of Columbia University notes that by this definition, the object is a planet. However, Hoy argues the hierarchical structure — object around brown dwarf around star — feels more like a moon, though he admits 'exomoon' is also inadequate. The find reignites debate over whether planetary classification should rely on intrinsic properties or orbital relationships.

Broader Implications

The CD-35 2722 system is not unique: in January 2026, another group reported a tentative sign of a similar Jupiter-half-mass satellite around a different brown dwarf. Motion has defined planets since ancient Greeks coined 'wandering star,' but the Copernican shift to heliocentric orbits still centered on motion. These discoveries suggest current taxonomy may need revision as more such systems are found.