An object with a mass comparable to a planet appears to be in orbit around a brown dwarf located 72 light-years away.
This detection, made with the Very Large Telescope of the European Southern Observatory, could constitute the first credible observation of a natural satellite outside the Solar System.
Artistic representation of the CD-35 2722 system.
Image ESO
The object orbits CD-35 2722 B, a brown dwarf that itself accompanies a star. Brown dwarfs occupy an intermediate position between giant planets and stars. Their mass is insufficient to sustain the nuclear reactions that make stars like the Sun shine.
The discovery is based on the radial velocity method. When one body orbits another, its gravitational attraction causes small oscillations. These movements slightly modify the lines present in the spectrum of the brown dwarf, as if it were periodically moving forward and backward.
The researchers used the CRIRES+ instrument, installed on the Very Large Telescope in Chile. Observations made between October 2023 and February 2024 reveal a signal consistent with the presence of an object in orbit around the brown dwarf.
The system, however, poses a vocabulary problem. The object is massive enough to be considered a planet, but it does not directly orbit a star. It accompanies a body, on the borderline of a star, itself in orbit around a true star, which blurs the usual boundary between planet and moon.
Astronomers therefore speak rather of an exosatellite, a broader term that refers to a natural satellite located outside the Solar System. The word exomoon is also considered, although no official definition precisely sets the characteristics needed to use this term.
This significant mass distinguishes this candidate from the known moons around planets in the Solar System. It especially shows that very different orbital architectures can exist. Extrasolar systems do not necessarily reproduce the familiar organization formed by the Sun, planets, and their moons.
The detection will still need to be confirmed by new observations. Other configurations could produce a similar signal, even if simulations render some alternative hypotheses unstable. Future instruments, particularly the Extremely Large Telescope, should allow the study of smaller satellites and clarify the diversity of these systems.