Researchers studying the substellar companion CD-35 2722 B have identified what may be the first detected natural satellite outside our solar system using radial velocity measurements. The host star at the center of the system has about half the mass of the Sun, while the brown dwarf orbiting it weighs in at more than 30 Jupiter masses—too heavy to be a planet and too light to sustain nuclear fusion.
The CD-35 2722 System and the Discovery of an Exosatellite
Orbiting that brown dwarf is an object at least as massive as Jupiter itself. Because it circles a companion rather than a star directly, the research team refers to the new body as an exosatellite.
CRIRES+ Spectrograph and the Radial Velocity Method
Led by Kevin Hoy, an ESO student based in Santiago, and coauthor Alice Zurlo of Universidad Diego Portales, the research team bypassed the sensitivity limits of the traditional transit method. Instead of waiting for an object to cross in front of a star, the team used the CRIRES+ spectrograph on the Very Large Telescope in Chile to measure small wobbles in the brown dwarf’s motion.
This radial velocity technique is the same method that detected the first exoplanet around a sun-like star in 1995. To confirm the signal came from a moon rather than other noise, the team evaluated potential errors from Earth’s orbital motion, atmospheric seasonal variations, and the rotation of the brown dwarf. Calculations involving the Hill radius and the Roche limit also verified that the satellite sits within a stable orbital range where tidal forces will not tear it apart.
Challenging Planetary Definitions and Cosmic Labels
The discovery sits awkwardly inside standard astronomical vocabulary. Neither conventional definitions of planets nor moons fit the hierarchy neatly, as the object possesses planetary mass yet orbits an intermediate brown dwarf.
“The satellite we report is a giant gaseous body orbiting a highly massive companion, itself several times the mass of Jupiter.”
Alice Zurlo, YEMS Director and collaborator on the study, via Boingboing
Zurlo noted that while our solar system maintains a clear separation between planets and the Sun, systems like CD-35 2722 blur those lines between stars, planets, and moons. The European Southern Observatory acknowledged in a footnote that no officially recognized definition for exomoons currently exists.
Previous Candidates and Future Prospects
While more than 6,000 exoplanets have been confirmed, definitive exomoon discoveries have remained elusive. Earlier this year, a separate team gathered hints of a satellite in the HD 206893 system using the VLT Interferometer without securing a firm detection, while other candidates like Kepler-1625 b and Kepler-1708 b continue to be studied. Concurrently, Notebookcheck notes that researchers exploring HD 206893 b—a gas giant discovered in 2021 that orbits an F-type star in 25,6 years at a distance of 9.6 AU with a mass 28 times that of Jupiter and located about 133 light-years away—have used astrometry with VLTI/GRAVITY to search for an object that seems to be orbiting HD 206893 b and could have a mass equivalent to 0.4 times that of Jupiter.
Astronomers believe the CD-35 2722 discovery will reshape theories of planet formation and celestial mechanics. If smaller rocky moons experience tidal heating from brown dwarfs, similar systems might support habitable environments at great distances from primary stars. Future instruments, such as the 39-meter primary mirror on the Extremely Large Telescope currently under construction at Cerro Armazones, are expected to enable the detection of even smaller exomoons.
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