Summary
ESO says observations with the Very Large Telescope have revealed plausible evidence for a Jupiter-mass object orbiting a brown dwarf in the CD-35 2722 system. If confirmed, it could be the first moon-like object detected beyond the Solar System.
Astronomers using the European Southern Observatory’s Very Large Telescope (VLT) report plausible evidence for a moon-like object in the CD-35 2722 system. The object appears to orbit a brown dwarf rather than a planet. ESO describes the finding as the first plausible detection of an exosatellite; if confirmed, it would be the first moon-like object discovered beyond the Solar System.
The system’s central star, CD-35 2722, has about half the Sun’s mass. A brown dwarf orbits it, and the newly reported object orbits that brown dwarf. The object is at least as massive as Jupiter, while its companion has more than 30 times Jupiter’s mass.
How the team looked for the object
The team used CRIRES+ on the VLT and applied the radial velocity method to the brown dwarf. This method detects the small motion of an object along our line of sight. In this case, the team looked for wobbles in the brown dwarf caused by the gravitational influence of an orbiting companion.
ESO says the observations revealed what the researchers consider strong evidence for the object. The release identifies the result as a plausible detection, rather than describing it as a confirmed exomoon. The study is titled “Planetary-Mass Exosatellite Detected Around a Star’s Substellar Companion” and is associated with a paper in Nature.
Why it is difficult to call it a moon
In the Solar System, a moon orbits a planet, which in turn orbits the Sun. CD-35 2722 has a different arrangement: the brown dwarf is the star’s companion, and the Jupiter-mass object circles the brown dwarf. Because the object is itself massive enough to count as a planet by mass, but does not orbit a star directly, familiar labels do not fit neatly.
The research team calls it an “exosatellite”. ESO notes that there is no officially accepted definition of “exomoon”; the term is generally used for a natural satellite orbiting a planet or another object outside the Solar System. In this system, the hierarchy of objects makes the distinction between a moon and a planet less straightforward than it is in our own.
Finding satellites around other planetary systems could help astronomers investigate how such systems form and evolve. ESO says its forthcoming Extremely Large Telescope, with a 39-metre mirror, is expected to enable searches for smaller exomoons. For now, the CD-35 2722 object is a promising candidate whose unusual orbit also tests the categories used to describe worlds.