Potential Exomoon Discovery Around Brown Dwarf CD-35 2722 B

A Signal from a Brown Dwarf

Recent observations made with the CRIRES+ instrument on the Very Large Telescope (VLT) of the European Southern Observatory (ESO) have detected a periodic signal associated with CD-35 2722 B. This signal suggests the presence of a companion body with a minimum mass of approximately 0.9 times that of Jupiter, orbiting with a period of about 170 days. Notably, this companion does not orbit the star CD-35 2722 directly; instead, it orbits the brown dwarf, which itself is a distant companion to the star.

This finding presents strong evidence, though it stops short of providing definitive confirmation of what could be the first exomoon. The researchers prefer to use the term "exosatellite," leaving the final classification open for discussion.

Why "Exomoon" Is Not Sufficient

The hierarchical structure of the system complicates the nomenclature inherited from our Solar System. In our Solar System, a moon is a natural satellite of a planet, a clear distinction since planets and stars are well-defined entities. However, CD-35 2722 B has a mass roughly 30 times that of Jupiter, making it too massive to be classified as an ordinary planet but not massive enough to undergo hydrogen fusion like a star. Thus, it is categorized as a brown dwarf.

The candidate body has a mass comparable to that of a giant planet. Referring to it as an "exomoon" captures the orbital relationship but may imply a smaller object akin to known moons. The term "exosatellite" is more neutral, indicating simply that a body orbits another body outside our Solar System without making assumptions about its nature or formal classification.

A New Application of Technique

Doppler variations of the brown dwarf reveal the gravitational influence of the companion. The method used measures the slight back-and-forth motion of an object caused by the gravitational pull of what it orbits. This motion periodically shifts the spectral lines, providing crucial data. Historically, this technique has been employed to discover numerous planets around stars; however, it is now applied directly to a previously imaged brown dwarf.

The measurement yields a minimum mass rather than a complete mass, as it is contingent on the inclination of the orbit relative to our line of sight. Further observations are needed to refine the orbital period, rule out alternative explanations, and better define the mass of the candidate. This caution enhances the significance of the result: it paves the way for observational pursuits aimed at detecting satellites around less extreme substellar companions.

A Discovery Awaiting Confirmation

The value of this finding lies in its orbital hierarchy rather than in a definitive label. To date, no exoplanet satellite has been confirmed without controversy. However, the CD-35 2722 system introduces an observable hierarchical configuration: a star, a companion brown dwarf, and a potential satellite orbiting the latter. If the signal is validated, it will allow for comparisons of the formation processes of planets, brown dwarfs, and satellites in a scenario that lacks a direct equivalent in our Solar System.

The most important takeaway is not to force a new term onto a single object but to employ definitions that align with the data. With more sensitive instruments, particularly the Extremely Large Telescope, this same technique could be extended to less massive companions, bringing us closer to the unequivocal identification of a true exomoon.

Sources

ESO study and scientific article