Reacción a "What could be the first exomoon orbiting a brown dwarf has been discovered"

This paper presents an interesting finding concerning the detection of a planetary-mass object orbiting a brown dwarf, which in turn orbits an M-type star. In my view, the significance of the study lies not so much in the classification of what has been detected, but rather in the technical capability provided by ESO’s CRIRES+ instrument to obtain precise radial velocities of very faint objects (such as this brown dwarf) and, furthermore, at a small angular separation from another star (in this case, the other component of the system, the M-type star). This capability is precisely what will enable us to obtain high-resolution spectra of increasingly faint exoplanets that are close to their stars, something that will be essential for directly studying planetary atmospheres using future instruments on the VLT (RISTRETTO) or the ELT (ANDES). In this regard, this discovery demonstrates the technological potential of current instrumentation and serves as a test case for these new instruments.

As far as nomenclature is concerned, in my view, the discovery does not represent the detection of the first exomoon or exosatellite, as I understand the concept. However, the absence of a definition by the International Astronomical Union (IAU) regarding what constitutes an exomoon or exosatellite opens the door for individual researchers to interpret these terms with varying degrees of flexibility. In fact, in a similar vein, there is not even a proper definition of an exoplanet, but only a working definition by the IAU set out in Lecavalier des Etanges & Lissauer (2022). But this working definition by the IAU is important in this case because it states that exoplanets are objects with an absolute mass of less than approximately 13 Jupiter masses. Consequently, the brown dwarf hosting the object discovered by Hoy’s team and colleagues (with a mass of 30 Jupiter masses) cannot be considered an exoplanet; therefore, the object found orbiting it would be nothing more than an exoplanet orbiting a brown dwarf which, in turn, orbits another star. Such configurations are commonly referred to as S-type planetary systems (from satellite), as opposed to circumbinary planetary systems (or P-type systems) where the planet orbits both stellar or substellar components.

For all these reasons, as I understand it, this is not the detection of the first exomoon, which would have to be a planetary-mass object orbiting another planetary-mass object, both of which are orbiting a stellar object or brown dwarf. Rather, we are witnessing the discovery of an interesting S-type planetary system that demonstrates the technological capabilities at our disposal and the potential of the next generation of instruments for exoplanetary research using direct light from the planet itself.