Thanks to the Very Large Telescope, a moon may have been detected in a different solar system

We return to writing about solar systems different from our own, particularly the research conducted by the Very Large Telescope of the ESO, following recent discoveries related to the Beta Pictoris system. This time, however, the focus is not on an exoplanet but rather on a moon or exosatellite that may be found within the system known as CD‑35 2722. This could be the identification of the first moon outside the Solar System. A result that, if confirmed, would mark a breakthrough in our understanding of extrasolar planetary systems.

This candidate exosatellite exists in a system very different from those we are accustomed to thinking about (and obviously different from the Solar System). In this case, the newly discovered object does not orbit around a planet, as all the moons known in the Solar System do, but around a brown dwarf, which in turn orbits around the star CD‑35 2722 (a young star with about half the mass of the Sun).

A new moon detected by the Very Large Telescope

The information on this new discovery is included in the study titled Satellite Detected Around a Star's Substellar Companion. According to reports, the system consists of three main elements: the star, the brown dwarf, and the moon (or exosatellite), whose nature challenges traditional definitions.

The definitions we usually give to moon and planet, based on what we see in the Solar System, "function poorly" for CD‑35 2722. For example, the brown dwarf has a mass of over 30 times that of Jupiter, while the exosatellite may have a mass similar to Jupiter (a value that would make it compatible with the definition of planet).

The satellite could have an eccentric orbit around the brown dwarf with a period of about 170 days. These conditions are therefore very different from the moons/satellites that are found in our vicinity. A second possibility, explored in the study, is that there could be two satellites around the brown dwarf, which would have circular orbits with periods of about 171 and about 87 days. However, this second possibility would be less likely and more unstable, favoring the option of one satellite.

The exosatellite could take this definition thanks to the fact that it does not orbit around a star but around a body that in turn orbits around a star. For this reason, it would not be associated with a planet, despite its characteristics.

Kevin Hoy (an ESO student in Chile and author of the study) stated, "It is quite difficult to define this system using terms based on the Solar System such as planet and moon. The exosatellite is clearly massive enough to be considered a planet, but it does not orbit around a star. Instead, it orbits around an object that in turn orbits around a star. Being the third element in this system, we would like to call it a moon, even though it does not resemble at all the small rocky moons we have in our system."

The search for natural satellites outside the Solar System is a goal pursued for years, but so far no exomoon has been confirmed with certainty. Despite the discovery of over six thousand exoplanets, the candidates as exomoons are very few and supported by limited evidence. Another example of an exosatellite could be found in the HD 206893 system, but there is still a lack of solid data before we can be certain.

As for the study of the CD‑35 2722 system, the researchers used the CRIRES+ (CRyogenic InfraRed Echelle Spectrograph Upgrade Project) instrument of the Very Large Telescope, applying the radial velocity method, which was previously used to identify the first exoplanet around a sun-like star. The scientists detected the small oscillations in the position of the brown dwarf caused by the orbiting object, thus indirectly detecting the presence of the exosatellite.

In the future, researchers hope to improve the understanding of these systems, their characteristics, and evolution with the use of the Extremely Large Telescope (ELT). This new system will feature a 39-meter mirror and advanced instruments. Thanks to more advanced features than the VLT, it should be possible to detect smaller moons and therefore obtain further data on the planetary systems that surround us.