Observations made with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) have revealed evidence for a moon-like object in the CD-35 2722 system. Unlike moons in our Solar System, the newly found object does not orbit a planet, raising questions about what to name it. Instead, it circles a brown dwarf, an object larger than a planet, that orbits the CD-35 2722 star. If confirmed, this could be the first ‘moon’ discovered outside our Solar System.
Kevin Hoy, an ESO student in Chile and lead author of the study published in Nature, stated:
“This system is somewhat hard to define using Solar-System-based words like ‘planet’ and ‘moon’. The exosatellite is clearly massive enough to be a planet, but it does not orbit a star, though it orbits an object that orbits a star. Being the third wheel in this system makes us want to call it a moon, even if it is nothing like the small, rocky moons we have in our system.”
The biggest and most massive object in this young system is the star CD-35 2722, which has about half the mass of the Sun. The star is being orbited by a brown dwarf, an object too massive to be a planet but too small to be a star. The newly discovered object orbits this brown dwarf.
The new object, which the team calls an exosatellite, is at least as massive as Jupiter, while the brown dwarf has more than 30 times the mass of Jupiter.
Alice Zurlo, YEMS Director and collaborator on the study, added:
“The satellite we report is a giant gaseous body orbiting a highly massive companion, itself several times the mass of Jupiter. We have a clear delineation between the planets and the Sun in the Solar System, so defining things like moons is simple. In the CD-35 2722 system, where we are blurring the lines between stars, planets, and moons, the whole thing becomes more complicated to describe.”
Astronomers have been trying to detect satellites outside our Solar System for years, but none has yet been confidently detected. Despite over 6,000 exoplanets discovered to date, only a few exomoon candidates have been spotted, and the evidence to support them is limited.
For the CD-35 2722 observations, the team used the CRIRES+ instrument on ESO’s VLT, employing the radial velocity method to detect small wobbles on the brown dwarf caused by the object orbiting it, finding what the team believe to be strong evidence for this ‘moon’.
Alice Zurlo concluded:
“As exotic as it is, this system is truly unique and represents a breakthrough: the first plausible detection of an exosatellite.”
With its 39-metre mirror and advanced instrumentation, ESO’s upcoming Extremely Large Telescope (ELT) will allow astronomers to detect smaller exomoons. Discoveries with the ELT will make us further reconsider how we label planetary objects from systems different from our own.
