Jupiter-mass ‘exomoon’ orbiting brown dwarf challenges cosmic labels

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 today in Nature, describes the system he spent months analyzing as “super weird” compared to our own. 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 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.

A system that defies labels

“This system is somewhat hard to define using Solar-System-based words like ‘planet’ and ‘moon’,” states Hoy, who is also affiliated with the Universidad Diego Portales and the Millennium Nucleus of Young Exoplanets and their Moons (YEMS) in Chile. 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.

“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,” says Hoy. “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.”

This exosatellite, or “exomoon,” a natural satellite outside our solar system, is difficult to label, given the differences in this system compared to our own. Alice Zurlo, YEMS director and collaborator on the study, explains, “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,” adds Zurlo, who is also an astrophysicist at Universidad Diego Portales.

A long search for exomoons

Regardless of what to call this object, astronomers have been trying to detect satellites outside our solar system for years, but none has yet been confidently detected. Despite more than 6,000 exoplanets discovered to date, only a few exomoon candidates have been spotted, and evidence supporting them is limited. Just a few months ago, a team led by Quentin Kral reported observations with ESO’s Very Large Telescope Interferometer in the HD 206893 star system that revealed hints of a satellite but no firm detection.

For the CD-35 2722 observations, Hoy, Zurlo and their team used the CRIRES+ instrument on ESO’s VLT, employing the method used to find the first exoplanet around a sunlike star. They applied this radial velocity method to detect small wobbles in the brown dwarf caused by the object orbiting it, finding what the team believes is strong evidence for this “moon.” “As exotic as it is, this system is truly unique and represents a breakthrough: the first plausible detection of an exosatellite,” says Zurlo.

The next test for detection

Beyond the excitement of discovering new types of objects, detecting satellites in other planetary systems can help us understand how diverse their formation and evolution might be. With its 39-meter (128-foot) 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.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

Full profile →

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

Full profile →