Scientists working at the European Southern Observatory’s Very Large Telescope (ESO’s VLT) in Chile have detected evidence for an unusual, ‘moon-like object’ in the CD-35 2722 system that doesn’t appear to orbit a planet.
The research team behind the discovery said the unusual object, which orbits a brown dwarf that orbits a large, Sun-like star, doesn’t fit normal astronomical categories, leaving them unsure what to name it.
If confirmed, the unusual space object would be the first moon without a host planet ever discovered. Such a confirmation would also make the newly termed “exosatellite” the first exomoon of any type discovered outside our Solar System.
Moon-Like Object Part of ‘Super Weird’ System
According to a statement announcing the moon-like object, ESO student Kevin Hoy had been monitoring the CD-35 2722 star system for months. The measurements that spotted the new, moon-like object were taken with the observatory’s CRIRES+ instrument using the radial velocity method that spotted the first planet around a Sun-like star.
During that time, the researcher and lead author of a study detailing the find said he noticed the system was “super weird” compared to our local Solar System. While the star at the center of the system, with a mass of about half of our Sun, was not unusual, they also determined that it is orbited by a brown dwarf. Brown dwarfs are astronomical objects too small to be a star, but too massive to be a planet, making the pairing unusual. Still, it was not unheard of.
However, according to Hoy, when the team spotted a third, smaller object in the system, they were left confused. Measurements indicated it is at least as massive as Jupiter, suggesting it may be a moon orbiting the Brown Dwarf, only without a host planet.
“The satellite we report is a giant gaseous body orbiting a highly massive companion, itself several times the mass of Jupiter,” explained Alice Zurlo, Director of the Millennium Nucleus of Young Exoplanets and their Moons (YEMS) in Chile project and collaborator on the study.
Mysterious ‘Exosatellite’ Defies Traditional Classification
The team has temporarily named the moon-like object an “exosatellite.” However, Hoy said the object’s status as a ‘third wheel’ in the system makes them want to classify it as a moon, “even if it is nothing like the small, rocky moons we have in our system.”
“This system is somewhat hard to define using Solar-System-based words like ‘planet’ and ‘moon’,” the researcher explained. “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.”
Director Zurlo said the differences between the observed system and our own Solar System have added to the difficulty in labeling the moon-like object.
“We have a clear delineation between the planets and the Sun in the Solar System, so defining things like moons is simple,” Zurlo explained. “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.”
“This System is Truly Unique and Represents a Breakthrough”
When discussing the significance of the moon-like object, Director Zurlo said detecting satellites in other systems will help researchers better understand the diversity in formation and evolution of planets, moons, and other moon-like objects. The director also noted the historical nature of the detection.
“As exotic as it is, this system is truly unique and represents a breakthrough: the first plausible detection of an exosatellite,” the researcher explained.
Moving forward, the team said the increased sensitivity of the 39-meter mirror on the ESO’s in-development Extremely Large Telescope (ELT) will allow for the detection of even smaller moons, including potentially spotting moons the size of Earth’s largest satellite. The researchers said such detections will likely force the astronomical community to reconsider how planetary objects are labeled, especially those in systems so radically different from our own.
The study “Planetary-mass exosatellite detected around the substellar companion of a star” was published in the journal Nature.
Christopher Plain is a Science Fiction and Fantasy novelist and has spent the last six years as Associate News Editor and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him at christopher@thedebrief.org.