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BY MARK WORGAN

Astronomers using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) have found evidence of what could be the first moon-like object discovered beyond the Solar System.

The object has been identified in the CD-35 2722 system, but unlike the moons in our own Solar System, it does not orbit a planet. Instead, it circles a brown dwarf – an object larger than a planet but too small to be a star – which itself orbits the star CD-35 2722.

The discovery, outlined in a paper published in the journal Nature, raises questions over how such objects should be classified if confirmed.

Kevin Hoy, an ESO student based in Chile and lead author of the study, described the system as “super weird” compared with our own.

The largest object in the young system is the star CD-35 2722, which has around half the mass of the Sun. Orbiting it is a brown dwarf with more than 30 times the mass of Jupiter. The newly identified object circles that brown dwarf.

“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 newly discovered object, which the research team refers to as an exosatellite, is thought to be at least as massive as 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.”

The researchers say the object could also be described as an exomoon – a natural satellite beyond the Solar System – although its unusual characteristics make it difficult to categorise.

Alice Zurlo, director of YEMS and a co-author of the study, said: “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.

Astronomers have been searching for moons beyond the Solar System for many years, but none has yet been confirmed with confidence. Although more than 6,000 exoplanets have been discovered, only a handful of exomoon candidates have been identified and the evidence for them remains limited.

Only a few months ago, another team led by Quentin Kral reported hints of a possible satellite in the HD 206893 system using ESO’s Very Large Telescope Interferometer, although that observation also stopped short of a definitive detection.

For the latest study, the researchers used the CRIRES+ instrument on ESO’s VLT and employed the radial velocity technique, which detects tiny movements caused by the gravitational pull of an orbiting object. The same method was used to discover the first exoplanet orbiting a Sun-like star.

By measuring subtle wobbles in the brown dwarf, the team found what they believe is strong evidence for the presence of the moon-like object.

“As exotic as it is, this system is truly unique and represents a breakthrough: the first plausible detection of an exosatellite,” says Zurlo.
Scientists say the discovery could improve understanding of how planetary systems form and evolve, particularly those that differ significantly from the Solar System.

They also believe future observations with ESO’s Extremely Large Telescope (ELT), which is currently under construction and will feature a 39-metre primary mirror, could enable the detection of even smaller exomoons.
Researchers say discoveries made with the ELT may further challenge existing definitions of planets, moons and other celestial bodies in planetary systems unlike our own.