Astronomers have found what may be the first convincing evidence of a giant “exomoon” beyond our solar system, but the unusual object is already challenging the way scientists classify worlds in space.
Using the European Southern Observatory’s Very Large Telescope (VLT) in Chile, an international team of researchers discovered evidence for a Jupiter-sized object orbiting a brown dwarf in the young CD-35 2722 star system.
Their findings were published in the journal Nature.
The discovery is unusual because the object does not orbit a planet like the moons in our solar system. Instead, it circles a brown dwarf, an object that is too large to be considered a planet but too small to become a true star.
The brown dwarf itself orbits the star CD-35 2722, which has about half the mass of our Sun.
Lead author Kevin Hoy, an ESO student in Chile, described the system as “super weird” because it does not fit neatly into the categories astronomers normally use.
The newly discovered object has at least the mass of Jupiter, making it large enough to qualify as a planet if it orbited a star. However, because it travels around a brown dwarf rather than a star, researchers believe it is more appropriate to describe it as an exosatellite, or exomoon—a natural satellite located outside our solar system.
“This system is somewhat hard to define using Solar-System-based words like ‘planet’ and ‘moon’,” Hoy explained. “The exosatellite is clearly massive enough to be a planet, but it does not orbit a star. Being the third wheel in this system makes us want to call it a moon.”
Alice Zurlo, director of the Millennium Nucleus of Young Exoplanets and their Moons (YEMS) and a co-author of the study, said the discovery highlights how difficult it can be to classify objects in planetary systems that are very different from our own.
In our solar system, the distinction between stars, planets and moons is straightforward. In systems like CD-35 2722, however, those boundaries become much less clear.
Astronomers have searched for exomoons for many years, but none has yet been confirmed with high confidence. Although scientists have discovered more than 6,000 exoplanets, only a handful of possible exomoons have been reported, and the evidence for them has remained uncertain.
To detect this new candidate, the team used the CRIRES+ instrument on the VLT and applied the radial velocity technique, which measures tiny movements caused by the gravitational pull of an orbiting object. By detecting a subtle wobble in the brown dwarf, the researchers found strong evidence that a massive companion is circling it.
If future observations confirm the discovery, it would mark the first reliable detection of an exomoon. Scientists are especially excited because studying these distant satellites could reveal new clues about how planets, moons and brown dwarfs form and evolve.
Future observations with the European Southern Observatory’s Extremely Large Telescope, now under construction, are expected to detect even smaller exomoons and may force astronomers to rethink how they define worlds beyond our solar system.
Source: KSR.
