Have We Finally Found an Exomoon?... Discovery of a Three-Tiered Celestial System Shakes Astronomy [Reading Science]

Jupiter-Mass Object Detected Orbiting Brown Dwarf: A Strong Candidate for the "First Exomoon"

Planet or Moon? New Classification Standards Up for Debate

For the first time, an object that could be considered an "exomoon" has been detected outside the solar system. However, because this object orbits a brown dwarf rather than a planet, a debate has emerged within the astronomical community over whether it should be classified as the first exomoon or as another form of exoplanet.

An international research team, including the European Southern Observatory (ESO), announced that they discovered an object with about 90% of Jupiter's mass orbiting the brown dwarf "CD-35 2722 B," which itself orbits the red dwarf "CD-35 2722 A," using the Very Large Telescope (VLT). The research results were published in the July 23 issue of the international journal Nature.

An imaginative illustration depicting a three-tier hierarchical structure consisting of a brown dwarf orbiting a red dwarf star, and a planetary-mass object orbiting the brown dwarf. This study presented a new observational possibility to identify potential exomoons. Photo by ESO/M. Kornmesser

Here, the term "dwarf" refers not to a small object like Earth, but rather to a star that is relatively small compared to others. Red dwarfs are stars smaller and cooler than the Sun, but still undergo nuclear fusion. Brown dwarfs lack sufficient mass to sustain hydrogen fusion like regular stars. In terms of size, brown dwarfs are much larger than Earth and are similar in diameter to Jupiter, about 11 times that of Earth.

The research team conducted precise observations of the brown dwarf's movement from October 2023 to February 2024, confirming regular wobbling with a period of about 171 days. This indicates that another object is exerting a gravitational influence on the brown dwarf, and their analysis showed that an object with a mass similar to Jupiter is orbiting the brown dwarf.

In other words, this means that yet another object exists around the brown dwarf orbiting a star. If confirmed, this discovery would mark the first time a "three-tier hierarchical structure"—a brown dwarf orbiting a star, and a planetary-mass object orbiting that brown dwarf—has been observed. Although more than 6,000 exoplanets have been found to date, there have been no confirmed detections of exomoons.

The Greater Significance: Advancements in Observational Technology

Experts cited progress in observational technology, rather than the identification of an exomoon candidate itself, as the most significant achievement of this study.

Kang Sungju, former senior researcher at the Korea Astronomy and Space Science Institute, stated, "It is important that the team was able to detect the companion object by measuring the brown dwarf's minute radial velocity with 10 to 100 times higher precision than before," and added, "This opens new pathways to discovering even smaller exomoons in the future."

Park Byunggon, principal researcher at the Korea Astronomy and Space Science Institute, also explained, "Because exomoons are low in mass, detection has been extremely difficult," and predicted, "Once the Giant Magellan Telescope (GMT) or the Extremely Large Telescope (ELT) is completed, it is highly likely that many more exomoons will be discovered."

However, some are cautious about immediately identifying this discovery as the first exomoon. Only 23 data points have been secured so far, so further observations are needed to confirm both the existence and the orbit of the object. The research team themselves have referred to this object as an "exosatellite" or a "planetary-mass exosatellite" rather than a "moon," showing a cautious attitude regarding its classification.

Spanish astronomer Jorge Lillo-Box stated, "This discovery is significant in that it demonstrates the ability to detect objects around very faint brown dwarfs using current technology," but also assessed that, "Rather than being strictly the first exomoon, it is more appropriate to view this as a new form of planetary system involving a brown dwarf."

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