Astronomers discover possible exomoon orbiting brown dwarf in the CD-35 2722 system

Why the classification is difficult

Astronomers now know more than 6,000 planets that orbit stars other than our Sun. The astrophysicists therefore assume that moons also orbit exoplanets – that is, planets around other stars. Six of the eight planets in our Solar System have moons – and the differences are large: Earth has only one, while Saturn, according to current knowledge, has nearly 300.

Yet despite all their efforts, researchers have so far not detected any such "exomoon". "Objects with planetary mass that orbit a brown dwarf", as the authors of the study also make clear, "are, according to the current nomenclature of the International Astronomical Union, planets." The term "exomoon" was previously reserved for companions of planets and smaller celestial bodies.

"In the CD-35 2722 system, where the boundaries between stars, planets, and moons blur, describing the whole becomes considerably more complicated." It is therefore necessary to more clearly define which celestial objects should count as moons and which should not. The European Southern Observatory is less cautious, referring in the headline of its press release to the discovery of a so-called "exomoon".

Significance for astronomy

The discovery shows that, with continuously improving astronomical instruments, it should be possible in the near future to also detect moons around planets orbiting other stars, the scientists write in the journal "Nature". "With the upcoming observational capabilities, many similar systems are likely to be discovered," the researchers write, "and with that, the establishment of clear categories will become increasingly important."

It remains unclear whether the newly discovered object is an exomoon or a planet. The question touches on fundamental definitions in astronomy: Is a low-mass companion orbiting a brown dwarf a moon – or a planet? The study authors plead for caution and therefore provisionally speak only of a planetary-mass exosatellite.

The findings are an important indication that future telescopes and instruments could indeed be capable of detecting exomoons directly. Should this succeed, astronomy would take a major step forward in better understanding the diversity of planetary systems beyond our cosmic neighbourhood.

The research team published its results in the journal "Nature". Among those involved was the astronomer Alice Zurlo of the Universidad Diego Portales in Chile. The precise nature of the celestial body is to be examined in more detail in further observations.