The search for a moon outside our solar system has moved into a new category of objects, but astronomers still aren’t sure if they’ve actually found one. Published Wednesday in the journal Nature, a new study describes an object orbiting a brown dwarf star. While researchers call it the first confirmed exosatellite, the discovery complicates the definition of what a moon actually is.
What makes a moon a moon? To be a satellite, something only needs to orbit something else, which makes for an easy definition. However, the object in question orbits a brown dwarf, an object larger than a planet but smaller than a star. The brown dwarf itself orbits the system’s core star. This makes the potential moon neither a planet nor a moon by traditional standards, though it certainly functions as a satellite.
The instrument that first spied the system is the ESO’s Very Large Telescope. The object itself is massive, roughly the size of Jupiter. This size presents a problem for the definition of a moon, which is specifically defined as orbiting a planet. The researchers make it clear that if their object was orbiting star CD-32 2722 directly, it would unquestionably be a planet.
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“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,” Kevin Hoy, the paper’s lead author and a student at the European Southern Observatory in Chile, said in a statement. “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 best the researchers can say for certain is that the object is definitely an exo-satellite. Its size and odd orbital characteristics mean it’s not exactly what people have been hoping for: proof of an exomoon.
Not only does this paper not prove that classical models for the evolution of rocky moons apply elsewhere in the universe, but it also doesn’t prove that modern astronomical techniques can discern extrasolar objects as small as classical moons. This distinction matters because the current detection methods rely heavily on the gravitational tug of a planet, which a brown dwarf simply does not provide.
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The search for a “real” exomoon continues. There has been debate about some past detections, but there are certainly no uncontroversial studies directly detecting exomoons. It’s taken for granted that they exist at all, but direct study has the potential to provide insights into the formation of solar systems. And, as mentioned, it would be a big step up in bragging rights for astronomers who only recently started finding exoplanets themselves.
Future observations might help clarify the object’s status. Technological improvements could allow for a clearer view of the system’s architecture. This would help determine if the object is truly a moon or if it falls into a new category of celestial bodies. The discovery highlights the complexity of our universe and the challenges of defining what we see.
