The First ‘Exomoon’ Ever Discovered Could Introduce a New Type of Celestial Body

What makes a moon a moon? To be a satellite, something only needs to orbit something else, which makes for an easy definition. But what makes a particular type of satellite a moon, specifically?

For the first time, a scientific paper has attempted to offer a practical answer to this question. Published Wednesday in the journal Nature, the paper presents data on what could be called the first-ever observation of an exomoon—or, it could be something else entirely.

That’s because the potential exomoon in question, found in a system called CD-35 2722, is orbiting a brown dwarf which is itself orbiting the system’s core star.

A brown dwarf forms like a star, but it never gathers enough mass to sustain hydrogen fusion in its core, and so continues to float “dead” in space. It’s larger than a planet like Jupiter, but much smaller than a real star, and definitely not a planet itself.

That presents a problem for the definition of “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. But since it’s orbiting a brown dwarf, it’s neither a planet nor a moon.

The ESO’s Very Large Telescope is the instrument that first spied CD-35 2722.
Credit: ESO/H.H.Heyer

The other issue is that our mold-breaking satellite is itself much larger than Jupiter, making it massive enough to defy a whole different aspect of the definition of a moon.

“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.

So 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.