Humans discovered the first exoplanet in 1992 and have since found approximately 6,000 other planets from outside our solar system. Not once have those researchers found a moon orbiting any of those planets, however. That may have changed with the recent discovery of the first moon-like object in another solar system. 

The findings were published in a recent study. Kevin Hoy, lead author of the study and PhD student at the Instituto de Estudios Astrofísicos, says that this potential moon, known as CD-35 2722 B, orbits a brown dwarf instead of a traditional planet. It’s also approximately 90% the size of Jupiter and bigger than Saturn. This moon-like object takes 170 days to fully orbit its host. 

Hoy and other researchers used a fairly old trick to discover this moon: radial velocity analysis, which is used to detect wobbles in stars and is an indication that an exoplanet is nearby. Planets get dimmer as they move away from their host star, and exoplanets are found by viewing the changes in spectral shift.

This is the same method scientists used to detect the first exoplanet, and it was used to detect this exosatellite, which is the name granted to objects that have not yet been defined as an exoplanet or an exomoon.

“This is the first time, to our knowledge, this technique has produced evidence of satellites around a companion brown dwarf,” Hoy said in the study.

Schrödinger’s Moon?

The study’s findings aren’t without their controversy. Arguably, the biggest question is whether this is actually a moon. Brown dwarves, often referred to as failed stars, are far larger than a standard planet but did not have the chemical reactions in their core necessary to become a star. The exosatellite orbiting this brown dwarf is also bigger than all but one planet in our solar system and larger than most of the exoplanets discovered by researchers so far. 

Complicating matters is the fact that no one actually knows what a moon is. There is no standard definition from the International Astronomical Union, the authority that usually comes up with such definitions. Most people understand a moon to be a naturally occurring object that orbits a larger object, like our moon orbits Earth. 

There are some rules to this. The host object can’t be a star because then the orbiting object would be a planet. It also can’t be made by humans, or it’s deemed a satellite. The definition has expanded as astronomers find additional, unique types of moons. One example is Zoozve, a quasi-moon that would look like a moon from the surface of Venus but is actually an asteroid that orbits the sun and is close enough to look like a moon. 

For CD-35 2722 B, Hoy told me that one problem is the special nature of brown dwarfs. Brown dwarfs are considered substellar objects because they’re too big and dense to be a planet but lack the hydrogen fusion needed to be a star. They exist in a middle ground between the two, so it’s unclear whether something orbiting a brown dwarf would be considered a planet or a moon. 

That isn’t the only roadblock. “The object itself is at least the mass of Jupiter, which is far larger than the rocky and icy moons in the solar system,” Hoy said. “It’s not clear that anything that big should be considered a moon.”

Hoy says that the currently accepted IAU definitions group brown dwarfs in with stars, in that an object orbiting a brown dwarf of sufficient size would be considered a planet. However, this brown dwarf orbits its host star like a planet and CD-35 2722 B orbits the brown dwarf like a moon; therefore, it fulfills both the behavior of a moon and the definition of a planet. 

“Maybe when a definition for exomoons is written, objects like this will meet both the definitions of planet and moon, but we’ll have to see,” Hoy said. “Those terms are mutually exclusive in the solar system, so I wouldn’t be surprised if they wrote the definition in such a way as to preserve that.”

Until that day comes, no one is quite sure what to make of CD-35 2722 B, and until the IAU opens up the metaphorical box and comes up with an official definition for what a moon is, CD-35 2722 B could realistically be considered both a moon and a planet.