Astronomers are grappling with a newly documented planetary system within the Milky Way galaxy that bears so little resemblance to our own solar system that they can’t quite agree on what to call the objects within it.
At the center of the system is a red dwarf star — roughly 40% of the mass of our sun — which is orbited by a brown dwarf, a celestial object that falls somewhere between a planet and a star. That brown dwarf is then orbited by a large, gas-filled world about the size of Jupiter. The Jupiter-like object was identified using the Very Large Telescope operated by the European Southern Observatory in Chile.
Under the definitions we use for our own solar system, any object that orbits another body that itself orbits a star would typically be called a moon — or, since this is outside our solar system, an exomoon. But the object circling this brown dwarf is nothing like the moons in our solar system, which tend to be relatively small, rocky or icy bodies. This one is far larger and composed very differently.
So what exactly do you call it?
“This will certainly be a matter of debate. In general, most astronomers are fine with the term exosatellite, at least as a placeholder until we formally adopt definitions for objects like the one we found,” said Kevin Hoy, a doctoral student in astrophysics at Universidad Diego Portales in Chile. Hoy is also affiliated with the exomoon research group YEMS and serves as lead author of the study, which was published in the journal Nature.
“We’re really hitting the limits of how far we can stretch the words we invented to describe the solar system to describe other systems in this case,” Hoy added.
The system sits approximately 71 light-years from Earth. To put that in perspective, one light-year — the distance light covers in a single year — equals about 5.9 trillion miles, or 9.5 trillion kilometers.
The exosatellite carries at least 90% of Jupiter’s mass and takes 170 days to complete one orbit around the brown dwarf, traveling at a distance roughly one-fifth of the gap between Earth and the sun.
Study co-author Alice Zurlo, an astrophysicist at Universidad Diego Portales and director of YEMS, described the find as a game-changer for the scientific community.
“I think this discovery has opened a Pandora’s box for the scientific community. This type of object is completely new and unusual, truly different from what we are familiar with in our solar system. Now we will have to understand, through theoretical models, how these kinds of objects form and whether they are common or not,” Zurlo said.
Zurlo also explained why the team chose not to label the object a moon. “We deliberately chose not to call it a moon. That’s because this object is unlike any moon in our solar system. It’s much more massive, and it doesn’t orbit a planet — it orbits a brown dwarf, an object that’s somewhere between a planet and a star. Since we’ve never seen a system quite like this before, we’re still figuring out the best way to describe it,” she said.
While scientists have catalogued more than 6,300 exoplanets — planets located outside our solar system — finding exomoons has been significantly more difficult, with only a handful of strong candidates ever identified.
Researchers are still working to determine how the exosatellite came to exist. “It may have formed from the disk of gas and dust around the brown dwarf, just as planets form around stars. Or it may have been captured gravitationally by the brown dwarf later on,” Zurlo said.
Brown dwarfs occupy an unusual middle ground in the universe — too massive to be classified as planets, yet never large enough during their formation to trigger the nuclear fusion that powers stars. The brown dwarf in this system is roughly 33 times more massive than Jupiter, about 50% larger in size, and considerably hotter. “Because the system is very young, the brown dwarf is still shining with the heat left from its formation,” Zurlo said.
Since exoplanets were first detected in the 1990s, scientists have observed a wide range of planetary systems — some resembling our own, others quite different. But this one stands apart even from those.
“Yep, this system is definitely weird. There was likely a pretty chaotic dynamical history that left it in its current state, but it’s really hard to tell exactly what kind of chaos it went through,” Hoy said.
