An artist’s impression shows the system around the star CD-35 2722, with the newly found moon-like object at the centre.
| Photo Credit:
ESO/M. KORNMESSER
Scientists are documenting a
planetary system in the Milky Way galaxy that is so starkly
dissimilar to our own solar system that they are struggling to
find the proper terminology to describe it.
The system includes a red dwarf star about 40% of the mass
of our sun that is orbited by a brown dwarf, a celestial body
that blurs the line between planet and star. The brown dwarf, in
turn, is orbited by a gaseous Jupiter-sized world, newly
discovered using the European Southern Observatory’s Chile-based
Very Large Telescope.
Using our solar system as a definitional frame of reference,
a body that orbits another body that orbits a star should be
called a moon — or in this case an exomoon, because it is beyond
our solar system. But the object that orbits the brown dwarf is
unlike any of the many moons in our solar system — all of which
are relatively small rocky or icy worlds — in terms of
composition and size.
So what should we 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 also
affiliated with the exomoon research group YEMS, lead author of
the study 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 said.
This system is about 71 light-years away from Earth. A
light-year is the distance light travels in a year, 5.9 trillion
miles (9.5 trillion km).
The exosatellite possesses at least 90% of the mass of
Jupiter, the gas giant that is our solar system’s largest
planet. It completes an orbit of the brown dwarf every 170 days
at a distance about a fifth of that separating Earth and the
sun.
‘A PANDORA’S BOX’
“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,” said study co-author Alice Zurlo, a
Universidad Diego Portales astrophysicist and YEMS director.
“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,” Zurlo said.
While more than 6,300 exoplanets — as planets outside our
solar system are known — have been discovered, finding exomoons
has proven more challenging, with only a few good candidates
identified.
The researchers are trying to figure out the origins of the
exosatellite.
“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.
A brown dwarf is neither a star nor a planet, but something
in between. They could be considered aspiring stars that during
their formative stages did not reach the mass necessary to
ignite nuclear fusion at their core like a star. But they are
more massive than the biggest planets.
The one in this system is about 33 times more massive, 50%
larger in size and much hotter than Jupiter.
“Because the system is very young, the brown dwarf is still
shining with the heat left from its formation,” Zurlo said.
Since the first detection of exoplanets in the 1990s,
scientists have spotted a wide variety of planetary systems,
some built much like our own and others quite different. The one
examined in the new research is an outlier.
“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.
Published on July 22, 2026
