Scientists may have found the first moon beyond our Solar System. Unlike every moon we know, this giant world does not orbit a planet.
Instead, it circles a brown dwarf – a strange object that falls between a planet and a star.
If confirmed, it would be the first moon-like object ever found beyond our Solar System.
A system that refuses to fit the rules
The system, known as CD-35 2722, is young and unlike anything in our Solar System. At its center is a star with about half the Sun’s mass.
Orbiting that star is a brown dwarf, an object too massive to be considered a planet but not massive enough to sustain the nuclear fusion that powers true stars.
The newly detected object circles the brown dwarf instead of the star. It is at least as massive as Jupiter, while the brown dwarf is more than 30 times Jupiter’s mass.
That unusual setup has left astronomers debating what to call it.
After studying the system for months, Kevin Hoy, an ESO student in Chile and lead author of the study, described it as “super weird” compared to our own.
A moon that might also be a planet
Scientists currently refer to the object as an exosatellite, meaning a natural satellite outside our Solar System.
The term exomoon is also commonly used for natural satellites beyond our planetary neighborhood, although there is no official definition that covers unusual systems like this one.
“This system is somewhat hard to define using Solar-System-based words like ‘planet’ and ‘moon’,” noted Hoy, who is also affiliated with the Universidad Diego Portales and the Millennium Nucleus of Young Exoplanets and their Moons (YEMS) in Chile.
“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.”
“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.”
Why brown dwarfs complicate things
Brown dwarfs occupy one of astronomy’s gray areas. They form much like stars but never gather enough mass to ignite long-lasting hydrogen fusion in their cores.
Because of that, brown dwarfs blur the line between stars and giant planets.
Alice Zurlo, YEMS Director and collaborator on the study, explained why that matters.
“The satellite we report is a giant gaseous body orbiting a highly massive companion, itself several times the mass of Jupiter,” said Zurlo.
“We have a clear delineation between the planets and the Sun in the Solar System, so defining things like moons is simple.”
“In the CD-35 2722 system, where we are blurring the lines between stars, planets, and moons, the whole thing becomes more complicated to describe.”
A long search may finally pay off
Astronomers have discovered more than 6,000 exoplanets over the past few decades. Finding moons around those distant worlds has been much harder.
Only a handful of possible exomoons have been reported so far, and none has been confirmed with strong enough evidence to settle the question.
Just a few months ago, another research team led by Quentin Kral found hints of a satellite in the HD 206893 star system using the European Southern Observatory’s Very Large Telescope Interferometer.
Those observations suggested a possible companion but stopped short of a firm detection. The new observations may change that.
Detecting an invisible companion
The research team used the CRIRES+ instrument on the European Southern Observatory’s Very Large Telescope.
They relied on the radial velocity method, the same technique that led to the discovery of the first exoplanet orbiting a Sun-like star.
Instead of looking directly at the suspected satellite, the researchers measured tiny back-and-forth movements in the brown dwarf.
Those small wobbles revealed the gravitational pull of an unseen object moving around it.
“This system is truly unique and represents a breakthrough: the first plausible detection of an exosatellite,” said Zurlo.
More moon-like worlds may be waiting
This evidence, while needing confirmation, is indicative of a time in which astronomers might discover an abundance of such peculiar systems.
Each such discovery allows scientists to understand better the development and evolution of planets, brown dwarfs, and their satellites.
With the launch of the European Southern Observatory’s Extremely Large Telescope and its huge 128-foot mirror and innovative equipment, the detection of even smaller exomoons will be possible.
As more of these distant worlds come into view, astronomers may have to rethink the labels that have guided planetary science for generations.
The full study was published in the journal Nature.
Image Credit: ESO/ M. Kornmesser
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