Since the discovery of the first world beyond the solar system in the 1990s, astronomers have become adept at spotting extrasolar planets, or exoplanets. But while NASA’s exoplanet catalog has burgeoned to over 6,000 confirmed entries, moons around these worlds, known as exomoons, have proved elusive.
In fact, we have failed to make a single confirmed discovery of an exomoon despite being certain that just as the planets of the solar system have their own natural satellites, worlds in other star systems must have them too.
Now, this controversial drought of exomoons may have become even more confusing and, frankly, weird. That is because while studying a strange star system called CD-35 2722 with the Very Large Telescope (VLT), astronomers have discovered an object that could be an exomoon, or it could be something that forces us to rethink our very definition of what a “moon” is and what a “planet” is.
The star CD-35 2722 is located around 73 light-years away and has around half the mass of the sun. It is orbited by a “failed star” or brown dwarf. These stellar bodies get their unfortunate nickname because they form like other stars but fail to gather enough mass to trigger the fusion of hydrogen to helium in their cores. In terms of mass, brown dwarfs are more massive than the largest gas giant planets, but smaller than the smallest stars, usually with around 13 to 80 times the mass of Jupiter, or around 0.013 to 0.08 times the mass of the sun.
The newly discovered object in CD-35 2722 is certainly moon-like, but rather than orbiting a planet as the moons in the solar system do, it orbits the system’s brown dwarf.
“This system is somewhat hard to define using solar-system-based words like ‘planet’ and ‘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,” team leader Kevin Hoy of the Universidad Diego Portales and the Millennium Nucleus of Young Exoplanets and their Moons (YEMS) 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 team’s research was published on Wednesday (July 22) in the journal Nature.
Erm… It’s definitely an exosatellite
The team currently isn’t able to definitively claim this object in CD-35 2722 is an exomoon, because that would require really nailing down a new definition of what a moon is.
“The satellite we report is a giant gaseous body orbiting a highly massive companion, itself several times the mass of Jupiter. We have a clear delineation between the planets and the sun in the solar system, so defining things like moons is simple,” team member Alice Zurlo of the Universidad Diego Portales said. “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.”
Zurlo and colleagues can, however, confidently claim this is an exosatellite, meaning it is a first-of-its-kind detection no matter what the future holds for its classification.
It is hoped that the Extremely Large Telescope (ELT) currently under construction in Chile, will make a massive impact in the hunt for exomoons, and now, other exosatellites.
Until these objects begin to be uncovered, one thing this discovery re-emphasizes is something that astronomers have been gleefully discovering ever since the first exoplanet was spotted: planetary systems come in a variety of diverse, weird and wonderful forms.