Astronomers May Have Discovered First Moon Outside Our Solar System

Astronomers studying the star system CD-35 2722 may have found the first known moon-like object outside our solar system. The classification is unusually tricky, however, because it orbits a brown dwarf rather than a planet, making it clearly an "exosatellite" but forcing scientists to rethink where the line between planet, moon, and failed star should be drawn. Space.com reports: 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 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.

The findings have been published in the journal Nature.

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