Astronomers Discover the First Known Moon Outside our Solar System

The brown dwarf “planet”—which orbits the star in the CD-35 2772 solar system—was discovered in 2011 using neither of these methods. Its huge, almost star-like bulk was great enough that astronomers could detect it with an ordinary optical telescope. The exosatellite, however, was a different matter. Even with its Jupiter-like mass it was too small to be seen from 71 light years away. Instead Hoy and his colleagues used the radial velocity method to look for wobbles in the brown dwarf. They spotted them and used the degree of the wobble to calculate the size of the world.

“Being the third wheel in this system makes us want to call it a moon,” said Hoy in a statement that accompanied the release of the study, “even if it is nothing like the small, rocky moons we have in our system.”

That fact raises the larger question in this discovery, which is essentially one of nomenclature. When is an almost-star actually a planet, and when is a Jupiter-like planet actually a moon? Astronomy typically has neater labels than that, but the CD-35 2772 solar system defies such tidiness.