WASHINGTON—Astronomers have pinpointed what appears to be the first moon detected outside our solar system, a large gaseous world the size of Neptune that is unlike any other known moon and orbits a gas planet much more massive than Jupiter.
The discovery was a surprise, and not because it showed that moons exist elsewhere—they felt it was only a matter of time for one to be found in another star system. They were amazed instead by how different this moon was from the roughly 180 known in our solar system.
