First exomoon may have been discovered

NASA’s Kepler space telescope has received signal of what could potentially be the first discovered “exomoon,” a moon orbiting a planet beyond our solar system. Astronomers believe the candidate moon, which lies some 4000 light-years from Earth, is about the size and mass of Neptune, and is orbiting a planet the size of Jupiter but with 10 times the mass. Further observations of the exomoon using the Hubble Space Telescope will take place in October.

October 2018 Update:

Hints of a potential alien moon strengthen with Hubble data