Our solar system is very well cataloged. We know all the planets, their moons and almost all the meteors that reside in our solar system. However, once you get out side of our solar system we have a few planets, but no moons...until now. Recently the first Exomoon (exo meaning outside our solar system) has been found. The host Exoplanet in question is called MOA-2011-BLG-262 and is about the size of 4 Jupiter's with its supposed moon smaller than the Earth.
All this "supposed" stuff is because the moon hasn't actually been seen, the only reason we think it is there is because of how the light from that region bends and distorts due to the gravitational pull of the planet. The process is called gravitational lensing and is what we use to find most exoplanets as most telescopes aren't powerful enough to actually see the planet.