-- When Galileo decided to point his toy telescope at things in the sky, he discovered moons circling the planet Jupiter. Right now, in 2016, something like 65 moons of Jupiter are known. Galileo saw four of them. You can see those too, if you have binoculars and you know how to find Jupiter in the sky.
-- The Moons aren't "on" Jupiter; they follow orbits around it, at some considerable distance from it.
-- Galileo didn't 'discover' Jupiter. Jupiter is quite visible to the unaided eye, and has been for several thousand years. That's a big part of the reason why Galileo decided to give it a look-see with his telescope.
