He didn't discover Jupiter. He "discovered" that it had moons. The significance of the latter is that it contradicted ideas that all things in the universe revolved around the Earth. He did spend some time trying to figure out the orbit and periods of those moons, which was pretty damned hard given the data and technology available to him. He later worked on whether the moons of Jupiter produced eclipses, which he thought was evidence of Copernicanism (I admit to not particularly following the logic there). He also attempted to use the study of Jupiter's moons as kind of a "clock" that could be used to determine longitude, with the idea that he would produce detailed tables on their motion and sell them to governments.

To my knowledge this was the extent of his work on Jupiter. The relationship between orbit and mass was not yet known; it was not until Newton that anyone was able to estimate it (Newton published estimates of Jupiter's and Saturn's mass), and even then there was considerable revision by later astronomers and physicists (e.g. Laplace).