You have it backwards. Kepler came first. He analyzed a lifetime of
measurements and observations that described the motions of the planets
as we see them move in the sky. From these observations, he came up
with a model of how the solar system could be organized, and he boiled
a planet's motion down to three simple-looking laws.
A century later, Newton proposed his law of universal gravitation, and showed
that IF gravity works according to his law, THEN planets MUST move according
to Kepler's laws.
