What did johannes kepler contribute to the study of planets? he formulated the universal law of gravitation. he identified the first moons of jupiter. he collected detailed data that led to the proposal of the heliocentric model. he provided mathematical support for the heliocentric model.

Kepler used the data collected by his teacher, Tycho Brahe, and showed how
beautifully it matched the heliocentric model.

Later, Newton came along and provided mathematical support for Kepler's work
and for that model.

-- Kepler used Tycho's data and showed that each planet moves in an elliptical
path, with the sun at one focus of the ellipse.

-- Kepler used Tycho's data and showed that the line from the sun to each planet
sweeps over equal areas in equal units of time ... each planet moves faster in its
orbit when it's closer to the sun, and slower when it's farther from the sun.

-- Kepler used Tycho's data and showed that the ratio of (the square of the
planet's orbital period) to (the cube of the planet's distance from the sun) is
the same number for every planet.

Kepler didn't say WHY the planets are organized like this, or why they move
like this.  And he didn't discuss any reasons why they SHOULD.  He just took
the observations, measurements, and data that Tycho Brahe spent HIS life 
collecting, and figured out what the planets ARE doing.

A hundred years later, Newton proposed his laws of motion and gravity.
Newton showed that IF his laws of motion and gravity are correct, then the
planets MUST do all the things that Kepler described.
(I used to be able to do that with Newton's laws, but it's been a really long time.)