Follow paths at different speeds

New to most of this animation stuff…but I made a simple “solar system”…just three planets. I have them “orbiting” the sun, but they all go around at the same speed. In reality, Mercury goes around the sun a lot faster than Venus, and Earth goes slower than Venus. I have given the three planets individual bezier curves and constrained them each to follow their own paths. When I play the animation, they move OK, but all at the same speed. Is there a way to adjust the speed of each planet individually?

In your graph editor you will find a curve for your evaluation time. You can slow it down, speed it up, go backwards, continue on past 100 frames. N key remove modifier. Control click to replace the curve on 0 0 then again on 100 for the numbers going up and wherever you want to to end/ the numbers left to right that is the frames. I believe. As long as the curve continues up the object will move along path. you will want a straight curve…Ya that is what I meant.
Edit Key interpolation mode linear. To make the speed the same. The " straight curve"

There is no F-curve in the graph editor, since there are no keyframes. All lines (X, Y and Z) remain flat throughout the animation. But what I want is for each planet to go around the sun at different speeds from each other. But each planet should go around at its own speed at a steady speed. For example, Mercury should go around once in 88 frames, Venus in 225 frames and Earth in 365 frames.

select the curve that controls the object.
look for the Properties->path animation->frame and change the number of frames on the evaluation for each planet.

Thank you! That is exactly what I wanted to do.

There is no F curve in the f curve editor? why do I wast my time? Keep your ignorance if you like it so much.

There’s no need to over react!
To complete Kazinger’s point, the rotation speed can also be controlled with the parameters of the modifier on each of the curves:

As you can see the inclination of the straight lines will make the orbiting go at different speeds.

The reason there was no F-curve in the graph editor was because I had the “planet” selected, and not the path (Bezier circle). When I selected the path as cegaton suggested, the F-curves were there. It was also very easy to control the speed(s) using the properties panel for each path. Thanks again, cegaton. Kazinger, I think you misunderstood my comment. But you should realize that some of us are new to Blender animation, so your mean reply was not appreciated.