Add a pushbutton on the same plot and provide a callback to that Pushbutton. In a callback function, plot the planet at a distance and size you want just like you displayed other planets moving around sun in original function. To specify the distance and size, you can make use of Edit box. Once the string of the Edit box is updated, you can use those values in Pushbutton callback to add the planet with appropriate distance and size. I hope this helps.