looks like you show
image(img,x,y) ;
only once when ?a key is pressed?
try to use the keyPressed function
and if key [a] is pressed
toggle a class boolean variable
boolean showit = false;
and use it for the show
if ( showit ) image(img,x,);
add:
void keyPressed() {}
if ( key == 'a' ) planets[0].showit = ! planets[0].showit;