your code
planet(400,400,"neptune");
alien(400,100,400,250);
alien(200,200,400,200);
planet(300,550,"venus");
alien(400,400,400,650);
fuel(300,400);
planet(200,400,"earth");
planet(550,450,"mars");
planet(550,550,"mercury");
fuel(500,550);
planet(250,550,"uranus");
alien(300,550,300,750);
mothership(200,600);