Four hundred years ago, the Italian scientist Galileo Galilei claimed that in the absence of any atmosphere, all bodies on earth (e.g. a hammer and a feather) would fall to the ground in the same speed, since there would be no air resistance. Watch this youtube video, taken by the Apollo 15 astronauts who landed on the moon (in 1971). Now imagine a planet (called “Planet X”) in which the same experiment is performed, with the result that the hammer reaches the ground before the feather. What would you deduce about Planet X?