Okay, hear me out – Mercury is the closest planet to every other planet in the Solar System, on average. When I read this it kind of blew my mind but after reading this article it makes a lot of sense. This is true because Mercury is on a very tight orbit around the sun. The other planets, because of their much larger orbits, spend a lot of time very far away from each other. Therefore, Mercury is the closest planet to all the other planets! The article hyperlinked above has a model that shows this quite clearly. The man runs a model over 10,000 years and calculates the distances each day and averages them out to find that Mercury is the closest planet to Earth around 45% of the time, Venus is 35% and Mars is the closest 20% of the time.