Mercury
Mercury is the closest planet in our Solar System to the Sun, and has the shortest orbital period of all the planets. Its rotation around the Sun is unique, for every two revolutions it makes it rotates on its axis exactly three times, this phenomenon is known as 3:2 spin orbit resonance. NASA’s MESSENGER visited Mercury in 2011 for four years but there is still muh more to be learnt about the planet. Mercury is the least explored planet of our Solar System, and has the oldest surface with the largest daily variations in temperature due in part to Mercury’s lack of atmosphere.