The crossword solutions for the term 'MERCURY, BUT NOT EARTH' are 'GOD', which is a 3 letter word. The term refers to the fact that Mercury is a planet in our Solar System, but Earth is not. Mercury is the closest planet to the Sun and is named after the Roman GOD of commerce, travel, and thievery. Earth, on the other hand, is the third planet from the Sun and is the only known planet to support life.