Far smaller and closer to the Sun than it should be, Mercury has long baffled astronomers because it defies much of what we know about planet formation. A new space mission arriving in 2026 might solve the mystery. At a cursory glance, Mercury might well be the Solar System's dullest planet. Its barren surface has few notable features, there is no evidence of water in its past and the planet's wispy atmosphere is tenuous at best. The likelihood of life being found amidst its scotched craters is non-existent. Yet, look closer and Mercury is a fascinating, improbable world that is shrouded in mystery. Planetary scientists remain flummoxed by the very existence of the closest planet to our Sun. This peculiar planet is tiny, 20 times less massive than Earth and barely wider than Australia. Yet Mercury is the second densest planet in our Solar System after Earth due to a large, metallic core that accounts for the majority of its mass.