From unexplained flashes in the night sky to flying saucers, this episode delves into the mysterious world of UFOs. How our drive to explain these bizarre phenomena, and desire to discover little green men, has in fact transformed our understanding of the universe.

How can two such similar planets have become so different? One is the crucible of life, the other an inferno with a surface scorched by raining acid, yet both began as almost identical bodies. With Venus prominent in the sky in May, the team explores our nearest neighbour, discovering how it formed and how ESA's Venus Express spacecraft has revealed the secrets of its atmosphere.