Over the past three decades, astronomers have discovered planets orbiting Sun-like stars throughout the universe. This discovery ended 2,500 years of debate about whether worlds existed beyond our solar system, but it came with a shock. The most common kind of planet in the universe is the type of world that doesn’t exist in our small corner of the cosmos: what astronomers call ‘Super-Earths’ and ‘Sub-Neptunes’, planets with much greater masses than ours and which could, in theory, sustain life.