To determine exactly why Kepler-107c is so dense, first the researchers considered what they already knew. Previous research has shown that intense stellar radiation can strip the atmosphere from a planet that sits too near its host star. But if the innermost planet lost its lighter atmospheric elements, it should be more dense than its twin, not less. According to the study, this would "make the more-irradiated and less-massive planet Kepler-107b denser than Kepler-107c," which is clearly not the case. However, there is another way that a planet can lose a lot of mass: by getting smacked with another planet....