For scientists who spend time thinking about how planets form, life would be simpler if gas giants like Jupiter and Saturn didn’t exist. According to the standard model of planet formation, called "core accretion," planets form over millions of years as enormous blocks of rock and ice smash together to form planetary embryos, called "protoplanets," and eventually full-fledged planets. Most scientists agree that core accretion is how terrestrial planets such as Earth and Mars were created, but the model can’t convincingly explain how gas giant planets like Jupiter and Saturn came to be. One major problem is that developing gas...