This illustration depicts the Sun-like star Kepler 51 and three giant planets that NASA's Kepler space telescope discovered in 2012–2014. These planets are all roughly the size of Jupiter but a tiny fraction of its mass. This means the planets have an extraordinarily low density, more like that of Styrofoam rather than rock or water, based on new Hubble Space Telescope observations. The planets may have formed much farther from their star and migrated inward. Now their puffed-up hydrogen/helium atmospheres are bleeding off into space. Eventually, much smaller planets might be left behind. The background starfield is correctly plotted as...