snow line" at a few AU and volatile-rich c outside. It will be interesting to see whether the usual migration mechanism that is invoked to alter planetary orbits-tidal interactions with the gaseous protoplanetary disk-could draw together two planets from such different regions of the disk. Or whether the compositions and densities of the planets could have changed with time, for example, because of the preferential erosion of the smaller planet's atmosphere by stellar irradiation (16) (fig. S21

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... ). Perhaps a combination of these factors will ultimately explain this puzzling pair of planets.