Two-part formation of the Solar System Measurements of meteorites have shown that the inner and outer Solar System formed from two distinct reservoirs of material. Existing models have proposed that these were split by Jupiter forming first, which would open a gap in the protoplanetary disc. Lichtenberg et al. instead argue that the snow line, the boundary between regions containing water vapor and solid ice, migrated first outward and then inward, forming two separate populations of planetesimals. Those planetesimals then grew through collisions to form the planets. Their simulation of this model explains the meteorite data and is consistent with astronomical observations of protoplanetary disks around other stars. Science , this issue p. 365