Two-step formation explains our solar system's dichotomy of planets By (0) Inner terrestrial protoplanets accrete early, and inherit a substantial amount of radioactive 26Al -- which researchers say explains the difference between smaller, dryer inner solar system planets and larger, wetter outer ones. Photo by Mark A Garlick Jan. 21 (UPI) -- The solar system features a unique dichotomy of planets -- the inner solar system features smaller, drier bodies, while the planets found in the outer solar system are larger and wetter. According to a study published Thursday in the journal Science, this divide is best explained by a two-step formation process.