These each react very differently to the forces present during the formation of planetary system, which the paper also defines as two different categories. Disk-drive migration (DDM) is a “gentle” process of dragging a planet toward the star using gravitational pull, and typically happens early in the formation of a system. It results from a gravitational nudge, either from a companion star or another planet in the system, that throws the entire system into gravitational chaos. Miller (Caltech/IPAC)The “ridge” that holes the overabundance of Neptunes, appears to be where many exo-Neptunes end their gravitational journey of both DDM and HEM. As they begin to, maybe they’ll come up with another great acronym for their planetary evolution theory - A Realistic Reckoning About Kool Interplantery System - ARRAKIS - anyone?