The word "protoplanetary" refers to something that is in an early stage of becoming a planet. It describes objects or processes that are related to the formation of planets in space. Full definition
We present a new method to estimate the maximum ionizing (13.6 - 100eV) luminosity reaching the surface of protoplanetary disks around Cool Stars. (www2.lowell.edu)
In order to answer this question, understanding the formation of protoplanetary disks as well as the associated chemical evolution in various star forming regions is essential. (sciencedaily.com)
The colour distribution observed is characteristic of material rotating in protoplanetary disks around the protostar. (almaobservatory.org)