Definition of protoplanetary in US English:

protoplanetary

adjective

Astronomy
  • Of, relating to, or of the nature of a protoplanet.

Origin

1950s. From proto- + planetary, partly after Russian protoplanetnyj, and partly after protoplanet.

Pronunciation

protoplanetary

/ˌprəʊtə(ʊ)ˈplanɪt(ə)ri/