1960s; earliest use found in Proceedings of the National Academy of Sciences of the United States of America. From proto- + solar, probably after protosun.
Definition of protosolar in US English:
protosolar
adjectiveˌprəʊtə(ʊ)ˈsəʊlə
Astronomy
Of or relating to the protosun.
Origin
1960s; earliest use found in Proceedings of the National Academy of Sciences of the United States of America. From proto- + solar, probably after protosun.