- the collection of matter, in the process of condensation, from which a planet is formed.
Origin of protoplanet
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
- a planet in its early stages of evolution by the process of accretion
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for protoplanet
Online Etymology Dictionary, © 2010 Douglas Harper