Usage History of 'protoplanet'

The following chart shows how frequently 'protoplanet' appeared in printed materials over time. For example, the word 'protoplanet' occurred 4 times in books in 1872 and 204 times in 2008. From 1872 to 2008, the word 'protoplanet' was used a total of 15,392 times in books scanned by Google. See Reference: Google Ngram project.

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