This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Of or pertaining to a protoplanet.

"In order to determine whether the surface of an extrasolar planet would contain water, scientists can look at what is called the protoplanetary disk - a disk of gas and dust surrounding a star during its early stages of development."

unformatted    white-yellow    unlubricated    protostar    magneto-optic