Yes, EARTHNUT is a Scrabble word!

Letter values in Scrabble US: E 1, A 1, R 1, T 1, H 4, N 1, U 1, T 1.

Is it a valid word?

EARTHNUT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

any of various highly prized edible subterranean fungi of the genus Tuber; grow naturally in southwestern Europe
a common European plant having edible tubers with the flavor of roasted chestnuts
pod of the peanut vine containing usually 2 nuts or seeds; `groundnut' and `monkey nut' are British terms
edible subterranean fungus of the genus Tuber

Also: earth-ball, conopodium denudatum, goober pea, monkey nut.