Well we can have our own preferences on this, but 'earnt' does exist, and even Dickens used it.
'There is conflicting advice on the origin of 'earnt'. Some internet sources state that it is an archaic (out of date/use) form of earned. They will point out that Charles Dickens used it in 1884, yet unlike learnt, dreamt and smelt, 'earnt' does not feature in the OED as a variant. A google search (March 2009) reveals 14,100,000 instances of earnt, but the British National Corpus (1980--1993), which has 100 million words, contains only eight instances of earnt.'