The default analyzer (StandardAnalyzer) uses the unicode word break algorithm (http://unicode.org/reports/tr29/).
The properties assigned to emoji are just "other", so they are treated no differently than "trash" like ^, , etc. Sorry this would be my personal opinion of them, too.
Anyway, yes you will need a custom analyzer if you want to make sense of emoji. You will have to decide how to make sense of them, e.g. if each should be its own word, or if someone writes 87 smileys in a row, what should happen then, and so on.