Confusion between "planet" and "star" in Cantonese?
we may assumed the listener know what the utterance means, according to the context.

another approach is to pronounce 恆星 as 
hang1　sing1

meanwhile, deliberately pronounce 行星 as haang4　sing1

this one is what i learned in primary school, a long time ago.

actually, in cantonese, we pronounce 行 as haang4 in 行街, 行船.