This is the way I look at it.
John studies harder than any other student - John is a student and I am comparing him to other students.
My husband is learning English by himself and he studies harder than any student (of mine) - John isn't a student and I am comparing his efforts to those of a different category of learners. Therefore I don't use "other".
Jupiter is a planet, which is being compared to other planets in the sentence above.
Hope it helps.