Which definition is the best for “semipermeable membrane”?

Ascientist discovers a new body between the orbit of neptune and the kuiper belt. the object is round and travels in an orbit around neptune with other space objects. the scientist claims that she has found a new dwarf planet. where is the scientist’s error? the object is an asteroid, not a dwarf planet. the object is a moon, not a dwarf planet. the object is not a dwarf planet because it travels with other objects. the object is not a dwarf planet because it is round.