WordNet records no synonym and no antonym for neptune. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(Roman mythology) god of the sea; counterpart of Greek Poseidon
a giant planet with a ring of ice particles; the 8th planet from the sun is the most remote of the gas giants
“the existence of Neptune was predicted from perturbations in the orbit of Uranus and it was then identified in 1846”
Synonyms, antonyms and the broader and narrower relations for neptune come from Princeton University’s WordNet. Full sources and licences.