Neptunian adj.

late 14c., "Roman god of the sea," from Latin Neptunus, the Roman god of the sea (son of Saturn, brother of Jupiter, later identified with Greek Poseidon), probably from PIE root *nebh- "cloud" (source of Latin nebula "fog, mist, cloud"), via a sense of "moist, wet."

The planet so named was discovered by German astronomer Johann Gottfried Galle (1812-1910) on the night of Sept. 23-24, 1846 and named by French astronomer Urbain Le Verrier (1811-1877), who had predicted its position based on anomalies in the motion of Uranus and sent the coordinates to Galle. It is too dim to be seen with the naked eye, but it had been seen by observers using telescopes as far back as Galileo, but they did not recognize and identify it as a planet. Until the identification of Pluto in 1930 (and since that planet's demotion), it was the most distant known planet of the solar system.

variant of suffix -an (q.v.), with connective -i-. From Latin -ianus, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective. In Middle English frequently it was -ien, via French.