海王星

Proper noun

 * 1) Neptune eighth planet of the solar system

Etymology
Appears to be borrowed from, applying the expected Japanese for the characters.

The planet itself was, so prior to that point, the planet had no name in any language. Following the convention at the time, it was, Neptune. After this, the term was probably first borrowed into Chinese as a calque and then borrowed into Japanese from there. and.

Proper noun

 * 1) Neptune