who named neotune the planet and why?

Answer 1

Neptune was first discovered in a means a little different from other planets. It's existance and location was hypothesized by observing slight variations in the orbit of Uranus (which had only recently been discovered and not even solidly named).

Urbain Le Verrier was an astronomer who crunched the numbers and was one of the first to suggest that Neptune existed. And for a while, it was just referred to as "Le Verrier's planet".

Until it was officially settled, though, there were LOTS of different names being flung around for both Neptune and Uranus, and those planets appeared under a variety of different names in different pieces of literature at the time. It was Le Verrier again who was the first to suggest 'Neptune', it was broadly accepted by others as a name they could agree on, and the Academy of Sciences put its official stamp on it.

It probably met with such broad acceptance because the name is very much in keeping with the naming system for the other planets which were all bestowed in ancient times. As to why he chose that specific god... it would be hard to say. Perhaps it was just one that others hadn't suggested yet and was thus free of partisan bickering, or perhaps it was because the bluish colour of Neptune was reminiscent of the sea so it seemed natural to name it for the god of the sea.