The original definition of neptunian is a gender linked to the void and to a soft, celestial, masculine energy. It is never hyper masculine. It is not male-aligned, simply linked. Neptunian people may feel different amounts of masculine energy at different times.
Under this definition the feminine equivalent to neptunian is venusarian.
Under the rework of the celestial gender system, neptunian is also known as merjuparian. It is defined as someone who is both juparian and mercurian. One could be both at the same time or they might be fluid between the two. They may feel different amounts of each gender at different times. Under the rework the original definition of neptunian is included under sonnian.
The term neptunian was coined by an anonymous ask on Juparian's Tumblr blog on November 9, 2017. The flag was created on the same day by Juparian. However the definition of many of the celestial genders were altered in a rework by System-Lgbt in an attempt to make it more consistent.
The flags colors of both flags are a reference to the colors of Neptune. The original flag's colors are also a reference to the void.