Neptune is the other sub-jovian planet, along with Uranus.   It presents a number of differences from either Jupiter or Saturn.  It is quite a bit colder and denser.  The blue color of the clouds is due to the presence of methane gas in the atmosphere.  We believe there is an appreciable layer of ice within the interior of the planet.  With no layer of metallic hydrogen to generate a magnetic field, Neptune apparently uses a layer of highly compressed water as the electrically conducting fluid.  The existence of the eighth planet was revealed to humans in the year 1846. Two men had independently calculated the supposed planet’s position using its gravitational influence on Uranus.  Who discovered Neptune? Was it John C. Adams or Urbain Leverier? I guess the answer to that depends on whether you are French or a loyal British subject. Although the first person to see Neptune, and know that it was a planet, was Johann Galle?
Carpe Caelum Planetary Astronomy