at 102.413kg, neptune's mass puts the planet between earth and the largest gas giants. neptune is the fourth largest planet in the solar system and third heaviest. neptune is 17 times the mass of earth, but just 1/18th the mass of jupiter. neptune is a little bit more massive than uranus, though neptune is denser and physically smaller than uranus. neptune and uranus are often considered to be part of a sub-class of a gas giant known as "ice giants". it is given their smaller size and big differences in composition compared to jupiter and saturn. in the search for extrasolar planets, neptune has been used as a reference to compare the size and structure of the discovered planet. some discovered planets that have similar masses like neptune are often called "neptunes".