Orbiting the Sun at a mean distance of 30 AU, with a revolution period of almost 165 years, Neptune is the outmost of the giant planets, and also the smallest. Its diameter is 49,530 km, i.e., 3.9 times the terrestrial value. Still, its mass (17 terrestrial masses) is slightly greater than Uranus’ and its density (1.64 g/cm3) is thus also larger. Both Uranus and Neptune are called “icy giants” because most of their mass is contained in the icy core from which they accreted. As all other giant planets, Neptune is surrounded by a system of rings and satellites.