Equatorial Radius of Neptune

Neptune presents the solar system's tidiest paradox with its neighbour: 800 km smaller in radius than Uranus, yet 18 per cent more massive. More mass compressed into less volume makes it the densest of the four giants, presumably richer in rock and ice relative to its gas envelope — although why two planets so alike in size ended so different in constitution is unresolved, one of many questions carrying the case for a return mission. The radius itself rests on Voyager 2's 1989 radio occultation, the only close measurement ever made.

The size held one more surprise: at the 1-bar level Neptune's disc is bounded by the fastest winds anywhere — retrograde equatorial jets near 2000 km/h, on the planet farthest from the Sun's driving heat. With an internal heat source radiating 2.6 times what it receives, Neptune runs its weather on leftover formation energy.