outer solar system

Pluto was reclassified as a dwarf planet in 2006, but the New Horizons flyby in 2015 showed a geologically active world with nitrogen-ice convection, water-ice mountains, hazy skies, and hints of a buried ocean. Beyond it, the Kuiper Belt holds the best-preserved leftovers of planet formation.

Uranus and Neptune are the outermost planets in the solar system, built mostly from hot, dense water, methane, and ammonia rather than hydrogen and helium. Uranus lies on its side and gives off almost no excess heat. Neptune radiates about 2.6 times the energy it absorbs and drives the fastest winds measured on any planet. Only Voyager 2 has ever visited either world.