kuiper-belt

Triton orbits Neptune backward, spews nitrogen geysers from a surface colder than anywhere else in the solar system, and is gradually spiraling toward a catastrophic end. It almost certainly came from the Kuiper Belt — and its violent capture may have reshaped the entire Neptunian system.

New Horizons passed 12,500 km above Pluto on July 14, 2015, the only spacecraft ever to do so. It found a nitrogen glacier churning in convection cells, water-ice mountains, layered blue haze, and a possible buried ocean, then spent 16 months sending the data home.

Why Pluto was demoted in 2006: the IAU's three-part planet definition, the discovery of Eris, New Horizons data, and the ongoing scientific debate.

Oort Cloud vs Kuiper Belt: two icy reservoirs at the edge of the solar system. Sources of comets, Planet Nine, interstellar visitors, and Voyager.

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.