new-horizons

Pluto has an atmosphere. In 2015, New Horizons measured its surface pressure at about 1 Pascal — roughly 1/100,000th of Earth's at sea level. That atmosphere is not stable. Over Pluto's 248-year orbit, it inflates and then freezes entirely out onto the surface, cycling between a thin gaseous envelope and nothing at all. We arrived just in time to see it while it still existed.

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.

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.