Pluto ⯓

Pluto has a thin, tenuous atmosphere composed primarily of nitrogen with traces of methane and carbon monoxide. This atmosphere expands when Pluto is closer to the Sun and partially freezes onto the surface as the dwarf planet moves farther away. New Horizons measurements in 2015 revealed the atmosphere extends much higher than expected and has multiple haze layers that create a blue-tinted sky when viewed from the surface.

Pluto's surface features a dramatic heart-shaped region called Tombaugh Regio, named after its discoverer Clyde Tombaugh. The western lobe, Sputnik Planitia, is a vast nitrogen ice plain that shows evidence of convection cells, suggesting the ice is still geologically active. Pluto has five known moons, the largest being Charon, which is so massive relative to Pluto that the two bodies orbit a common center of gravity located between them -- essentially forming a binary system.

NASA's New Horizons spacecraft made its historic flyby of Pluto on July 14, 2015, after a nine-year journey. The encounter transformed our understanding of Pluto from a blurry dot into a complex, geologically diverse world. New Horizons revealed towering mountains of water ice, flowing nitrogen glaciers, and a surprisingly young surface with very few craters. After passing Pluto, New Horizons continued into the Kuiper Belt and flew past Arrokoth (2014 MU69) on January 1, 2019.