Pluto

Pluto sits at the threshold between the giant planets and the icy Kuiper Belt. Discovered by Clyde Tombaugh in 1930, it was the ninth planet for 76 years before the IAU's 2006 redefinition demoted it to dwarf-planet status. NASA's New Horizons flyby on July 14, 2015 transformed Pluto from a fuzzy dot into a complex world: nitrogen-ice glaciers flowing across the heart-shaped Tombaugh Regio, water-ice mountains rising 3.5 km, and a hazy blue atmosphere extending hundreds of kilometres into space.

A rock-and-ice body, roughly two-thirds rock and one-third water ice. The surface is dominated by frozen nitrogen with patches of methane and carbon monoxide ice. A thin atmosphere of N₂, CH₄, and CO freezes onto the surface as Pluto recedes from the Sun and sublimates back into vapour as it nears perihelion. Models suggest a possible subsurface liquid-water ocean kept warm by radiogenic heat — a tantalising target for future astrobiology.

Pluto's atmosphere is mostly nitrogen with about 0.5% methane and trace carbon monoxide. Surface pressure is roughly 10 microbars — a hundred-thousandth of Earth's. Sunlight breaks methane apart in the upper atmosphere, building haze layers that blue-tint backlit images. As Pluto's 248-year orbit carries it from perihelion (32.9 AU) to aphelion (48.7 AU), the temperature drops enough that most of the atmosphere freezes onto the surface, only to evaporate again 124 years later.

Sputnik Planitia, the western lobe of the Tombaugh Regio "heart", is a 1,000-km-wide basin of nitrogen-ice convection cells with no impact craters — its surface is renewing on geological timescales. The al-Idrisi Montes border the basin, with peaks of water ice rising 3-4 km. Cthulhu Macula is a long dark equatorial band stained by tholins (organic chemistry from UV-driven methane chemistry). The whole world is gravitationally locked to Charon, its largest moon — they always show each other the same face.

Pluto remained an unresolved disc to even the largest telescopes until Hubble produced the first crude maps in the 1990s. NASA's New Horizons launched in January 2006, flew past Jupiter in 2007 for a gravity assist, and screamed past Pluto and Charon at 14 km/s on July 14, 2015. The flyby returned 50 GB of imagery and spectra over 16 months. New Horizons continued into the Kuiper Belt, performing the most distant flyby in history at the contact-binary Arrokoth on January 1, 2019. There is currently no follow-up Pluto orbiter on the books.