Solar Exploration

Pluto was considered the ninth planet from its discovery in 1930 until 2006, when the International Astronomical Union reclassified it as a dwarf planet after the discovery of similarly sized objects in the same distant region, the Kuiper Belt. Despite the reclassification, Pluto remains one of the most beloved and scientifically interesting bodies in the outer solar system.

Pluto's largest moon, Charon, is so massive relative to Pluto — about half its diameter — that the two don't quite orbit a point inside Pluto at all; they orbit a shared center of gravity in the space between them, making the pair function more like a double dwarf-planet system than a planet-and-moon.

NASA's New Horizons flyby in 2015 revealed a far more active world than expected: towering water-ice mountains, nitrogen-ice glaciers, and a striking heart-shaped plain called Sputnik Planitia, a basin of frozen nitrogen so large it's thought to gently reshape Pluto's rotation over time. Pluto's thin atmosphere isn't constant — as it moves away from the Sun on its 248-year, sharply elliptical orbit, its atmosphere gradually freezes onto the surface, only to sublimate back into a haze as it swings closer again.