Pluto
The exiled dwarf planet
Summary
Pluto is an icy dwarf planet in the Kuiper belt, beyond Neptune, in one of the most remote corners of the Solar System. For 76 years it was counted as the "ninth planet"; in 2006 it was reclassified as a dwarf planet. In 2015 NASA's New Horizons spacecraft photographed it up close for the first time, revealing a vast heart-shaped plain of ice on its surface.
In depth
Pluto was discovered in 1930 by the 24-year-old Clyde Tombaugh at Lowell Observatory, the only "planet" ever found by an American. Its diameter is just 2,376 km (smaller than the Moon!), and its mass is 0.2% of Earth's. Its orbit is very eccentric (e ≈ 0.25) and crosses Neptune's, so from 1979 to 1999 it was closer to the Sun than Neptune. In 2006 the International Astronomical Union's new definition of a "planet" (which requires it to have cleared its orbital neighbourhood) removed Pluto from the list: it shares its orbit with thousands of similar bodies in the Kuiper belt. The New Horizons flyby in 2015 completely renewed our knowledge of it: active geology, a thin nitrogen atmosphere, a complex climate and huge mountains of ice. Its moon Charon is so large that the two orbit a shared centre of mass (barycentre) that lies between them; it is the only system in the Solar System that can truly be called a "double dwarf planet".
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