Pluto
Discovered in 1930 by Clyde Tombaugh at Lowell Observatory. Demoted from planet to dwarf planet by the IAU in 2006 — a decision that remains genuinely controversial among planetary scientists, if not astronomers generally.
New Horizons flew past on July 14, 2015 after a 9.5-year journey. The heart-shaped nitrogen ice plain (Tombaugh Regio) was not expected. Neither were the mountain ranges of water ice reaching 3,500 m, the hazy nitrogen atmosphere, or the evidence of recent geological activity on a body this far from the Sun.
| Property | Value |
|---|---|
| Diameter | 2,377 km (66% of Moon) |
| Mass | 0.00218 Earth masses |
| Distance from Sun | 39.5 AU (avg) |
| Orbital period | 248 Earth years |
| Surface temp | −223°C |
| Moons | 5 (Charon, Styx, Nix, Kerberos, Hydra) |
Charon is so large relative to Pluto (half its diameter) that the two orbit their common barycenter — a point in space between them, not inside either body. Some astronomers classify them as a double dwarf planet system.