Pluto 

A different sort of animal altogether....




Diameter: 2274 km (smaller than the Moon)
Mass: 1.27x1022 kg (5.6x smaller than the Moon)
Density: 2000 kg/m3 (how does this compare with other objects in the solar system?)
Average Distance from Sun: 39.5 AU
Orbital Period: 248 years!

Discovery: Variations in Neptune's orbit suggested another planet beyond Neptune. Calculated positions first given in 1905, but Pluto not discovered until 1930. Why so long?
 
 

Pluto has very eccentric, inclined orbit, unlike the rest of the planets. It actually crosses the orbit of Neptune (although collisions will not happen).
 
 

Estimates of mass, size, density all extremely poor until 1977, when Pluto's satellite Charon was discovered:

Charon is a big moon (compared to Pluto) -- 50% the size of Pluto, 14% by mass.
 

Best (computer generated) images of Pluto's surface (from the Hubble Space Telescope):

Atmosphere: Pluto seems to have a very thin atmosphere of methane - perhaps from methane ices evaporating when Pluto closer to Sun (like it is now?).


What is Pluto?