Pluto 

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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?

• A planet?
• it is denser than the icy moons of the outer planets -- higher rock content.
• it has its own moon.
• but it doesn't follow the orbital, structural, or chemical patterns of the planets.
• An ejected moon of Neptune?
• similar in size and mass to the moons of the gas giants.
• orbit crosses that of Neptune.
• dynamically difficult to do, though.
• A trapped proto-comet?
• density very similar to that of comets
• locked in a 3:2 orbital resonance with Neptune -- Neptune may have pulled and trapped Pluto into its current orbit.
• we are now finding more small icy objects out at the distance of Pluto ("Kuiper Belt objects"). Pluto may be a big example of these objects.