Pluto

• Region: Kuiper Belt
  
  

• Diameter: 2,376 km
  
  

• Mass: 1.31 × 10²² kg
  
  

• Orbit: 248 Earth years, semi-major axis ~39.5 AU, eccentricity 0.2488
  
  

• Rotation: 6.387 Earth days (sidereal day)
  
  

Surface & Geology:
Pluto has a highly varied surface. Its most famous feature, Tombaugh Regio (“the heart”), is a large, roughly heart-shaped nitrogen-ice plain. Surrounding it are mountain ranges composed of water ice, some rising 3–6 km high, indicating a rigid bedrock capable of supporting such heights. Other regions, like the dark, reddish Cthulhu Macula, are coated in tholins, complex organic molecules produced by solar radiation interacting with methane. The surface also includes nitrogen glaciers, cratered terrains, and possible cryovolcanic structures.

Atmosphere:
Pluto has a very thin atmosphere composed mainly of nitrogen (N₂), with traces of methane (CH₄) and carbon monoxide (CO). The atmosphere expands and contracts depending on its distance from the Sun. Sublimation of surface ices contributes to seasonal changes in atmospheric pressure, ranging from microbars to a few tens of microbars.

Moons:
Pluto has five known moons:

• Charon: Diameter 1,212 km, nearly half the size of Pluto. Orbits at 19,570 km; the system is sometimes considered a double dwarf planet.
  
  

• Nix, Hydra, Kerberos, Styx: Small irregular moons, discovered via Hubble observations, with diameters between 10–50 km.
  
  

Internal Structure:
Models suggest a differentiated interior: a rocky core beneath an icy mantle. Evidence from surface geology and tectonics indicates the possibility of a subsurface ocean, likely composed of water and ammonia, which could remain liquid due to residual heat.

Temperature:
Surface temperatures range from –240°C to –210°C, cold enough to keep volatiles like nitrogen and methane frozen.

Discovery:
Discovered in 1930 by Clyde Tombaugh, Pluto was initially considered the ninth planet until 2006 when it was reclassified as a dwarf planet after the discovery of Eris and other similar-sized Kuiper Belt Objects.