Is Pluto a Planet?

In order to answer this question, we need to know what criteria we will use to make our decision.  We previously established reasonable criteria for the definition of a planet.  Does Pluto qualify?

Let's start with what we know about Pluto:

  1. orbits the Sun at a distance of 39 AU with a period P = 247.7 yr
  2. has a very eccentric (elliptical) orbit: perihelion = 29 AU, aphelion = 49 AU at high inclination (i = 17) [most eccentric, highly inclined planet]
  3. mass = 0.0021 Mearth (1/500th Mearth) [smallest mass of any planet; Mercury is 0.05 Mearth]
  4. diameter = 2274 km (0.18 of Earth; 2/3 of Moon)
  5. surface gravity = 1/14th of Earth (moon = 1/6)
  6. rotation period = 6.39 days backwards (or upside down)
  7. has a moon (Charon).  D = 1300 km.  distance = 20,000 km.  orbital period = 6.39 days. rotation period = 6.39 days. [If Earth's Moon were only as distant as 15 times its diameter (50,000 km), then it would be six times bigger in angular extent and 36 times bigger in apparent size!]
  8. was closer to Sun than was Neptune from 1989-1999; this repeats once per orbit. But when this occurs, Pluto is always at least 60 degrees away from Neptune so no collision is possible due to 3:2 resonance (Pneptune = 164.8, so Ppluto/Pneptune = 247.7 / 164.8 = 1.503 = 3 : 2)
  9. has an atmosphere for 40 years out of each orbit, when temperature is warm enough for CO and CH4 and Argon to sublimate off surface during day, freeze back out at night.

  10.  
Given our knowledge, of Pluto, Is it a planet?
 answer