Hello, my practical knowledge of inheritance is thoroughly lacking and I've hit a bit of a roadblock, would like to know how to proceed.

My project's Main class is tracking an array of planets in a solar system:

public Planet[] planets = new Planet[_pNum];

The way my code is set up at the moment is that Planet is a base class for different planet types, Gas Giants, habitable, whatever. The way I'm trying to instantiate is as follows:

for(int i = 0; i < _pNum; i++)
       {
         if(i <= 1)
          planets[i] = ScriptableObject.CreateInstance("MP") as Planet;

         if(i == 2)
         {
          planets[i] = ScriptableObject.CreateInstance("Hab") as Planet;
         }

         if(i == 3)
         {
          if(0 == Random.Range(0,5))
              planets[i] = ScriptableObject.CreateInstance("Hab") as Planet;
          else
          {
              planets[i] = ScriptableObject.CreateInstance("TP") as Planet;
          }
         }

         if(i < 4)
          planets[i] = ScriptableObject.CreateInstance("GG") as Planet;

         if(i < 7)
          planets[i] = ScriptableObject.CreateInstance("IP") as Planet;
       }

So far every index in the array is returning null reference exceptions. Is there a way to make my current setup work or is there a correct way of doing this?