Thanks to this question, I now know what 4x stands for, so thanks :)
Anyhow, I would say no, but it depends.
I think a classic example of classes is cars. Cars have several properties (color, make, model, weight, whatever suits you...) and several methods - say, gas, break.
Now, in the programming representation, if you were to, say, create a bunch of cars that share a certain set of properties and don't add more, then you would stick to a single class. For example, if cars had only a color and no other properties unique to each, then you would want one class. But if a specific car has special properties/behaviors - let's say, some cars can transform into some flying thing and some cannot. Flying objects will have different properties then the normal one, and so a new derived class is appropriate.
IF you think about inheritance, the general concept is, the base class is what they all have in common. Derived classes each need to differ in some respect.
So consider your planets. If each planet has, say, only a texture, position, rotationSpeed, and nothing else - then you can just craft different planets (like different colored cars) from this one class. On the other hand, if planets have special properties or methods, then you could make special classes for each planet, though they should still have SOMETHING in common and thus inherit from a Planet class. For example, if the gas planet can explode, dissapear, or has new fields (say, something about the atmosphere), then you can make a specific class for it, assuming a BarrenPlanet cannot do the same things.
In conclusion, it depends, but from what I can gather you could get by with a single Planet class (choose based on your circumstance, though), and as Josh said, needlessly adding a bunch of specific Classes with the inheritance will just make it confusing.