c# What makes the difference between the two this keywords in the constructor below?

I don't understand the following: In the code found below, Visual Studio tells me, that I can simplify my code by deleting the second this keyword in the constructor.

But why can't then I do away with the first this keyword? Both variables were declared outside the constructor, in the class, so both will be "overridden"* for the instance.

If I remove both this keywords, then VS will complain that the first assignment is made to same variable, but not for the second. The only obvious difference to me is the second variable is an array, but I do not understand how would that make it any different?

*I suspect override is not the correct term here.

public class CelestialObject {

    CelestialBody[] celestialBodies;
    int celestialBodyCount;

    public CelestialObject(int celestialBodyCount = 2) {
        this.celestialBodyCount = celestialBodyCount;
        this.celestialBodies = new CelestialBody[celestialBodyCount];
    }
}