If your planet truly needs to be 100k + pixels and you need to be able to see the entire thing (be it through zooming or panning)... then use a mesh with a repeating planet texture, scale the texcoords accordingly.
Generating the points of a circle looks somewhat like this:http://codepen.io/anon/pen/orGCD
To determine an approximate number of steps given your radius, you can use something like this:
int steps = (int)(12 * Math.pow(radius/2f, 1/3f));
int steps = (int)(12 * Math.cbrt(radius/2f));
You can read about LibGDX meshes here:https://github.com/mattdesl/lwjgl-basics/wiki/LibGDX-Meshes
You can use triangles or triangle fan. Triangle fan is easy; just put a point in the center, then points for each circle segment (just like in the Code Pen). Triangles is not too much more difficult; the benefit of them is that you can place many in the same batch (in this case it doesn't seem necessary, since you've only got one planet).
Texcoords would be calculated kind of like this:
float u = (((x-centerX) / radius) / 2f + 0.5f) * repeatCount;
float v = (((y-centerY) / radius) / 2f + 0.5f) * repeatCount;
Which leads to 0.0 - 1.0 values, multiplied by the number of times you want the texture to repeat. Make sure TextureFilter.Repeat is set! https://github.com/mattdesl/lwjgl-basics/wiki/LibGDX-Textures#texture-parameters
If you only ever see the tip of the planet, then forgo the above and just use a half-circle texture that makes the ship look small in comparison (SpriteBatch).