Question How do I put an object BEHIND all the rest of the scene?

I would like to put a huge planet as a background to the scene.
As the mountains would cover it, I can't just splat it there in the sky.
I need to render it behind everything that is in the scene, except the skybox.
How do I achieve this?

 

The common method for doing this would be to use multiple cameras. Render the skybox and the planet in one camera, render the rest of the scene with another camera using Clear Flags > Don't Clear.