// Create a planet in orbit around center object with semi-major axis a
    public static GameObject CreatePlanetInOrbit(GameObject center, float mass, float a)
    {
        // position will be trumped by orbit
        GameObject planet = CreateNBody(mass, new Vector3(1, 0, 0));

        OrbitEllipse orbitEllipse = planet.AddComponent <OrbitEllipse>();

        orbitEllipse.a = a;
        orbitEllipse.SetCenterBody(center);
        return(planet);
    }