Making a child object circulate/orbit around another object using mouseposition

So, what i'm trying to accomplish is my childobject circulating/orbiting my mainobject, but in the direction my mouse cursor points to. The problem here is it has to stay at a certain set radius, much like a planet circulating another planet. But in this case i want to chose the direction with my cursor.

The code i've written so far tracks the mouse/cursor position perfectly but only rotates the childobject itself, it's not making the object rotate around the mainobject

Any help is appreciated

 using UnityEngine;
 using System.Collections;
 
 public class RotationScript : MonoBehaviour 
 {
     
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void FixedUpdate () 
     {
         float test = angleInDegree (screen (), mouseposition ());
         Debug.Log (test);
 
         this.transform.eulerAngles = new Vector3 (transform.eulerAngles.x, transform.eulerAngles.y, -test);
     }
 
 
     Vector3 mouseposition()
     {
         return Input.mousePosition;
     }
 
     Vector2 screen ()
     {
         return new Vector2 (Screen.width / 2, Screen.height / 2);
     }
 
 
     public float angleInDegree(Vector2 origin, Vector3 target)
     {
         var n = 360 - (Mathf.Atan2 (origin.y - target.y, origin.x - target.x)) * 180 / Mathf.PI + 270;
         return n % 360;
     }
 
     private float toRadians(float x)
     {
         return (x * Mathf.PI) / 180;
     }
 }