Find with tag not working

I am trying to make a solar system and i am trying to set the stars and find them with "FindWithTag". Every time run the game it generates this error:

 NullReferenceException: Object reference not set to an instance of an object
 ShowInfoScript.Update () (at Assets/Scripts/ShowInfoScript.cs:42)

It points to the place where i set the rotation of the star with a Quaternion. Code:

 using UnityEngine;
 using System.Collections;
 
 public class ShowInfoScript : MonoBehaviour {
 
     public GameObject camera;
 
     private GameObject mercury;
     private GameObject venus;
     private GameObject earth;
     private GameObject mars;
     private GameObject jupiter;
     private GameObject saturn;
     private GameObject uranus;
     private GameObject neptune;
 
     void start(){
         mercury = GameObject.FindWithTag ("Mercury");
         Debug.Log( "find");
         venus = GameObject.FindWithTag ("Venus");
         Debug.Log( "find");
         earth = GameObject.FindWithTag ("Earth");
         Debug.Log( "find");
         mars = GameObject.FindWithTag ("Mars");
         Debug.Log( "find");
         jupiter = GameObject.FindWithTag ("Jupiter");
         Debug.Log( "find");
         saturn = GameObject.FindWithTag ("Saturn");
         Debug.Log( "find");
         uranus = GameObject.FindWithTag ("Uranus");
         Debug.Log( "find");
         neptune = GameObject.FindWithTag ("Neptune");
         Debug.Log( "find");
     }
     void  Update (){
         if (Input.GetMouseButtonDown(0)) {
             RaycastHit hit;
             Ray ray= Camera.main.ScreenPointToRay(Input.mousePosition);
 
             if (Physics.Raycast(ray, out hit)) {
                 if (hit.transform.name == "Earth" ){
                     mercury.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
                     venus.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
                     earth.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
                     mars.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
                     jupiter.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
                     saturn.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
                     uranus.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
                     neptune.transform.rotation = Quaternion.AngleAxis(0, Vector3.up);
                     OrbitScript.exeleration = 0;
                     Debug.Log( "Earth");
                 }
             }
         }
     }
 }

TLDR: Can't find my objects using FindWithTag.

Please help me i need it for school....

Ethan