Instantiate and remove objects according to the number of objects in an Array

hello everyone , i got a bit of a problem , here it is:

i want to create a radar , where it have a range , and some planets , what i want is as follows:

1:-when a planet enters the range (lets say 100) its added to an array (Done)

2:-spawn a UI image object for each planet in range , so if there is 3 planets in range then spawn only 3 , if 2 then spawn 2 .....etc

3:-if a planet leaves the range , then its corresponding icon gets remove (maybe Disabled).

4:-make each icon follows its corresponding planet (will try to do it on my own at first)

problem: using the script below , its just adds unlimited number of Icons.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class Radar : MonoBehaviour {
 
     public float radarRange;
     public LayerMask planets;
     public Collider[] hits;
     public Slider range;
 
     public Image icon;
 
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void FixedUpdate () {
         //Collider[] hits;
         hits = Physics.OverlapSphere(transform.position, radarRange, planets);
         radarRange = range.value;
 
         foreach (Collider planetObject in hits)
         {
             if (Vector3.Distance(transform.position, planetObject.transform.position) < radarRange)
             {
                 Instantiate(icon, planetObject.transform.position, Quaternion.identity);
             }
         }
     }
 }

i know it might look like a beginner script , but i am just learning to script.

thanks for any help i get.