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.

Look at this: Unity - Scripting API: Collider.OnTriggerEnter(Collider)
Your code dont work because the collider is called every fixed update and look if ther is a collsion but the collision dont stop after one collison so many instanzien get startet. If you would use Collider.OnTriggerEnter and Collider.OnTriggerExit it would just called once.