Planets.cs

using UnityEngine;
using System.Collections;

public class Planet : MonoBehaviour {

    public Transform sun;
    public float speed;

    public void Init(Transform _sun)
    {
        sun = _sun;

        if (Random.Range(1, 10) >= 5)
        {
            speed = Random.Range(-30, -0.000001f);
        }
        else
        {
            speed = Random.Range(30, 0.000001f);
        }

        transform.GetComponent<Renderer>().material.color = new Color((int)Random.Range(0, 255), (int)Random.Range(0, 255), (int)Random.Range(0, 255));
    }

    // Update is called once per frame
    void Update () {
        transform.RotateAround(sun.position, Vector3.up, speed * Time.deltaTime);
    }
}