Please help finding center of object

I am trying to write a code for a gravity pull towards a planet. I want to use a sphere collider as a trigger to start adding force to the object its manipulating towards the planet.

So OnTriggerStay I can retrieve the planets position to use with AddForce but its not the center of the planet. I tried and it is the same as the transform.position vector3. Now I tried to put a empty gameobject on the planet and I would like to change the planets center to that of the empty gameobject so I can therefor retrieve that data without the use of getcomponent. Heres some code I was trying.

using UnityEngine;
using System.Collections;

public class PlanetGravity : MonoBehaviour {

	public Rigidbody bullet;
	public GameObject center;

	// Use this for initialization
	void Start () {
		Vector3 centervector = new Vector3(center.transform.position.x, center.transform.position.y, center.transform.position.z);

		this.transform.position.Set(center.transform.position.x, center.transform.position.y, center.transform.position.z);, center.transform.position.y, center.transform.position.z);

		Debug.Log("Center = " +;
		Debug.Log("Position = " + this.transform.position);
		//Debug.Log("Mesh = " + 


	void OnCollisionEnter(Collision collision){

		//bullet = collider.rigidbody;

	// Update is called once per frame
	void Update () {

You want to add a force in the DIRECTION of the planet. You need a direction vector which points towards the planet.

To get the direction you can do this:

Vector3 direction = (orbiter.transform.position - planet.transform.position).normalized;

then multiply by the float force to point the force in that direction

Vector3 directionalForce = direction * force;

then you can do:


Which should push your object towards the planet.