2D planet orbit

Im trying to make a game which contains multiple planets/moons orbiting around each other in 2D but I must not be doing it right because I keep running into weird problems.

Im new to unity and I have no idea on how to use the libraries and stuff.

anyways Im using this script to rotate a sphere to face another sphere and applying a constant 2d force of -10 on the Y axis.

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class faceObject : MonoBehaviour {
5.    
6.     public Transform Target;
7.     // Use this for initialization
8.     void Start () {
9.    
10.     }
11.    
12.     // Update is called once per frame
13.     void Update () {
14.         Vector3 LookAtPoint = new Vector3(Target.transform.position.x, Target.transform.position.y, Target.transform.position.z);
15.         transform.LookAt(LookAtPoint);
16.     }
17. }
18.  

And I get the following result, the sphere orbits correctly at first but then suddenly stops orbiting and crashes into the bigger orb.

I am probably completely doing this wrong and I should probably use an entirely different method, I'm open to suggestions.

(I have searched the internet pretty thoroughly but I could not figure out my problem by myself)

 

Perhaps you should also post the code where you are adding that constant force in the Y direction.

 

Its just a constant force 2d component