i need help making this code work with 2D physics.

i found this code that works wonderful with 3d physics, but i am making a 2d game. what i need is for objects to find the closest point on any planet's collider so i can add forces toward that planet. the reason i want it to find the closest point and not the closest planet is because i also want objects to attract to the closest surface. this will allow things such as walking on walls that are attached to a circular object we will call a planet.

here is the code:

i am mainly focused on just finding a Vector2 point to attract the object to. remember, it needs to be 2D

 using UnityEngine;
 using System.Collections;
 
 //OverlapSphere Test
 public class OverlapSphere : MonoBehaviour {
 
     public float maxSearchDistance = 50.0f;
     public Collider thisOne;
     public LayerMask lm = ~(1<<8 | 1<<9); //Player and Terrain
 
     public Vector3 temp;
 
     void Update ()
     {
         Collider[] myColArray;
         float lastLength = maxSearchDistance;
         bool flagSet = false;
 
             myColArray = Physics.OverlapSphere(transform.position, 50.0f, lm);
             for(int i = 0; i < myColArray.Length; i++)
             {
                 temp = myColArray[i].ClosestPointOnBounds(transform.position);
                 Vector3 tempLength = temp - transform.position;
                 float sqrLength = tempLength.sqrMagnitude;
 
                 if(sqrLength < lastLength*lastLength)
                 {
                     thisOne = myColArray[i];
                     lastLength = tempLength.magnitude;
                     flagSet = true;
                 }
             }
             if(flagSet)
             {
                 Debug.Log(thisOne.gameObject.name);
             }
     }
 }