Disabling Gravity on touching layer C#

I’ve been working on this game for a few days and I want to be able to make the FPS Controller disable gravity once it exits the planets outer object, this is my current code.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;

public class IsOnPlanet : MonoBehaviour {

	public GameObject other;
	public Rigidbody rb = other.GetComponent<Rigidbody> ();

	// Use this for initialization
	void Start () { 
	}

	void OnCollisionEnter(Collision col)
	{

		if (col.gameObject.layer == 8)
		{

			this.rb.useGravity = true;

		}
	}
}

Here’s the error : Assets/Scripts/IsOnPlanet.cs(9,24): error CS0236: A field initializer cannot reference the nonstatic field, method, or property `IsOnPlanet.other’

Delete " = other.GetComponent ()" and add “public Rigidbody rb = other.GetComponent ();” to void Start()