instantiating object from Resources folder via script

Question

'm trying to instantiate an object that I imported from blender (and a prefab the same file), but, I'm getting the error

The object you want to instantiate is NULL.

I can set the game object via GUI but I'd like to know how to set it via C#. The gameobject I created in blender is called Sphere.

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

public class Create_Planet : MonoBehaviour {
    public GameObject planet;
    // Use this for initialization
    void Start () {
        planet = Resources.Load("Sphere") as GameObject;
    }

    // Update is called once per frame
    void Update () {
        if (Input.GetKeyDown(KeyCode.Space)) {
            Instantiate(planet, new Vector3(0, 0, 0), Quaternion.identity);
            Debug.Log("spcae");
        }
    }
}