Drag to rotate with forces

I want to be able to rotate a planet by dragging on the screen. I am already using a script for this however you have to drag on the object rather than anywhere on the screen. In addition to this I am trying to find a way to make it so that after you spin it it keeps on moving for a bit.

Here is the code I am using. Does anyone know how it could be changed to achieve what I want?`

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

public class DragtoRotate : MonoBehaviour { public float rotSpeed = 20;

 void OnMouseDrag()
 {
     float rotX = Input.GetAxis("Mouse X") * rotSpeed * Mathf.Deg2Rad;
     float rotY = Input.GetAxis("Mouse Y") * rotSpeed * Mathf.Deg2Rad;

     transform.RotateAround(Vector3.up, -rotX);
     transform.RotateAround(Vector3.right, rotY);
 }

}