Removing a background

Hi guys!
Hope you can help (newbie to photoshop).
Okay.
What Im trying to do is the following..............................

I have a bitmap image of the earth. This bitmap has a white background.
What I want to do is to cut out the earth so there is no white background, the insert what's left (eg.just the earth) into another program as an image.

I have used the wand tool and varied the tolerance, feathering etc, but find this tedious.

Can you do the following?

Add a circle over the earth, so that just the earth is covered.
Then somehow select the circle, invert the selection, so that the background eg.white background is selected, then delete the background?

Any suggestions would be much appreciated.
Thanks
Simon