In 2009 I created a tutorial on how to create the “small planet” effect using Panoramic Photographs. This post was very popular and picked up by various other blogs so you have probably found your way here via one of them.
However – this post was/is woefully outdated now so I would suggest looking at one of the following for more up to date information
If you are interested in the original tutorial the text for parts one and two can be found here.