Lens? Seems like it must have been a fisheye.... Looks absolutely fantastic, I'd buy that poster.
Yea, looks like it was a fisheye, pointing at an angle towards the ground, swung around the tripod and then stitched together somehow?
Why fisheye? Regular panorama. Very old website (2006 ) http://content.photojojo.com/tutorials/create-your-own-panorama-planets/
There are many panorama software options nowadays that make creating "little planets" even easier...
Oh really...how interesting. Thanks for the link, I'll have to try some of that out