King's College Circle of University of Toronto.

Update: Thanks for the comments, I'm glad a lot of people seem to like this image. As requested here's a brief explanation on how to make your little planets using photographs. There are different ways of doing this, but here's one way that's probably the most straight forward. I will include a tutorial with images later but this should be helpful for now.

1- First you need a 360 panorama image. You can use partial non-360 panoramic images too but a full 360 will look more natural. For that you can use commercial software like PTGui or Photoshop's Photomerge, or use freeware applications like AutoStitch.
1a- note that you can use a wide landscape shot instead of a panorama too, but it's best if the width is at least twice the height of the image, and also make sure the left side and right side of the photo match more or less. To check that you can try using Filter>Other>Offset command in Photoshop.

2- Open the image in Photoshop and resize it to a 1:1 square image. To do that you need to uncheck Constrain Proportions in Image Size panel. I recommend scaling up the smaller size to match the bigger length and not the other way around. So for example if your photo is 2000x1000 pixels resize it to 2000x2000 pixels.

3- Rotate the image 180 degrees.

4- Go to Filter>Distort>Polar Coordinates and select Rectangular to Polar.

5- Your little planet should be ready now! You can start painting/healing the center of the frame to get rid of the extreme lines if you like.

Tips:
-To make a clear sky like the photo above, you can slightly over-expose your photos or take photos in an overcast day to get a white sky.
-Try different cropping variations to get different effects and sizes for your planet. For example cropping more of the grass at the bottom of the above image will result a smaller green center.
- If you skip step 3 (rotating the image) you'll get a reverse effect that is quite interesting too.
- There is a plug-in from Flaming Pear called Flexify that can help achieve similar images.

Update 2: There are many resources about this technique. Here is a small list of more resources, tutorials and galleries:
- Dirk's tutorial
- Create your own planets group on flickr
- making planets group on flickr
- Frans' tutorial
- Patt's Piks Little Planets flickr set
- Pisco Bandito's Little Planets flickr set