This is not officially a supported function. But one possibility would be to put your camera *inside* the planet (good call sboerner!), give the planet a negative radius, and render a spherical image. The trick is giving it a negative radius (equal to its original radius; just put a - at the beginning of the radius value you had, after you put the camera inside).
For displacement it's a bit more complicated than that, you need to disable GI and other lighting and turn the height values of the planet into color values (black to white). You also would ideally remap the black and white point values so you get a more manageable ramp of height values you can use. Then render out spherical and save to 32 bit EXR. I've attached an example project file which shows a basic setup. Note that the Color Adjust values need to be changed to reflect the minimum and maximum displacement values of your terrain for the color adjustment/remap to work right. And you'll need to fiddle with this in your other app(s) to get the height remapped correctly. If you take it into Photoshop for example, it will appear all black, but increase Exposure by a good amount and you'll see the height values.
- Oshyan