Imagine providing millions of images of the earth and Mars to a deep neural network so that it can study and learn what the surfaces, colours, patterns and shades of these surfaces are like. 
Imagine that this network learns them so well, that it is able to generate images that are as real as the real ones, with the same textures, only they are not real but synthetic.
Imagine that this network can now generate new combinations, maps that go beyond reality, but look like reality. This is neural planets.