EXPERIENCE THE PLANETS is a stunning collective art project that aims to "illustrate [the planets'] wonders, weaving art and science together."
Here are some of the great HD wallpapers they offer.
From the Project Page:
The cornerstone of ETP is its process, which holds each artist responsible for understanding and representing the science behind their work. Combining research with creative exploration and fact-based critique, the ETP community strives to make every piece an artist produces the best they’ve ever done.