Desert planets

There are currently 29 class G desert planets in the Multiverse Database. This is roughly one twelfth the number of class M worlds. The main difference between a class G desert planet and the Earth clone class M is the amount of water on the planet. The class G worlds lack the large connected oceans that typify a class M world. If there are oceans in a desert world, they tend to be small and isolated. In fact, these oceans are more properly classified as inland seas.

Presumably the large deserts, and corresponding lack of plant life, should inhibit an oxygen nitrogen atmosphere from forming. This is certainly the case on Mars, the only “real” desert planet we know of. And yet in the Star Trek classification system, Mars is a class K planet, due to the lack of a breathable atmosphere.

So we have to ask how realistic is a desert planet? The most famous class G planet in science fiction is Frank Herbert’s Dune, (followed closely by its Star Wars knockoff, Tatooine). Herbert did such a good job of creating his world that readers completely suspended their disbelief. And yet, readers who plow through the entire Dune saga learn that Dune was once a much wetter planet. Careful readers will note that many of the salt plains on Dune are ancient sea beds. In fact it was the introduction of sandtrout on Arrakis that slowly changed the ecology and created the vast desert. This explanation gets Herbert off the hook. Otherwise how do you explain the evolution of humanoid life on Dune? (Of course “Dune” takes place so far in the future that we don’t really know whether humanoids are indigent to Dune or the descendants of colonists. The latter is more probable).

It’s hard to see how humanoid life could evolve in a desert world. Without substantial water, there are no jungles. Without jungles, there are no trees to hide in, so early hominids get eaten by carnivores. Except without water, there are no carnivores, because any amphibians that crawl out of the tiny seas would have to live close to shore and would never get very large. Life would be pretty basic in a desert world. Small fish and simple plant life hugging the shores of small seas, maybe insects and a few amphibians on land. The highest order of life might be lizards and other reptiles sunning themselves in the desert sand. And that’s it. Certainly not much diversity due to the scarcity of niches to fill. So not much competition to drive evolution. In short, a desert world would be a boring place from a biological perspective. And as an aside, there’s no way a water filled slug like Jabba could ever evolve on a planet like Tatooine. He too came from somewhere else.

On the other hand, if a planet starts out class M and then slowly changes to class G, life has a chance to adapt. This is presumably what happened on Dune. We know, for example that the Fremen have a longer small intestine and can thus reabsorb more water from wastes. Similar evolution has probably changed their kidneys. On Earth for example, desert animals and birds have kidneys which expel a more concentrated form of ammonia than the mammalian kidney's urea.

So let us hypothesize that there are no desert planets where advanced life forms evolve. Instead life evolves on wetter class M worlds, and then as the oceans slowly dry up, (for whatever reason), life adapts to the realities of a desert world. Mars, our own class K world, seems to fit this hypothesis. We know from the current Rover missions that Mars once had oceans. Perhaps simple life evolved and an oxygen/nitrogen atmosphere formed. If Mars were the size of Earth, perhaps the higher gravity could have held on to a denser atmosphere, even as the oceans slowly evaporated. So if Mars were bigger it could have turned into a “real world” Dune or Tatooine. Or if it had tectonic plates to recycle carbon dioxide so the planet didn't cool off. Or not. Planetary ecology is no doubt much more complicated than the simple explanations offered. For example, exactly how do you lose an ocean, anyway? But at least these ideas are one explanation for the exotic life-bearing desert worlds of science fiction.