It’s one of the great cliche responses to science fiction. “Earth has dozens of different biomes, but every planet in sci fi is just one thing, either it’s all desert or all frozen or…”
It’s easy to see why this bothers so many people watching science fiction movies. It’s very true that Earth has a wide range of climates and terrain, from snowswept mountains to dense, fetid swamps to arctic deserts and mist forests. When a planet in a work of science fiction is homogeneous from pole to pole, it feels like the creators are taking a shortcut. They’re being lazy.
But there are very good reasons why we keep seeing single-biome worlds in fiction. For one thing–as shortcuts go, it’s a great one. If you have multiple planets to depict, as in Star Wars, it allows you to instantly set them off from each other. You know by simple lighting cues and color palettes if a scene is set on Tattooine or Dagobah or Hoth. You don’t need to keep putting titles on the screen telling us where we are.
Furthermore it allows you to develop a landscape even if the characters move from place to place on the same planet. A consistent setting can grow and develop depth, whereas you just don’t have room to describe fifty different biomes in the same book, say.
This concern over single biome worlds didn’t start with Star Wars. It was old even when Dune was at its heyday. Arrakis is desert from pole to pole. It’s literally called the “Desert Planet”. This makes it the butt of one of the oldest tropes in sf criticism. Yet Herbert wasn’t trying to create a lushly diverse world in Dune. He was specifically trying to create a world which appears to be empty and hostile to life. What J.G. Ballard would call “psychic zero”–the same desolate landscape that is the setting for most of the Bible, a place where his zensunni warriors could test themselves both physically and spiritually. He also wanted to show the life cycle of the sandworms, which is so complex they may be the only living things on Arrakis, pre the arrival of humans. It’s funny that Dune, which is often called the first ecological science fiction novel, started this trend of worrying so much about biological diversity.
Looking outside the science fiction world, we can find a pretty solid defense of the single-biome planet as well–we need only look at the actual universe we live in. While Earth is home to multiple habitats and giant variations in its weather patterns, it turns out that it’s the exception, not the rule.
Wherever we look in our own solar system, we find nothing but single-biome worlds. Mars is cold, dry, and dusty. The polar caps are slightly different, but only slightly. Mercury is even more homogeneous, with maybe a few patches of water ice in craters at its poles–otherwise it’s nothing but sunblasted rock. Venus is shrouded in such a dense atmosphere that its entire surface is just molten rock and maybe liquid metals.
Beyond the orbit of Mars, diversity in planetary surfaces drops off considerably. The four giant planets are nothing but unitary weather patterns. Their moons are almost universally made of ice and rock in equal measure. Titan has methane lakes and deserts of electric sands, but again, it sticks out for its incredible wealth of biomes (two, maybe three). Beyond Neptune, there is nothing but an endless succession of Hoths. Worse than Hoths, because they lack the atmosphere to even have variable weather.
The criticism that worlds in science fiction are too samey, too much of a piece, is perfectly legitimate when we’re talking about planets exactly like Earth–planets with incredible genetic diversity, planets where life has sculpted the environment into biomes that favor individual species’ reproductive success. But please, when you’re creating worlds for your own sf stories, don’t feel obliged to throw rain forests onto your alpine planet. You don’t need them, and despite what anyone says, they aren’t all that realistic anyway.