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Posts Tagged ‘xenobiology’

Old McDonald Had a Planet

Old McDonald had a planet*,
And on this planet he had a…

Maybe it all starts with a planet: big, small, hot, cold, wet, dry. But a planet needs things living on it, right?

Or maybe it starts with the creatures: plants, animals, something else entirely, and they need a place to live.

Possibly you’re reading some interesting science fiction aliens, and want to know if they make sense.

It’s all about the biology. All three scenarios can be looked at for biological plausibility. We don’t yet# have any non-terrestrial biologies to examine, so I’m extrapolating a bit, but there’s a lot of physical and chemical properties involved that will have to be the same for any physical biological life forms##.

If I’m thinking about whether a fictional ecology can work, I look at three questions.

1. Where does the stuff come from? Can the living creatures get enough basic building blocks to maintain their bodies? Terrestrial life absolutely requires six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur (abbreviated CHNOPS), and while it’s possible that one or more of those might be swapped for something else, those six are likely to be important to any other biologies we encounter. They are simply the most effective and appropriate elements for their roles in metabolism and genetics.

If one or more is scarce, the organisms might not be able to get very large or grow very fast, and competition for resources could be extreme. (Biology driving plot, perhaps, something I love to see in my science fiction.)

2. Where does the energy come from? The sun? Chemistry? Heat? How is it processed? Our planet runs mostly on photosynthesis, converting solar energy into usable biochemical forms. Gas giants like Jupiter radiate more heat than they receive from the Sun. This could fuel an ecology that doesn’t rely on solar energy.

Does the amount of energy available match the activity and speed of the species? Photosynthetic organisms are either small, immobile, or both, because that’s what the energy available to them supports. Terrestrial animals almost all rely on plants to concentrate energy into a more efficient form so that they can move quickly. Large fast terrestrial animals all need to breathe oxygen. Anaerobic organisms are very small and slow, and that’s likely to be universally true.

3. Do the evolutionary antecedents make sense? Could this system have developed gradually? (And if not, how was it created? Here’s another good place for story to develop.) Are there obvious relatives, and not organisms that seem entirely unlike everything else on the planet? The movie Avatar was bad at this: every animal had six legs. except the Na’vi. So how’d they lose their other pair of limbs? And why?

I don’t expect every story to meet all three points^. Sometimes that level of detail is irrelevant; sometimes it actively impedes telling the story the author intends. But a little bit of thought about how the whole ecosystem works will help avoid any glaring errors in biology, and choosing to depart from known science is a whole lot different in effect than doing through ignorance.

What do you think? Any fictional biologies that particularly annoy you, or that you think are wonderful?


* EIEIO! I had all sorts of stupid titles for this post, including: “Dammit Jim, I’m a doctor not an ecologist,” and “It’s aliiiiiveee!” Sorry**.

** Not really.

# Yet. I hope I get to see it.

## Leaving out energy-only life, and mechanical life, and some of the wilder Star Trek creatures.

^ I love Sheri Tepper’s Grass, even though it fails at least one of these.