Science for SciFi: Astrobiology

Rubber-forehead aliens are basically a meme at this point. They make perfect sense in terms of production costs and limitations imposed by special effects technology at the time. The good news is that writing for print gives us far more options than would be possible otherwise. I think that they make it easier to relate to characters on screen but making all of your aliens look like humans with a few extra bits glued on requires a lot of worldbuilding to explain away. If you try to explain it that is.

The point of this post is not so much to provide an explanation of how life on other worlds could work but rather why it’s so hard to envision what life on other worlds could be like. This is because a) I am not a biochemist and b) it’s somewhat difficult to pin down just what “life” is. Once you’ve wrapped your head around this second idea and thought about some of the strange chemistries that are possible on alien worlds you will feel much freer to imagine strange new forms of life.

At this point, you are probably getting ready to type an angry comment or tweet along the lines of “WHAT DOES HE MEAN? OF COURSE WE KNOW WHAT LIFE IS. I’M ALIVE AREN’T I?” It’s actually a lot more complicated than that. Here on Earth we still have trouble deciding whether viruses are alive. Sure they can infect hosts and they use the same DNA/RNA coding that we and the rest of life here on Earth do, but they lack the machinery needed to make copies of themselves so they have to hijack ours. NASA has a definition of life that they use in the search for extraterrestrials and it’s probably the best one available to us, but there are still likely to be some who disagree with the definition.

“Life is a self-sustaining chemical system capable of darwinian evolution” – NASA

I’d be interested to hear whether readers think that viruses are included in this definition or not.

The reason that the definition of life is so hard to pin down is that we only have our own world to serve as a reference. In our solar system of eight planets and who knows how many planetoids and moons only one body, Earth, is known to support life. Yet there are two other planets, Mars and Venus, that might have once supported life and several moons that could even harbor life this very moment (I’m looking at you Europa).

We can look around at our own planet and describe how life works here. We can explain how DNA works, how organisms obtain energy, and how one organism gradually evolves into another over time. We know all that but we still do not know how life began on this planet. Without knowing how life began it is hard to definitely say whether a planet could support life or not. We tend to get excited when we find planets around other stars that could support conditions similar to those here on Earth, but when we talk about whether a planet is in a star’s “Goldilocks Zone” what we are really saying is that the planet could support life that is like us, and that’s a little arrogant I think.

Fortunately, our knowledge of chemistry and physics allows us to envision other ways in which life might arise. We are, after all, just bags of chemical reactions that happened to develop egos.

Excuse Me, Is This Life Organic?

These days a lot of people think that if something is organic it was produced from “natural” materials or grown without the use of certain fertilizers or pesticides. What they don’t realize is that oil is both organic and naturally occurring, but you wouldn’t want to eat it. When scientists say something is organic all that means is that it is composed of primarily carbon and hydrogen along with a smaller proportion of other elements.1

We and all the living organisms that we know of here on Earth are built out of carbon. Our DNA, our proteins, our hormones, our cell walls. Every bit of biochemical machinery that makes us is built on a scaffolding of carbon. Organic compounds are so prevalent in living things that the distinction between organic vs inorganic chemicals was originally based on whether they had come from a living thing or not.

Carbon is useful in all these ways because each carbon atom can form up to four bonds with other atoms. Carbon can form long chains with other carbon atoms and can also form double and triple bonds not only with itself but with other elements important to Earth life including both oxygen and nitrogen. Silicon is often suggested as a possible substitute for carbon on alien worlds, but silicon is less versatile than carbon and many of its compounds are unstable. This combined with the prevalence of carbon among molecules found in space does not bode well for silicon’s chances. There is however the clay hypothesis that has to do with the beginning of life on Earth.

Another point in carbon’s favor is that by now many complex organic molecules have been detected in space in molecular clouds around stars and on the surfaces of comets. Many of them being the same molecules used by living things here on Earth. With ready-made materials out there in the cosmos, why not take advantage of them?2

Water? I Hardly Know Her

Water is a really great solvent for life on our planet. Besides being everywhere and thus the most logical choice for life solvents, its properties allow both acid and base chemistry to take place. When we begin to consider different temperatures, pressures, and chemical makeup, a number of other solvent options become clear.

All it takes is a solvent that allowed for acid-base chemistry to take place. Water allows this, but there are other solvents that could, under different conditions, or with different commonalities. Waters is ubiquitous on Earth, but it doesn’t have to be on other planets.

Take a look at different solvents. Or familiar gases that would be liquid at other temperatures. The possibilities might surprise you.

Eating Sunshine (And Other Things)

Here on Earth, most ecosystems arise from the energy provided by the Sun. Just about everything either harvests light through photosynthesis or eats something that does. But even on Earth, we know that this is not the only option. In the deepest parts of our oceans, we have found extremophiles that feed off the heat and chemicals released by volcanic vents.

That is just on Earth. There are many sources of energy in the universe including gravity and magnetic fields. Alien life forms could catalyze the synthesis of vital metabolites using alpha and beta particles released by radioactive minerals to catalyze reactions or construct molecular machines on their cell membranes that harvest hydroelectric power.

The Galactic Habitable Zone

It’s weird to think that there might possibly be a shortage of resources on a galactic scale but once you get an idea of how elements are made it begins to make sense.

Basically, there is a band with indeterminate boundaries somewhere between the center of the galaxy and the edge that makes up the galactic habitable zone, a region that is determined by metallicity, the age of the stars, and how often stars in the area go supernova.

The center of the galaxy with its high concentration of stars is considered unsuitable for life, as the frequent supernovae release bursts of radiation that would sterilize nearby worlds and make the development of life difficult, if not impossible. Meanwhile, the edge of the galaxy is full of younger stars that have not had time to transmute the heavier elements that life needs.

All this results in an uncertain band looping around the galaxy where planets are more likely to be habitable. The boundaries of this band are uncertain, but

Turning The Weirdness Dial Up to Eleven

A lot of us tend to base alien species on the species we are familiar with here on Earth. A quick look at some extinct species will show that there are many, many variations on how weird life can get just on a single planet. Just because it didn’t happen HERE doesn’t mean it can’t happen SOMEWHERE.

In all honesty, it just depends on how strictly you want to adhere to known science. You could have aliens with bones like ours with gelatinous flesh or stationary mollusks that spend their days exploring complex algorithms in their minds.

Hard science fiction is all about finding plausible explanations based on what we know now. That doesn’t mean it can’t be weird.


If anything should be clear by now it’s that nothing is. The Milky Way is a big place and we can envision so many possibilities that it is just about impossible to anticipate all the possible variations of life. In fact, I came across so much information while reading this post that I wasn’t able to include it all in this one post. So stay tuned for future posts on the origins of life, panspermia, and whatever else catches my eye in the process.


  1. Unfortunately for myself and other inorganic chemists. Our field is literally defined as “not organic” which makes it more than a little hard for someone to guess what we do.
  2. I’ve come across a wealth of information on this topic and it’s all incredibly facinating. Enough so that I’ll probably have future posts on both on possible origins of life and on the wonderful hydrocarbons that have been found out there in the universe.


Some of these resources may be behind a paywall. Consult your friendly neighborhood librarian for help. Or in the case of research papers, it never hurts to email the author, they may just send you a copy!

Astrobiology: The Study Of The Living Universe by Christopher F. Chyba and Kevin P. Hand. Annu. Rev. Astron. Astrophys. 2005

Astrochemistry: From Astronomy to Astrobiology by Andrew M. Shaw

Beloved Wikipedia.

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