Realism When Writing About Aliens

Note: to avoid having to explain myself every time I use a big word and potentially boring readers who already know it, this time I've started using footnotes. If you don't know a word and there's a number next to it, the footnotes will either have a definition/explanation or a link to something that explains it better than I can. I also use footnotes for sources.

There's still a great amount of debate among most people about whether or not there are life forms on planets other than ours. Sometimes this theory is shot down as unrealistic altogether, especially when it comes to intelligent life forms. Most evidence to disprove the existence of intelligent extraterrestrial life is within the categories of "we haven't found life on any of the other planets in our solar system" or "we've never been contacted by aliens." But the universe is a lot bigger than most people realize. The furthest distance that we can see is about 46 billion lightyears away.¹ That means it would take 46 billion years to get there at the speed of light. The furthest distance humans have been from the Earth is 248, 655 miles,² or 0.00000004 lightyears. We have no idea how big the entire universe actually is, especially because it's constantly expanding. So given how much of the universe we haven't explored, humanity's experience (or lack thereof) with extraterrestrial life means just about nothing.

So are there aliens?

The short answer is probably.

Life on Earth started with chemical reactions between elements that were already on the planet, like Nitrogen and Oxygen and, of course, Carbon. These reactions were all spontaneous, meaning they undoubtedly would happen anywhere given the same materials, temperature, etc, and they formed things like nucleotides³ and amino acids,⁴ which formed things like nucleic acids⁵ and proteins. All the building blocks of life were there and eventually became single cells, protected by a membrane of molecules huddled together to protect themselves from water.⁶ Some cells ate other cells, leading to more complicated cells--for example, both our mitochondria and the cloroplasts in plant cells were originally on their own before they were consumed by other cells--that then evolved into multi-cellular organisms.

This all happened naturally. Over billions of years, sure, but it didn't need any help. It was just a long series of spontaneous reactions.⁷ So is it really that hard to believe that if there are an estimated 100-200 billion planets in the Milky Way⁸ and an estimated 2x10²³ planets in the Universe,⁹ some of them could have started life the same way?

If aliens exist, why have we never seen any?

Humans have never travelled outside of our solar system. The only planets we've been close enough to to tell if they had life on them have conditions that aren't capable of containing life, or no longer are. They need to be within a certain range of distances from the star they orbit to have the right climate and they need to have water. Some believe that Mars has small life forms on it (whether or not they're currently alive) and just doesn't have enough water or the right climate to sustain higher life forms. My biology professor from a couple of quarters ago even theorized that Venus once had as much life on it as Earth does, but suffered climate change in the way we're beginning to, and that all life forms on it went extinct as the Ozone layer or whatever was protecting them from the Sun completely depleted. But as it is, none of the planets in our solar system could support much life, at least without a lot of work. Mercury is too close to the Sun, and Venus might be as well. The gas giants are too far from the Sun and also, considering they're mostly made of gas, a bit difficult to walk on. Pluto is way too far and doesn't have much to it--it may not even have all of the elements.
As far as space travel, if we haven't travelled outside of our own solar system, how can we expect that other life forms would have made it all the way to us from other solar systems or galaxies? The Earth has been around for approximately a third of the universe's age, so some planets may be significantly older than ours and could have more advanced technology, but the time it would take to travel to the closest solar system outside of ours would be about 81,000 years.¹⁰ Even if an intelligent species is billions of years older than us (which is unlikely because most species don't seem to survive that long), they probably haven't reached a life expectancy that could survive that trip. It might be possible for aliens from another solar system to travel to ours, but it would require generations upon generations to be born in whatever ship is taking them here. Space has shown to have deleterious effects on health in humans,¹¹ so I can't even imagine what would happen to aliens who lived their whole lives in a space ship. It would have to mimic their planet in almost every way in order to keep the travellers the same species as they were when they left.
But science fiction isn't about what we know is possible. It's about the "what if"s. And a whole lot of science fiction revolves around the question of "what if we could meet aliens?"

Unrealistic vs Realistic Aliens

Maybe someday humans will meet an intelligent form of extraterrestrial life. But what would it be like? Certainly nothing like those presented in most movies. Honestly, the most realistic idea I've seen for an alien was that in E.T. He was obviously different from humans, but shared some of the traits that gave us an evolutionary advantage, like being bipedal or having two eyes. By his lack of hair, feathers, or scales, we can tell that he evolved to different environments than most humans, but all of his differences were still within the realm of plausibility. He also doesn't speak any English until Eliot teaches him a few key words. What's wrong with all of the other aliens, though? Here are are some things I've noticed:

Human-ness

So many aliens in mainstream media, whether it be the Doctor or Superman, look identical to humans. I know it's just the creators being lazy or wanting their characters to blend in with those around them, but if you think about it from a scientific standpoint, the chances that life on another planet would evolve into what looks like a human is infinitesimally small. The environment on their planet would have to be pretty much identical, including the lengths of days and years, and they would need to have all the same events that impacted our evolution. Neanderthals were the same biological species as humans (we could interbreed) and even they looked different enough to be classified as something else, so how could aliens look exactly like us? Pair this with having two hearts or skin that deflects bullets under the light of a yellow sun, and it really makes no sense. A species wouldn't look like humans if they didn't also function like humans. And they wouldn't evolve to do either on most other planets.

Skin color

Mentioning human-like aliens, another thing is that most aliens I've seen that resemble us in shows or movies about aliens either look like white people or have skin that's blue, green, or some other color that human skin doesn't come in.

If another planet were to somehow evolve a human-like species, they definitely wouldn't all (or even mostly) be white. The human variance as far as skin color has to do with the weather in different parts of the world. Before colonization, people closer to the poles tended to have paler skin, and people closer to the equator tended to have darker skin. That wasn't the only criteria, though, because the weather also has to do with things like mountains and water. The big unique thing about our planet, though, is its 23.5 degree tilt from its axis. This influences how sunny it is near the equator and how devoid of light it is around the poles. If a planet like ours didn't have that tilt, just about everywhere would have half of a day in the sun and half of a day out of it. The equator would always be even more sunny and hot than ours, and the poles would have very little sunlight but no sense of night and day, as it would always be dim daylight. This would probably result in the people close to the equator being darker than our darkest, and the people close to the poles being paler than our palest. The thing is, though, pale skin is a mutation. It just adapted better in colder and less sunny climates than in brighter and hotter climates, because, well, have you ever seen a pale person after spending the day in the sun without sunscreen? Something made it better for survival in colder, darker places, though--perhaps something to to with camouflage. But if no one on that other planet ever had that mutation, all of their aliens would be black, or at least a lot darker than those we're shown.

As far as brightly colored skin goes, it really doesn't work for a land predator species. Any aliens with blue or green skin would be very visible to prey and would be terrible at hunting. Bright colors are helpful for some species of frog or insect because they warn predators--whether this is true or not--that the animal is poisonous and not a good idea to eat. Humans or human-like species wouldn't need this warning because we're predators, and at "the top of the food chain," so most animals aren't going to be eating us anyway. The only situations in which bright skin would be helpful to a species of our size and power are if 1) that's actually what would blend in the best with their environment or 2) most animals on their planet were actually a great deal larger than them. Both of those situations involve a drastically different environment, though, so they wouldn't end up looking like us.

Elements

I went to see Captain Marvel when it was in theaters with my girlfriend and her family. It was a good movie, for the most part, but what really drove me nuts was their representation of aliens. When undergoing an examination of an alien, a doctor said something along the lines of "he's made with elements we haven't even heard of!" That's, in the best words I can think of, absolute bullshit. Elements are differentiated by the amount of protons they have, and we know of all elements with as few as one proton and as many as 118. That means for an element to be new to us, it would have to have more than 118 protons and electrons and an atomic mass of more than 236μg (probably more than 294, as that's the atomic mass of Oganesson, the 118th element¹²). The more protons and electrons an atom has, the more unstable it is because electron transfers have more and more energy with every higher electron shell.¹³ An atom of Carbon, which is our primary element, has two electron shells. An atom of Oganasson has seven.¹² Elements we haven't heard of would probably have more. Our bodies are made primarily of Carbon compounds because a) Carbon has 4 out of 8 electrons in its valence shell¹⁴ so it can make exactly 4 covalent bonds and b) it's a small enough atom with few enough electrons and electron shells that its bonds forming and breaking don't wreak havoc in our bodies with the amount of energy they use and produce. Scientists have theorized that Silicon could be used similarly, but an atom with 7 or more electron shells would be far too powerful and energetic to build a living thing out of, and larger elements tend to be more unstable and prone to radioactive decay, as well. I'm sure you've heard of Uranium, and Oganasson has a half-life of less than a thousandth of a second.¹² No living thing could survive being composed of radioactive elements or compounds with enough energy stored to break everything around them.

Language

Another thing I've noticed is that in most media, aliens are portrayed either already knowing our language or with a universal translator that translates all languages for them and what they say back into the native language of the human they're talking to--English for the most part in whatever I've watched or read, but that has more to do with that being the only language I'm fully fluent in, and I'm sure this applies to other languages as well. The problem with the first portrayal is obvious, because how would beings from another planet become fluent in our languages without first spending time here? However, the same applies to a universal translator. One can only make technology with the knowledge and/or learning capacities of its maker. So either whoever made the translator would have to know the languages already (so it would have to be a human who knows the alien language(s) or an alien who knows human languages) or the translator would be able to learn languages over time. Either way, either the aliens or a super-intelligent AI translator would have to be exposed to humans for a while to learn the language(s). Most media doesn't account for this, and has an alien or a group of aliens showing up already able to communicate with humans even though they've never been here before. Of course, last time I told someone this, they responded with, "But what if the aliens staked out the Earth at first to learn human languages?" If they did that, either we would have seen them already or they would be too far away to learn anything. It's unlikely that aliens would learn anything about our languages unless they were closely listening to human conversations and taking notes, then moved onto actually talking to us themselves. If you didn't have teachers or web services like Duolingo or Rosetta Stone, how would you learn a new language? I guarantee you couldn't learn it from a satellite without someone who already speaks it there with you.

Conclusion

The main scientific issue with the aliens portrayed in the media is that they evolved in ways that are nearly impossible for them to evolve. A species from another planet could never be exactly like humans unless they were in an environment exactly like ours that went through the same evolutionary events. If they did look like humans, they wouldn't have weird differences like two hearts, superpowers, or green skin, because to be so different means they evolved for different circumstances, which would mean they couldn't evolve to look like us either. Their skin could be a variety of different colors, but which colors make sense fully depend on what environment they live in, which also dictates how similar to us they'd be, so if they're bright blue, they shouldn't be bipedal with opposable thumbs and two forward-facing eyes. They also would have to be made up of some specific elements, even if their molecules were built in an unexpected way, because life on our planet is made with a Carbon backbone for a reason. Finally, they would only speak human languages (or have a universal translator with our languages built in) if they had spent a certain amount of time on Earth first. If aliens exist--which they probably do--they're nothing like the Kree.

1. https://phys.org/news/2015-10-big-universe.html
2. https://www.space.com/11337-human-spaceflight-records-50th-anniversary.html
3. Parts of DNA or RNA, sometimes simplified as "bases" (although bases aren't necessarily all of the nucleotide). 
4. Parts of proteins, coded by DNA. Each possible set of 3 bases codes for one amino acid.
5. DNA or RNA.
6. https://www.ncbi.nlm.nih.gov/books/NBK26871/
7. https://chem.libretexts.org/Courses/University_of_Kentucky/UK%3A_CHE_103_-_Chemistry_for_Allied_Health_(Soult)/Chapters/Chapter_11%3A_Properties_of_Reactions/11.5%3A_Spontaneous_Reactions_and_Free_Energy
8. https://www.space.com/19103-milky-way-100-billion-planets.html
9. https://skiesandscopes.com/how-many-planets/#5_How_many_planets_are_there_in_the_universe
10. https://www.universetoday.com/15403/how-long-would-it-take-to-travel-to-the-nearest-star/
11. https://www.nasa.gov/hrp/bodyinspace
12. https://www.chemicool.com/elements/oganesson.html
13. https://revisionscience.com/a2-level-level-revision/chemistry-level-revision/atomic-structure-bonding-periodicity/shells-and-subshells
14. Outermost electron shell. In smaller elements like Carbon (excluding Hydrogen and Helium, which can only have up to 2 electrons in their valence shell), it can fit 8 electrons and is constantly trying to have 8, which is what drives covalent bonds and ionization.

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