Which sci-fi aliens might actually be out there waiting for us?
We talked to astrobiologist Dr Lewis Dartnell ā aĀ UK Space Agency research fellow at the University of Leicester ā to get the science behind alien life. If you want to read more, Dr Dartnell has also written a book,Ā Life in the Universe: A Beginnerās Guide.
Humanoid aliens?
Everywhere with a low costume budget, from Star Trek and Star Wars to Doctor Who.
Thanks to something in evolution called āconvergence theoryā, we often see very different lifeforms finding similar solutions to the same problems. This includes things like developing eyes and being able to see, or lungs to extract oxygen from the atmosphere, or a heart and circulation system.
Other developments seem to be more random. This includes the number of fingers on a hand, and perhaps even the number of limbs.
āYou wouldnāt expect to meet an alien that looks exactly like a human but with green skin or slightly pointier ears. But you would expect some recognisable features, just from the basic principles of evolution trying to solve a problem and find the best form for survival.ā
Certainly possible, but thereās no guarantee.
Giant invertebrates?
Ā āArachnidsā in Starship Troopers or the āBugā in Men In Black.
Lower gravity and higher concentrations of oxygen on other planets might allow things with exoskeletons and less efficient respiration systems to grow larger than they do on Earth. Of course, alien invertebrates might also have more complex respiration like us, and then oxygen intake would be less of a problem.
Growth is difficult with an exoskeleton because itās like being locked in a rigid box. On Earth, invertebrates regularly malt their exoskeletons ā leaving them vulnerable while their new exoskeleton hardens.
āIn order to have giant invertebrates on another planet youād need firstly to have lower gravity to help support their weight, but if you have a planet which is too small with too little gravity then its atmosphere can start to bleed away.ā
Totally plausible, but only to a reasonable size.
Shape-shifting aliens?
Changelings (Star Trek: Deep Space Nine) or the ice-trapped alien in The Thing.
Caterpillars change their entire structure when they metamorphosise into butterflies ā although this takes a long time. A quicker solution might be to tightly control facial muscles in order to mimic other individuals.
Rearranging cells very quickly (such as when a Hollywood alien just touches something and assumes their shape) seems fairly difficult. Completely changing shape and size goes entirely against the laws of science as we know them.
āShape-shifting to a totally different shape ā and especially a different size ā would be impossible. You canāt just change the amount of stuff that youāve got.ā
Maybe, but not like in the movies
Galaxies full of similar-looking aliens
Star Wars, Star Trek, Doctor Whoā¦
Just within our galaxy we already know of many different stars with Earth-like planets orbiting them. Thereās no reason sentient life hasnāt evolved on lots of them.
There generally needs to be a shared starting point for very similar life forms to develop separately on different planets. When human civilisations started stumbling across each other centuries ago we were the same because weād all come from the same place.
āIf you had an original species that colonised lots of planets, then that might explain how youād get lots of very similar aliens. But you wouldnāt expect it to have arisen independently due to evolution.ā
Could definitely happen elsewhere, but probably not to us unless weāve missed something huge in our past.
Ancient races?
From the Ancients (Stargate) to myths about Atlantis.
Thereās always a slim chance that weāve missed something in the fossil record, or a previous race might have been entirely destroyed without a trace.
It seems very unlikely that a clever ancient race would not have left any evidence of their existence ā just think how much rubbish we generate ourselves. And if theyād wiped themselves out with nukes, for example, then weād be able to see evidence of radioactive fallout.
āThereās good reason to think animal life was actually impossible before it did start. You need things like a certain level of oxygen in the atmosphere, and that didnāt happen before about 550 million years ago. In fact, itās almost like animal life started to evolve as soon as it was able to.ā
Doesnāt look very plausible on Earth.
Robotic aliens?
The Cybermen (Doctor Who) or the Cylons (Battlestar Galactica).
If we accept that there might be life out there just as clever as us, and that we ourselves might soon be capable of making ālivingā robots, then thereās no reason to think it hasnāt happened elsewhere.
You need that clever robot-building alien race first.
āTo have robotic or mechanical life you need to have biological life beforehand to build it. And if the machines are then intelligent and reproduce you might start calling those robotic forms ālifeā.ā
Very likely somewhere.
Aliens living on gas giant planets?
The Beldon of Bespin (found in Star Wars spin-off fiction)
Organisms living in gas clouds would have to be buoyant to rise above high temperature and pressures at a gas giantsā core. They could possibly live off energy from the sun or organic particles generated in the gas.
It seems unlikely that life could develop without a rocky surface or an ocean of liquid for essential minerals to dissolve in.
āWe donāt quite know how life might get started in a cloud ā but life could survive there if it had already gotten started somewhere else.ā
Possible, but where would they come from?
Gas-based aliens?
Ā The Gelth (Doctor Who) or the Dikironium āvampireā cloud (Star Trek).
Honestly, scientists arenāt really sure it is.
Molecules move around too fast in gas, and collide too infrequently, to create the rich chemistry needed for life.
āIt seems most sensible for life to include a liquid to dissolve things into. But this might not necessarily be water. Maybe youād have ammonia-based life or something else that serves the same basic solvent function.ā
Basic chemistry says not.
Energy-based aliens?
TheĀ Prypiatosian-B (Ben 10).
Ā In a sense all life on Earth is energy based, we all rely on energy.
Does something without structure or matter even fit our concept of life?
āTo have something that is only energy with no matter ā so thereās no structure or stuff to it ā youāre basically talking about fantasy now.ā
What does energy-based even mean?
Aliens from another dimension?
Chaos and the Daemons (Warhammer 40,000) or Krang (Teenage Mutant Ninja Turtles).
Some mathematical theories about the universe incorporate other dimensions beyond the four that we inhabit.
The human brain canāt even conceive what these dimensions might be like.
āAliens from other dimensions are really just sci-fi shorthand for āsomething from somewhere elseā. But why would you start inventing other dimensions in which they could have originated?Ā Thereās so many planets and stars out there that aliens could have come from.ā
Pure fantasy.
