Which sci-fi aliens might actually be out there waiting for us?

With Doctor Who back on our screens this weekend, fans will be looking forward to a whole range of new alien adversaries to enjoy and fear. But how many aliens imagined by science-fiction might actually exist somewhere out there in our universe?

We talked to astrobiologist Dr Lewis Dartnell – a UK Space Agency research fellow at the University of Leicester – to get the science behind alien life. If you want to read more, Dr Dartnell has also written a book, Life in the Universe: A Beginner’s Guide.

Humanoid aliens?


Sci-fi examples: Everywhere with a low costume budget, from Star Trek and Star Wars to Doctor Who.

Why it’s possible:Thanks to something in evolution called “convergence theory”, 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.

The problems: Other developments seem to be more random. This includes the number of fingers on a hand, and perhaps even the number of limbs.

Expert analysis: “You wouldn’t 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.”

Verdict: Certainly possible, but there’s no guarantee.

Giant invertebrates?

close-up of praying mantis

Sci-fi examples: ”Arachnids” in Starship Troopers or the “Bug” in Men In Black.

Why it’s possible: 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.

The problems: Growth is difficult with an exoskeleton because it’s like being locked in a rigid box. On Earth, invertebrates regularly malt their exoskeletons – leaving them vulnerable while their new exoskeleton hardens.

Expert analysis: “In order to have giant invertebrates on another planet you’d 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.”

Verdict: Totally plausible, but only to a reasonable size.

Shape-shifting aliens?


Sci-fi examples: Changelings (Star Trek: Deep Space Nine) or the ice-trapped alien in The Thing.

Why it’s possible: Caterpillars change their entire structure when they metamorphosise into butterflies – although this takes a long time. A quicker solution might be to tightly control facial muscles in order to mimic other individuals.

The problems: 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.

Expert analysis: “Shape-shifting to a totally different shape – and especially a different size – would be impossible. You can’t just change the amount of stuff that you’ve got.”

Verdict: Maybe, but not like in the movies

Galaxies full of similar-looking aliens

Spiral galaxy

Sci-fi examples: Star Wars, Star Trek, Doctor Who…

Why it’s possible: Just within our galaxy we already know of many different stars with Earth-like planets orbiting them. There’s no reason sentient life hasn’t evolved on lots of them.

The problems: 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 we’d all come from the same place.

Expert analysis: “If you had an original species that colonised lots of planets, then that might explain how you’d get lots of very similar aliens. But you wouldn’t expect it to have arisen independently due to evolution.”

Verdict: Could definitely happen elsewhere, but probably not to us unless we’ve missed something huge in our past.

Ancient races?


Sci-fi examples: From the Ancients (Stargate) to myths about Atlantis.

Why it’s possible: There’s always a slim chance that we’ve missed something in the fossil record, or a previous race might have been entirely destroyed without a trace.

Problems: It seems very unlikely that a clever ancient race would not have left any evidence of their existence – just think how much rubbish we generate ourselves. And if they’d wiped themselves out with nukes, for example, then we’d be able to see evidence of radioactive fallout.

Expert analysis: “There’s 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 didn’t happen before about 550 million years ago. In fact, it’s almost like animal life started to evolve as soon as it was able to.”

Verdict: Doesn’t look very plausible on Earth.

Robotic aliens?

Cybermen on Tube escalator
(Matt Crossick/PA)

Sci-fi examples: The Cybermen (Doctor Who) or the Cylons (Battlestar Galactica).

Why it’s possible: 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 “living” robots, then there’s no reason to think it hasn’t happened elsewhere.

Problems: You need that clever robot-building alien race first.

Expert analysis: “To 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 ‘life’.”

Verdict: Very likely somewhere.

Aliens living on gas giant planets?

Jupiter being orbited by Galileo

Sci-fi examples: The Beldon of Bespin (found in Star Wars spin-off fiction)

Why it’s possible: Organisms living in gas clouds would have to be buoyant to rise above high temperature and pressures at a gas giants’ core. They could possibly live off energy from the sun or organic particles generated in the gas.

Problems: It seems unlikely that life could develop without a rocky surface or an ocean of liquid for essential minerals to dissolve in.

Expert analysis: “We don’t quite know how life might get started in a cloud – but life could survive there if it had already gotten started somewhere else.”

Verdict: Possible, but where would they come from?

Gas-based aliens?

Gas clouds in space
(Hubble Heritage/Flickr)

Sci-fi examples: The Gelth (Doctor Who) or the Dikironium “vampire” cloud (Star Trek).

Why it’s possible: Honestly, scientists aren’t really sure it is.

Problems: Molecules move around too fast in gas, and collide too infrequently, to create the rich chemistry needed for life.

Expert analysis: “It seems most sensible for life to include a liquid to dissolve things into. But this might not necessarily be water. Maybe you’d have ammonia-based life or something else that serves the same basic solvent function.”

Verdict: Basic chemistry says not.

Energy-based aliens?

Solar flare energy

Sci-fi examples: The Prypiatosian-B (Ben 10).

Why it’s possible: In a sense all life on Earth is energy based, we all rely on energy.

Problems: Does something without structure or matter even fit our concept of life?

Expert analysis: “To have something that is only energy with no matter – so there’s no structure or stuff to it – you’re basically talking about fantasy now.”

Verdict: What does energy-based even mean?

Aliens from another dimension?

hands reaching to other dimension

Sci-fi examples: Chaos and the Daemons (Warhammer 40,000) or Krang (Teenage Mutant Ninja Turtles).

Why it’s possible: Some mathematical theories about the universe incorporate other dimensions beyond the four that we inhabit.

Problems: The human brain can’t even conceive what these dimensions might be like.

Expert analysis: “Aliens from other dimensions are really just sci-fi shorthand for ‘something from somewhere else’. But why would you start inventing other dimensions in which they could have originated? There’s so many planets and stars out there that aliens could have come from.”

Verdict: Pure fantasy.

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