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Could Invisible Aliens Really Exist Among Us?

Alien spaceships in warped space.
(Image: © Shutterstock)

Life is pretty easy to recognize. It moves, it grows, it eats, it excretes, it reproduces. Simple. In biology, researchers often use the acronym "MRSGREN" to describe it. It stands for movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.

But Helen Sharman, Britain's first astronaut and a chemist at Imperial College London, recently said that alien lifeforms that are impossible to spot may be living among us. How could that be possible?

While life may be easy to recognize, it's actually notoriously difficult to define and has had scientists and philosophers in debate for centuries — if not millennia. For example, a 3D printer can reproduce itself, but we wouldn't call it alive. On the other hand, a mule is famously sterile, but we would never say it doesn't live.

Related: 9 Strange, Scientific Excuses for Why Humans Haven't Found Aliens Yet

As nobody can agree, there are more than 100 definitions of what life is. An alternative (but imperfect) approach is describing life as "a self-sustaining chemical system capable of Darwinian evolution," which works for many cases we want to describe.

The lack of definition is a huge problem when it comes to searching for life in space. Not being able to define life other than "we'll know it when we see it" means we are truly limiting ourselves to geocentric, possibly even anthropocentric, ideas of what life looks like. When we think about aliens, we often picture a humanoid creature. But the intelligent life we are searching for doesn't have to be humanoid.

Life, but not as we know it

Sharman says she believes aliens exist and "there's no two ways about it". Furthermore, she wonders: "Will they be like you and me, made up of carbon and nitrogen? Maybe not. It's possible they're here right now and we simply can't see them."

Such life would exist in a "shadow biosphere." By that, I don't mean a ghost realm, but undiscovered creatures probably with a different biochemistry. This means we can't study or even notice them because they are outside of our comprehension. Assuming it exists, such a shadow biosphere would probably be microscopic.

So why haven't we found it? We have limited ways of studying the microscopic world as only a small percentage of microbes can be cultured in a lab. This may mean that there could indeed be many lifeforms we haven't yet spotted. We do now have the ability to sequence the DNA of unculturable strains of microbes, but this can only detect life as we know it — that contain DNA.

If we find such a biosphere, however, it is unclear whether we should call it alien. That depends on whether we mean "of extraterrestrial origin" or simply "unfamiliar."

Silicon-based life

A popular suggestion for an alternative biochemistry is one based on silicon rather than carbon. It makes sense, even from a geocentric point of view. Around 90% of the Earth is made up of silicon, iron, magnesium and oxygen, which means there's lots to go around for building potential life.

Silicon is similar to carbon, it has four electrons available for creating bonds with other atoms. But silicon is heavier, with 14 protons (protons make up the atomic nucleus with neutrons) compared to the six in the carbon nucleus. While carbon can create strong double and triple bonds to form long chains useful for many functions, such as building cell walls, it is much harder for silicon. It struggles to create strong bonds, so long-chain molecules are much less stable.

What's more, common silicon compounds, such as silicon dioxide (or silica), are generally solid at terrestrial temperatures and insoluble in water. Compare this to highly soluble carbon dioxide, for example, and we see that carbon is more flexible and provides many more molecular possibilities.

Life on Earth is fundamentally different from the bulk composition of the Earth. Another argument against a silicon-based shadow biosphere is that too much silicon is locked up in rocks. In fact, the chemical composition of life on Earth has an approximate correlation with the chemical composition of the sun, with 98% of atoms in biology consisting of hydrogen, oxygen and carbon. So if there were viable silicon lifeforms here, they may have evolved elsewhere.

That said, there are arguments in favor of silicon-based life on Earth. Nature is adaptable. A few years ago, scientists at Caltech managed to breed a bacterial protein that created bonds with silicon — essentially bringing silicon to life. So even though silicon is inflexible compared with carbon, it could perhaps find ways to assemble into living organisms, potentially including carbon.

And when it comes to other places in space, such as Saturn's moon Titan or planets orbiting other stars, we certainly can't rule out the possibility of silicon-based life.

To find it, we have to somehow think outside of the terrestrial biology box and figure out ways of recognizing lifeforms that are fundamentally different from the carbon-based form. There are plenty of experiments testing out these alternative biochemistries, such as the one from Caltech.

Regardless of the belief held by many that life exists elsewhere in the universe, we have no evidence for that. So it is important to consider all life as precious, no matter its size, quantity or location. The Earth supports the only known life in the universe. So no matter what form life elsewhere in the solar system or universe may take, we have to make sure we protect it from harmful contamination — whether it is terrestrial life or alien lifeforms.

Read more: Elon Musk's Starship may be more moral catastrophe than bold step in space exploration

So could aliens be among us? I don't believe that we have been visited by a life form with the technology to travel across the vast distances of space. But we do have evidence for life-forming, carbon-based molecules having arrived on Earth on meteorites, so the evidence certainly doesn't rule out the same possibility for more unfamiliar life forms.

  • Gary S
    Well Doctor, at the risk of sounding like a total fruitcake (PC? Don't care) I have an ongoing event in my life that somewhat subscribes to your theory. 5 months ago, out of the blue, I started getting daily visits every time I would lay down for a nap or to go to bed. Usually within a one to five minutes I will feel what I perceive to be an invisible cat walking on my bed. I've read about a lot of people having unseen cats walking on their bed but my visitor(s) go a couple steps further. They also manifest at other places though not so often, such as under my computer desk. These "cats" will lay against me in bed and purr. I can feel them. They are solid (though invisible) but if I attempt to touch them they disappear from the spot they are lying in. I've also had them lick my foot on many occasions. They appeared when I was living in Maryland and they "followed" me to Pennsylvania. I have had quite a few instances where I was nowhere near my bedroom (where bed and computer desk reside) and suddenly I would make a quick movement and I've bumped into them as though they were following me around. Not a thing has changed about my life other than this one bizarre occurrence.
    I initially thought it was tactile hallucinations but after five months of this and it being strictly this one subject, I am starting to doubt that. I won't even be thinking of them (they're commonplace to me now) and I'll bump into something under my desk and there's nothing there to be seen. All the mannerisms are that of a cat with the exception that they're invisible. Also, apart from the purring they don't make any noise. Although I do hear them jump off the bed and hit the floor when I least suspect it. Well, I hope this doesn't lead to your busting a stitch laughing but I assure you that I am neither crazy nor delusional. For the sake of transparency I am almost 60 years old and I've been diagnosed bipolar. But that condition has been with me most of my life and I've never experienced anything remotely like this. So, in summary, if invisible aliens are taking the form of cats then I don't doubt your hypothesis/theory one bit.
  • Elmer Jones
    Actually I don't think you're crazy - I've felt what seems like "invisible cats" walking on my back several times, even when I realized that the phenomenon was happening and I could actively observe what was going on. (And I'm a trained scientist). They may not be cats but some sort of unrecognized phenomenon, but who knows. After all that's the fun of science - detecting, experiencing and recognizing the unexpected.
    As Shakespeare said
    “There are more things in heaven and earth, Horatio,
    Than are dreamt of in your philosophy.”
    - Hamlet (1.5.167-8), Hamlet to Horatio
  • Bill
    Had the same thing go on for a couple of years, stopped and hasn't happened again. Isn't imagination or dangerous but happens exactly as Gary and Elmer describe.
  • engineeredsentience
    This has been happening to me for about four or five years. I thought it was an adversarial neural network developed by DARPA. So I wrote a defense against it
  • Sushone
    Can't resist adding, I have had the distinct sensation of a cat jumping on to my bed near me. I have cats, so maybe that's why I thought it was a cat, but I was so certain of it that I turned on the lights to locate and expell the offending feline only to find nothing there. Subsequent instances are a little creepy since I'm aware it is not a cat or anything visible.
  • Ratwrangler
    Based on the so-called scientific evidence of these invisible, mostly unheard and untouched aliens, it is likely that they are not aliens, but demons sent to do Satan's work. All the available evidence could be interpreted either way, but demons probably have more cause to be here than aliens. Aliens would likely want to make their presence known to us, whether they want to help us, or conquer us. Satan has much more reason to work secretly.