One of the great grey areas of astrobiology is just how alien biochemistry might work.
Conventional wisdom would be to look at life on earth, we know for a fact it can exist,
and then imagine how that might apply to life in the universe.
And while there doesn’t seem to be any reason to doubt that carbon-based biochemistry couldn’t
be common in the universe, it’s also possible that other types of radically different biochemistry
could exist.
So here are 10 ways alien life could be radically different from that of earth.
Number 10.
Alternative Chirality
One odd aspect of life on planet earth is that everything is apparently of one variety
when it comes to chirality, effectively how biomolecules can exist in mirror configurations.
But there doesn’t appear to be any good reason why life couldn’t also function in
alternative chirality, and that examples of it just happen to be missing here on earth.
Or at least appears to be missing, as amino acids of opposite chirality have been found
here, and while they are thought to be a product of the decay of normal organisms, it also
can’t be ruled out that life of opposite chirality does indeed exist here and we simply
haven’t detected it.
Given what we know, it could be possible that life on other worlds might be based on some
configuration of alternate chirality.
While different in a chemical sense though, these type of configuration wouldn’t be
all that alien, at least in the sense of it being markedly different from what we see
on earth.
In short, we would recognize it as life.
But it would be alien in the sense that it would have no relation to life on this world,
so far as we know.
Number Nine.
Different Solvents
We often discuss the search for life in the universe on the presence of liquid water on
the surface of an exoplanet.
There’s a reason for this, it’s crucial to life here on earth.
But, our planet is a sampling of one.
But water isn’t the only liquid that life might use as a solvent and we have to be careful
of water bias when evaluating the possibilities for life.
Perhaps the strongest alternative to water would be ammonia.
Like water, there is no shortage of it in the universe.
Ammonia can support a wide range of chemical reactions, and can dissolve organics.
Other possible alternatives to water include formamide, hydrogen flouride, and even hydrocarbons
opening the way for very low temperature life, and also liquid in low temperatures is nitrogen.
But, as with anything, there are tradeoffs and the alternative solvents seem more limited
for life than liquid water, but even water actually does have some disadvantages, one
being that it has high albedo, contributing to the severity of earth’s ice ages.
Also possible are a few possible mixes of water and other liquids, such as ammonia and
water.
This is thought to be possible at Saturn’s moon titan, where a subsurface mix of ammonia
and water could remain liquid at much lower temperatures than water alone.
The solvent in which alien life is based would also profoundly affect things like evolution,
for example water being transparent to sunlight helped guide the evolution of oceanic creatures,
and ultimately the formation of eyes.
In other conditions in other fluids, the equations can change and life based in an alternate
solvent could be very different from that of earth indeed.
Number 8.
Arsenic Life
Even though arsenic is usually considered poisonous, at least to humans, it’s thought
that phosphorus and arsenic are chemically similar enough that it may be possible that
alien life could use arsenic in its biochemistry and even its DNA.
And, arsenic might figure prominently as far life in the universe at large because phosphorus
has somewhat of a problem.
Astronomers studying the abundance of phosphorus in the universe have found that it’s actually
not that common, due to differences in how it’s created in supernovas.
If this is the case, life, at least as we know it on earth, wouldn’t be able to get
started in some areas of the universe just from a lack of phosphorus.
This is enough of a problem, that the idea of phosphorus simply being rare and unevenly
distributed in the galaxy could be a solution to the Fermi Paradox.
While contentious, if arsenic can indeed function similarly to phosphorus in biochemistry, then
there may well be more arsenic based life out there than phosphorus based life.
Number Seven.
Non-Green Photosynthesis
Plant life on earth is predominantly green in color due to the plants inability to absorb
some of the green wavelengths of light, yet the sun emits strongly in green light.
Rather plants tend to absorb red and blue light, and it could be that green isn’t
absorbed simply because the plant has all the light it needs, at least those on land.
But when you start looking at the classes of stars that might also be suitable for alien
life, the profiles of their light emissions change.
As a result, photosynthesizing plant analogues on exoplanets may not tend towards green,
rather yellow, red or even black.
It’s also possible that alien photosynthesizing analogues might not even be based on chlorophyll,
but rather retinal.
If so, this kind of life would appear purple.
Number Six.
Azotosomes
It’s possible, perhaps even likely, that the very simplest microbial forms of life
in the universe will tend to look a lot alike no matter where it’s found.
Some very likely will be similar to organisms present on earth.
But looks can be deceiving, if functionally similar, their biochemistry could still be
radically different.
Take Titan, the only other body in the solar system to sport large amounts of liquids on
its surface other than earth that could support some kind of biology.
The problem being, Titan also happens to be extremely cold, far colder than anything life
on earth can withstand.
But what of a different configuration of microbial life.
Researchers envisioned what this might look like by thinking about different kinds of
chemistry, in this case nitrogen compounds that could be used to create a kind of cell
membrane that could function on Titan.
Called an azotosome, this hypothetical cell membrane could function at temperatures where
methane is liquid, about 292 degrees below zero fahrenheit.
Such a microbe could use the nitrogen, carbon and hydrogen that are present on Titan in
its biology, and use no oxygen at all.
Number Five.
The Long Sleep
With planet earth, we essentially got lucky as far as stable climate conditions.
Mostly.
Our planet undergoes seasonal changes that make certain periods harder on some animal
life than the rest of the year.
But this could actually get much worse on exoplanets where life, at least as we envision
it, might come to a near standstill for long periods of time.
Life on earth has proven resilient to cold periods, for example there are species that
can completely dry out, freeze, and then effectively come back to life when conditions allow for
it.
Essentially, this isn’t just dormancy, but a period of death and resurrection, and it’s
not just simple life, there are species of frog that can do this.
As to what’s possible elsewhere in the universe, there may even be intelligent species that
have evolved in harsher conditions than earth that hibernate, or even outright die in order
to survive periods of adverse conditions only to rehydrate when times get better and effectively
come back to life.
Such a species might even use that to their advantage, making long journeys across space
in a state of natural suspended animation or even temporary death.
Number Four.
Silicon Based Life
Most elements on the periodic table are not good candidates for life due to their chemistry.
An element like gold for example doesn’t have the ability to react with other elements
to create the complex chemistry needed to sustain an analogue of biology.
But there are a few that might, and chief among these is silicon.
Silicon is in the same group as carbon on the periodic table and can form complex molecules
that could conceivably be a basis for life.
But it’s much more limited than carbon is, but on the other hand it’s been suggested
that silicones could be more stable than hydrocarbons in atmospheres rich in sulfuric acid, such
as venus-like worlds.
And while carbon is extremely common in the universe, more so than silicon, terrestrial
planets like earth are very rich in silicon, yet poorer in carbon, yet despite that, life
on earth is carbon based and we have not found any strictly silicon based life on this planet,
though plenty of life uses it, such as diatoms and one hypothesis holds that the presence
of silicon may have been necessary for carbon based life to have its genesis.
Silicon based life might also be well adapted for very low temperatures, or high ones, and
it has to be said that it may be the case that silicon ends up being the basis for intelligent
life, not in a biological context, but a technological one as species may choose to transition to
become machine civilizations.
But silicon isn’t the only possibility, boron-based life is also in principle thought
possible, though it’s somewhat rare in the universe.
Sulfur also has hypothetical scenarios where life based on it might be possible, but perhaps
most interestingly are some of the metals.
Life might also, under very strict conditions, exist using metal oxidation, even at very
high temperatures where carbon based life probably cannot exist.
Number Three.
Neutron Star Life
One of the biggest problems when discussing the possibilities of alien life are biases.
Carl Sagan once stated that he was a carbon chauvenist, and to a degree a water chauvenist
when speculating about what alien life can be like.
But it could also be surprising, and nothing like what we’re used to.
One possibility, though a long shot, would be life based on a neutron star.
First proposed by Frank Drake, and explored in the books of Robert Forward, this speculative
form of life would exist on the surface of a neutron star.
They would be very tiny, sub-microscopic, existing based on a kind of alternate type
of molecule where tighly packed atomic nuclei form random molecules of sorts.
It’s a complete unknown if this is even possible, neutron stars are made up of extraordinarily
dense material.
But what can be said is that if this type of life indeed does exist, it might exist
in fast forward due to the speed at which nuclear reactions occur.
They would experience their lives conceivably millions of times faster than you do.
Number Two.
Plasma Life
So far, we’ve taken the subject of alternative life mostly from the standpoint of straightforward
chemistry, but that may not be the only possibility for life.
In 2003, researchers were able to create plasmas that exhibited life-like behaviour, in that
they could grow, replicate and even communicate to some degree with each other.
This plasma phenomenon featured a distinct layer, much like a cell wall, that separated
it from its outside environment, a key feature of biological cells.
They could also replicate by splitting in two, again like living cells.
And they demonstrated the ability to grow under certain circumstances.
The communication came in the form of electromagnetic emissions.
If this can indeed be considered a form of life, which is fully open for debate, we may
already have seen an alternative form of life.
Number One.
The Shadow Biosphere and RNA life
While speculative, the concept of panspermia, the idea that life could hitch a ride on rocks
blasted off a planet and then fall on another planet and survive to colonize it, isn’t
all that far-fetched and may well have occured within our own solar system.
Indeed, life on earth might have originally been seeded here from some other body, such
as Mars, where conditions in the distant past may have made it an easier place for life
to arise, than earth was at the time.
But the idea of panspermia can be extended to interstellar scales.
While carbon based life as we know it may not have great chances of surviving interstellar
travel, it’s possible that some of the hypothetical alternate life on this list might fare better,
such as silicon based life.
If this is indeed possible, it might be here undetected, or if silicon-based simple indigenous
life could have formed on earth, then would we even know about it?
The answer is not necessarily.
The shadow biosphere is a hypothesized second biosphere, or even a second separate genesis
of life on earth based on some other biochemistry.
Given that our methods of studying microbial life typically depend in some way on familiar
means of biochemistry, it’s possible that an entirely different microbial world could,
at least in principle, exist alongside our standard microbial biosphere undetected.
Another variant on this theme is that it’s possible that there were once organisms on
earth that may have been based on RNA instead of DNA.
If so, the methods typically used to detect microorganisms wouldn’t detect them, should
they still be living among us.
While the shadow biosphere is hypothetical, there is a phenomenon in geology known as
desert varnish that’s not well understood.
It forms as deposits on the surface of rocks and its origin is more likely to be geological
rather than biological.
But one odd feature of this material is that it’s very high in manganese, about 50 times
more abundant than is normal for earth’s crust.
One possibility that’s been advanced is that a shadow microorganism could be depositing
it.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently eyeing my office
suspiciously wondering if there’s a shadow biosphere in here and what it’s currently
doing.
It’s all very cloak and dagger, the microbial world and be sure to check out my books at
your favorite online book retailer and subscribe to my channels for regular, in-depth explorations
into the interesting, weird and unknown aspects of this amazing universe in which we live.