Is there alien life in the shadow biosphere?

The search for alien life is one that has preoccupied scientists
for decades – Helen Sharman, Britain’s first astronaut and a
chemist at Imperial College London (https://bit.ly/3wYbHy7),
is the latest figure to weigh in on the debate. She says she
believes that aliens exist and “there’s no two ways about it”,
before further adding that they may be living among us and
simply impossible to spot. Her suggestion leads to some
interesting science on how we define life, and how we can
possibly find something we are not properly equipped to look
for.
Thinking about alien life, Sharman asks: “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.” In this case,
life would exist in something called a ‘shadow biosphere’ –
essentially, it’s a hypothetical biosphere that contains undiscovered
creatures with different biochemical and molecular processes to
ordinary life. Sharman is thinking in terms of Earth but, if an
alternate form of life was found, it’s entirely possible that this
shadow biosphere could exist on other planets too.
It’s possible they’re here right now and we simply can’t see them.
So, why haven’t we found such biospheres? One answer is that, if
it exists, it will probably exist on a microscopic scale, and our
tools are not ready to deal with it. We can only culture a small
percentage of microbes in a lab and although it is possible to
sequence the DNA of certain microbes, doing so assumes that this
alien life will contain DNA. This is the other major issue – we have
no conception of what an alternate form of life may look like. Our
ideas of life are informed by our own understanding of life on
Earth and the chemical processes that drive it, but such ideas may
be completely useless in practice.
However, despite any practical evidence, scientists have theorised
how an alternative biochemistry may look: the most popular
suggestion is one based on silicon rather than carbon. Around 90%
of the Earth is made up of silicon, iron, magnesium and oxygen
(meaning there is a lot to go into building potential life, even
though much of the planet’s silicon is contained within rocks),
and it has certain similarities with carbon. It has four electrons
available for creating bonds with other atoms, but it is much
heavier (with 14 protons to carbon’s six) and is not as good at
creating strong bonds, thus rendering its long-chain molecules
much more unstable. Furthermore, common silicon compounds
tend to be solid at terrestrial temperatures and insoluble in water,
granting them much less flexibility in comparison to carbon.
We can only culture a small percentage of microbes in a lab and
although it is possible to sequence the DNA of certain microbes,
doing so assumes that this alien life will contain DNA.
The search for alien life is one that has preoccupied scientists for
decades – Helen Sharman, Britain’s first astronaut and a chemist
at Imperial College London, is the latest figure to weigh in on the
debate. She says she believes that aliens exist and “there’s no two
ways about it”, before further adding that they may be living among
us and simply impossible to spot. Her suggestion leads to some
interesting science on how we define life, and how we can possibly
find something we are not properly equipped to look for.
Thinking about alien life, Sharman asks: “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.” In this case, life
would exist in something called a ‘shadow biosphere’ – essentially,
it’s a hypothetical biosphere that contains undiscovered creatures
with different biochemical and molecular processes to ordinary life.
Sharman is thinking in terms of Earth but, if an alternate form
of life was found, it’s entirely possible that this shadow biosphere
could exist on other planets too.
It’s possible they’re here right now and we simply can’t see them.
So, why haven’t we found such biospheres? One answer is that, if it
exists, it will probably exist on a microscopic scale, and our tools
are not ready to deal with it. We can only culture a small percentage
of microbes in a lab and although it is possible to sequence the
DNA of certain microbes, doing so assumes that this alien life will
contain DNA. This is the other major issue – we have no conception
of what an alternate form of life may look like. Our ideas of life are
informed by our own understanding of life on Earth and the chemical
processes that drive it, but such ideas may be completely useless in
practice.
However, despite any practical evidence, scientists have theorised
how an alternative biochemistry may look: the most popular suggestion
is one based on silicon rather than carbon. Around 90% of the Earth
is made up of silicon, iron, magnesium and oxygen (meaning there is
a lot to go into building potential life, even though much of the
planet’s silicon is contained within rocks), and it has certain
similarities with carbon. It has four electrons available for
creating bonds with other atoms, but it is much heavier (with 14
protons to carbon’s six) and is not as good at creating strong bonds,
thus rendering its long-chain molecules much more unstable.
Furthermore, common silicon compounds tend to be solid at
terrestrial temperatures and insoluble in water, granting them
much less flexibility in comparison to carbon.
We can only culture a small percentage of microbes in a lab and
although it is possible to sequence the DNA of certain microbes,
doing so assumes that this alien life will contain DNA.
Although silicon is the most probable solution, it’s not the only
one that has been touted. Boranes (a class of boron molecules) are
stable in a reducing environment, although they are dangerously
explosive in Earth’s atmosphere. Various metals can form complex
and thermally stable structures when combined with oxygen, and
sulphur is able to form long-chain molecules (although these are
typically only linear chains, whereas life tends to require branched
ones).
Another major question is whether life can exist in a state that
does not require water as a solvent – other touted solvents include
ammonia, sulphuric acid, liquid nitrogen and hydrofluoric acid.
Water is important for our life processes for many reasons: it is
complex; it does not combust in oxygen; it is liquid over a large
temperature range; it has a high heat capacity; and its solid form,
ice, has a lower density than its liquid form (thus explaining why
it freezes over, rather than freezes solid). It is cosmically
abundant, but it is not the most useful solvent on many planets
because it cannot exist in liquid equilibrium over the planet’s
normal temperature range (something that is also contingent on
pressure).
Various metals can form complex and thermally stable structures
when combined with oxygen, and sulphur is able to form long-chain
molecules.
Much of this science is highly hypothetical, because imagining an
entirely different form of life is a challenge even for advanced
scientific minds. But there is nothing to discount the possibility
of unfamiliar life on Earth being discovered in the future – the big
question, however, is how we will actually be able to find it. How
do you find alien life when you don’t know what you are looking
for, and your equipment isn’t built to register it anyway?