Britains first astronaut Helen Sharman recently hit the headlines after announcing impossible-to-spot aliens may already be living among us. Life as we understand it is easy to recognise: living things move, grow, eat, excrete and reproduce.

However, while life might be easy to spot, it is actually notoriously difficult to define and has had scientists and philosophers in debate for millennia.

The intelligent life we are searching for doesnt have to be humanoid

Dr Samantha Rolfe

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.

Dr Samantha Rolfe, the astrobiology and principal technical officer at Bayfordbury Observatory, believes it is this lack of definition that presents problems when searching for life in space.

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She wrote in The Conversation: Not being able to define life other than well 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 doesnt have to be humanoid.

Ms Sharman recently courted controversy after announcing aliens exist and theres no two ways about it.

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The pioneering space explorer said: Will they be like you and me, made up of carbon and nitrogen?

Maybe not. Its possible theyre here right now and we simply cant see them.

Dr Rolf added: Such life would exist in a shadow biosphere, meaning this alien life would exhibit different biochemistry.

This alien life would consequently be outside of the realm of comprehension for humans.

Assuming it exists, such a shadow biosphere would probably be microscopic.

The Bayfordbury Observatory expert believes this alien life has not already been found due to the limited ways of studying the microscopic world.

She said: This may mean that there could indeed be many lifeforms we havent 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.

One popular suggestion for alternative biochemistry is one based on silicon rather than carbon.

Proponents for this position argue approximately 90 percent of the Earth is made up of silicon, iron, magnesium and oxygen, meaning there is plenty of material for building potential life.

Silicon is similar to carbon, it has four electrons available for creating bonds with other atoms.

Silicon is, however, heavier, with 14 protons 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.

Silicon struggles to create strong bonds, meaning long-chain molecules are far less stable.

In addition, common silicon compounds, such as silicon dioxide are generally solid at terrestrial temperatures and insoluble in water.

When compared to soluble carbon dioxide, 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.

The chemical composition of life on Earth has an approximate correlation with the chemical composition of the sun, with 98 percent of atoms in biology consisting of hydrogen, oxygen and carbon.

Therefore, if there were viable silicon lifeforms on this planet, they may have evolved elsewhere.

However, silicon-based life cannot be ruled-out, the researcher revealed.

Dr Rolfeuch wrote: Only 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 Saturns moon Titan or planets orbiting other stars, we certainly cant rule out the possibility of silicon-based life.

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Alien bombshell: Why 'it's possible' 'invisible' aliens are hiding somewhere on Earth - Express.co.uk