Life is pretty easy to learn. He moves, he grows, he eats, he secrete, he multiplies. Simply. In biology, researchers often use the abbreviation MRSGREN to describe it. It refers to movement, breathing, sensitivity, growth, reproduction, excretion and nutrition.

But Helen Sharman, the first British astronaut and chemist at Imperial College London, recently said that alien life forms that cannot be found can live among us. How could this be possible?

Although life can be easily recognized, it is actually difficult to define, and scientists and philosophers have argued for centuries, if not millennia. For example, a 3D printer can reproduce itself, but we would not call it alive. On the other hand, the mule is famously sterile, but we would never say that he does not live.

As no one can agree, there are over 100 definitions of what life is. An alternative (but imperfect) approach describes life as a self-sustaining chemical system capable of Darwin evolution, which works in many of the cases that we want to describe.

(Read: aliens exist, but we are not open enough to see them)

Lack of definition is a huge problem when it comes to finding life in space. Failure to define life other than we will know it when we see it means that we really confine ourselves to geocentric, perhaps even anthropocentric ideas about how life looks. When we think of aliens, we often portray humanoid creatures. But the intelligent life we seek does not have to be humanoid.

Sharman says that she believes that aliens exist, and there are no two ways. She also wonders: Will they look like you and me, made up of carbon and nitrogen? Probably no. Perhaps they are here now, and we simply cannot see them.

Such a life will exist in the shadow biosphere. By this I do not mean the realm of ghosts, but undetected creatures, possibly with a different biochemistry. This means that we cannot study or even notice them, because they are beyond our comprehension. Assuming that it exists, such a shadow biosphere is likely to be microscopic.

So why didnt we find it? We have limited opportunities to study the microscopic world, since only a small percentage of microbes can be cultured in the laboratory. This may mean that there really can be many life forms that we have not yet noticed. Now we have the opportunity to sequence the DNA of uncultured strains of microbes, but this can only be detected by the life that we know which contains DNA.

However, if we find such a biosphere, it is not clear whether we should call it a stranger. It depends on whether we mean extraterrestrial origin or simply unfamiliar.

A popular alternative biochemistry proposal is based on silicon, not carbon. This makes sense even from a geocentric point of view. About 90 percent of the Earth is made up of silicon, iron, magnesium and oxygen, which means there are many opportunities for creating potential life.

Artists impression of silicone life form. Zita

Silicon is like carbon, it has 4 electrons to create bonds with other atoms. But silicon is heavier, with 14 protons (protons make up the atomic nucleus with neutrons) compared with six in the carbon nucleus. Although carbon can create strong double and triple bonds to form long chains, useful for many functions, such as building cell walls, silicon is much more complex. He is struggling to create strong bonds, so molecules with long chains are much less stable.

Moreover, conventional silicon compounds, such as silicon dioxide (or silicon dioxide), are usually solid at terrestrial temperatures and insoluble in water. Compare this, for example, with highly soluble carbon dioxide, and we will see that carbon is more flexible and provides much more molecular possibilities.

Life on Earth is fundamentally different from the basic composition of the Earth. Another argument against the silicon-based shadow biosphere is that there is too much silicon in the rocks. In fact, the chemical composition of life on Earth has an approximate correlation with the chemical composition of the Sun, with 98 percent of the atoms in biology being made up of hydrogen, oxygen, and carbon. So, if there were viable life forms of silicon, they could have developed elsewhere.

However, there are arguments in favor of silicon-based life on Earth. Nature is adaptable. A few years ago, scientists at the California Institute of Technology managed to bring out a bacterial protein that created bonds with silicon essentially driving silicon. Thus, although silicon is inflexible compared to carbon, it may be able to find ways to assemble into living organisms, potentially including carbon.

And when it comes to other places in space, such as the moon of Saturns Titan or planets orbiting other stars, we certainly cannot rule out the possibility of life based on silicon.

To find it, we must somehow think outside the framework of terrestrial biology and find ways to recognize life forms that are fundamentally different from carbon-based forms. There are many experiments to test these alternative biochemical methods, for example, from Caltech

Regardless of the belief of many that life exists elsewhere in the universe, we have no evidence of this. Therefore, it is important to consider all life as precious, regardless of its size, quantity or location. Earth supports the only known life in the universe. Therefore, no matter what form life can take elsewhere in the solar system or the universe, we must be sure that we will protect it from harmful pollution whether it be earthly life or alien life forms.

So can aliens be among us? I do not believe that we were visited by a living form with the technology to travel through vast outer spaces. But we have evidence that carbon-based life-forming molecules arrived on Earth on meteorites, so the data certainly does not preclude the same possibility for more unfamiliar life forms.

This article is reprinted from Conversation by Samantha Rolfe, lecturer in astrobiology and chief technical specialist at the Bayfordbury Observatory, University of Hertfordshire, licensed under Creative Commons. Read the original article.

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Astrobiologist explains why there may be invisible aliens among us. - Tech Ballad