The earth offers the only example of life we know of. But the theory finds other options - creatures living in the molten mantle, in the icy seas of liquefied gas, and even plasma inhabitants of outer space.
Silicon. Living stones
Among the chemical elements, carbon stands out for its amazing ability to form bonds with itself. Its atoms fold into rings and chains, linear and branched molecules.
Organic substances are extremely diverse, their transformations form the chemical basis of life known to us. However, silicon, a neighbor of carbon on the periodic table, has similar properties and can also create rather complex compounds. They are far from being as stable and diverse as organic ones, but they are quite capable of serving as the basis for life of a completely different, "silicate" type. From our point of view, it will be rather slow and meager. Silicon organisms should resemble rock, in which an uncomplicated and unhurried metabolism takes place. If they use oxygen to breathe, it is not carbon dioxide that is emitted as a by-product, but silica - quartz. But, most likely, such a life prefers an oxygen-free environment, high temperature and pressure, at which silicon compounds are more mobile and stable. Such conditions can be found in the bowels of planets, including under the surface of the Earth. Unfortunately, it is not yet clear how and by what criteria it is possible to check the existence of life in hot magma.
Space. Plasma creatures
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One of the most daring hypotheses about extraterrestrial life was put forward several years ago by scientists from the General Physics Institute of the Russian Academy of Sciences.
By simulating the motion of a plasma, they discovered that its constituent particles are capable of spontaneously forming microscopic chains that immediately twist into spirals. Electric charges cause them to be attracted to each other and interact. These interactions were strikingly similar to what we usually call life. The spirals changed and copied, so that over time in their "populations" could persist and spread more and more stable and "adapted" forms. It is hard to imagine that these short-lived structures are capable of evolving into something truly complex and diverse. However, ionized plasma accounts for 99.999% of all ordinary matter in our universe. If there is still something like life in it, then this "biosphere" can be amazingly large. Against her background, and ordinary earthly organisms,and hypothetical inhabitants of other planets, even taken together, will turn out to be a rare exotic, capable of surviving only in some secluded corners of space.
Arsenic. Shadow biosphere
Phosphorus is a component of nucleic acids, cell membrane lipids and other compounds that make up the familiar life. Arsenic, chemically close to it, can interfere with many important reactions involving phosphorus, which makes it such a dangerous poison. At the same time, individual organisms still involve arsenic in their metabolism. Microbes are known that use its compounds to generate energy. And who knows if there are cells in which arsenic plays an even more important role, partially or completely replacing phosphorus?
Ten years ago, proteobacteria were isolated from the sediments of the extremely salty Lake Mono in California, containing so much arsenic that scientists first suspected the microbes using unusual forms of DNA and RNA, where this element replaces phosphorus. Later, the bold version had to be abandoned, but not the idea of "arsenic life" in general. It can inhabit isolated niches that are dangerous for other organisms. There is even a hypothesis about the existence of a whole "shadow biosphere", which constitutes an important part of life on Earth. Organisms whose biochemistry is incompatible with ours can hide in hot alkaline springs deep in the thickness of the lithosphere. Unsurprisingly, we don't overlap and still don't have the right tools to study them.
Anhydrous. Gases instead of liquids
Like carbon, water has a number of unique properties that make it such a suitable medium for the development of life. Water is an excellent, widespread solvent. But this does not mean that there is nothing to replace it in another part of the Universe and in other conditions. Methane, ammonia, hydrogen sulfide and even ordinary carbon dioxide can act as water. At high pressure and temperature, it becomes a liquid and very effective solvent that is used industrially to extract caffeine from coffee.
“Lakes” of liquid CO2 accumulate at the bottom of the Earth's oceans, and some extremophile microbes have adapted to life here. Vast seas of carbon dioxide can exist on super-Earths - distant exoplanets several times more massive than ours, capable of holding a much denser and heavier atmosphere. It is curious that proteins of ordinary terrestrial organisms, apparently, retain their structure and activity in liquid CO2. Therefore, it is possible that the distant hypothetical creatures with which carbon dioxide replaces water differ from us not so radically.
THE NEIGHBORHOOD
In a distant galaxy
It is difficult for Earthlings not to consider themselves special, although in the development of life on our planet there was not so much accidental. Carbon compounds are more diverse than substances based on all possible competitors. DNA is more stable than other suitable macromolecules, water is more versatile than alternative solvents. Therefore, many scientists believe that if someday life is found outside the Earth, then in general it will be the same as ours. However, biologists will not be bored. Developing in a different gravity, in a different atmosphere, under different stars, distant creatures are unlikely to be like us, although they will use some familiar elements. This is the densely populated universe of "Star Wars", where there are hundreds of different species. For all their differences, they retain a basic similarity to each other - and to us.
Hatta type
Slime-like creatures that grow to gigantic proportions and live for many centuries. Hutts have three lungs, firm thick skin, and a muscular tail with a bony skeleton.
Planet Nal Hutta
The hot climate, swampy soils and incessant mud rains are favorable for the fetid microflora and Hutts.
Talza type
Sentient creatures covered in thick white fur. They have two pairs of eyes: small - "daytime" and large - "night". They feed through a thin proboscis that does not allow cold air to pass through.
Planet Ortho Plutonium
A celestial body with a breathable atmosphere. The surface is covered with ice.
Wookiee view
Thick-furred humanoids 2–3 m tall. The Wookiee language seems to be an inarticulate lowing, although in fact the representatives of this race are skilled engineers, diplomats and warriors.
Planet Kashyyyk
A jungle with a temperate climate and trees as tall as a skyscraper. It revolves around a single sun, but has three moons.
Mon Calamari view
Amphibious humanoids have much in common with fish: bulging streamlined heads, large eyes, webbed "fins" and salmon-colored skin.
Planet Duck
The blue planet is completely covered by the ocean. Its inhabitants have built cities both on the surface of the water and at depth.
Geonosian species
Winged insect-like individuals nest in vertical colonies resembling termite mounds with pointed spiers.
Planet Geonosis
The planet rich in minerals is deserted. Its rocky surface is covered with radioactive dust, which forces the inhabitants to strive for the upper atmosphere.
Tuskena view
The people of the sands, or the Tusken raiders, are nomadic tribes, belligerent towards all living things, which is not surprising: in the desert, any resources are extremely expensive.
Planet Tatooine
The once beautiful world of forests and oceans, which has become a continuous dusty desert. Tatooine revolves around paired suns.
Author: Anton Monakhov