When we think about whether aliens exist or not, we usually imagine them on an Earth-like planet orbiting somewhere around a distant star. Hardly anyone thinks that they live in space itself. But this idea has a right to life. In April 2016, scientists became even more convinced that the key elements of life can emerge from simple substances in questionable conditions for life in interstellar space.
Cornelia Meinert of the University of Nice in France and her colleagues have shown that a mixture of frozen water, methanol and ammonia - all of which are abundant in the "molecular clouds" where stars form - can be transformed into a wide variety of sugar molecules when exposed to ultraviolet rays. also fill space. Among these sugars and ribose, part of a DNA-like RNA molecule.
It follows that the fundamental molecules of life can be formed in outer space, and then hitchhike to planets like Earth, along with ice comets and meteorites. So what, you ask? We've known for decades that other building blocks of life can emerge from chemical reactions like this, and then fall into comets, asteroids and planets. But it's not that simple. Perhaps life itself does not need a warm and cozy planet bathed in the rays of the sun to be born. If raw ingredients are suspended in space, can life originate from them?
Ideas about the origin of life rarely consider this scenario. It is already difficult to figure out how life originated on the early Earth, not to mention conditions in which temperatures are close to absolute zero, and instead of the atmosphere, there is almost a complete vacuum.
Creating the basic building blocks of life, sugars and amino acids is as easy as ever. There are many chemically possible ways to do this, having at least simple molecules of young solar systems available.
It is much more difficult to get these complex molecules to assemble into something that can support life processes such as reproduction and metabolism. Nobody has ever done that. No one suggested a possible way to do this - even in the most comfortable laboratory environment, let alone in outer space.
And yet there is no reason why life could not have appeared far from any star, somewhere in the barren desert of interstellar space. Quite the opposite.
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But first, we need to agree on what counts as "life." After all, it is not at all necessary to look for something familiar. For example, you can imagine something like the Black Cloud in the classic science fiction novel of the same name by Fred Hoyle in 1959: a kind of living gas that floats in interstellar space and is surprised to find life on the planet. True, Hoyle did not offer a clear explanation of how a gas without a certain chemical composition could become reasonable. Perhaps we will represent something more solid.
While we cannot be certain that all life is carbon-based, as we are on Earth, there is every reason to believe that it is. Carbon is a much more flexible building block for complex molecules than silicon, the second most popular theoretical basis for life. Scientists love to speculate about how silicon-based alien biochemistry might live in the first place.
Astrobiologist Charles Cockell of the University of Edinburgh in the UK believes that the basis of life on Earth - carbon and the need for water - "reflects a universal norm." He admits that his view is somewhat conservative, which science, as a rule, rejects. But let's take a conventional life on carbon. How could it originate in deep space?
With the chemical base, everything is clear. Like sugars, life on Earth needs amino acids, the building blocks of proteins. But we know that they can be formed in outer space, because they are found in "primitive" meteorites that have never seen the surface of the planet.
They can appear in ice granules in a chemical reaction called Strecker synthesis, named after the 19th century German chemist who discovered it. This reaction involves simple organic molecules, ketones or aldehydes, in combination with hydrogen cyanide and ammonia. Chemistry combined with ultraviolet light is proposed as an alternative for initiation.
At first glance, these reactions appear to have no place in deep space, since there are no sources of heat or light to propel them. Molecules that collide in cold, dark conditions don't have enough energy to start a chemical reaction. They seem to be trying to jump over a barrier that is too high for them.
But in the 1970s, Soviet chemist Vitaly Gol'dansky showed the opposite. Some chemicals can react even when cooled to four degrees above absolute zero - almost like the temperature of space itself. All they need to do is help high-energy radiation like gamma rays or electron beams - cosmic rays that sweep through space.
Under these conditions, as Goldansky discovered, formaldehyde, a carbon-based molecule common in molecular clouds, can assemble into polymer chains several hundred molecules long. Goldansky believed that such cosmic reactions could help the molecular building blocks of life assemble from simple ingredients, hydrogen cyanide, ammonia and water.
Making such molecules merge into more complex forms is much more difficult. The high energy radiation that might have helped initiate the first reactions is now becoming a problem. Ultraviolet light and other forms of radiation can cause reactions like those demonstrated by Meinert. But Cockell says they will break molecules as well as assemble them. Possible biomolecules - precursors of proteins and RNA, for example - will break apart faster than they can be produced.
“The end result is the question of whether a completely alien environment can support the emergence and growth of self-replicating chemical systems that can evolve,” says Cockell. "I see no reason why this could not happen in very cold conditions or on the surfaces of ice granules, but in general, I doubt that very complex molecules can appear under such conditions."
The planets offer two softer sources of energy: heat and light. Life on Earth depends on sunlight, so it would not be superfluous to assume that life on "exoplanets" near other stars will also rely on the energy reserves of their own luminaries.
Vital warmth is also everywhere. Some scientists believe that the first life on Earth did not rely on sunlight, but on volcanic energy that came out of the interior of the planet, as well as on hot springs in the deep sea. Even today, these springs spew a warm brew rich in minerals.
There is also heat on the large moons of Jupiter. It is born in the course of the action of powerful tidal forces exerted on the satellites by a giant planet, compressing the bowels of the moons and heating them in the process of internal friction. These tidal energies cause the oceans to melt on the icy moons Europa and Ganymede, and Io generally has the most powerful volcanic system in the solar system.
It is difficult to imagine how molecules, forced to hide in the ice grains of interstellar space, could find this nurturing energy. But there may be other options?
In 1999, planetary scientist David Stevenson of the California Institute of Technology suggested that galaxies may be full of "rogue planets" that float outside stellar environs, too far from their parent star to sense its gravity, heat, or light.
These worlds, Stevenson said, could have formed like an ordinary planet, close to a star, in its environment of gas and dust. But then the gravitational tug of large planets like Jupiter or Saturn led to the fact that the planets left their trajectories and were thrown into the empty space between the stars. It may seem that they face a cold and barren future. But Stevenson argued that, on the contrary, these rogue planets may be "the most abundant living worlds in the universe" - because they can remain warm enough to support the existence of liquid water underground.
All solid planets in the inner solar system have two internal heat sources.
First, each planet has a fiery core that is even hotter after formation. Secondly, there are radioactive elements. They heat up the interior of the planet in the process of decay - a piece of uranium is warm to the touch. On Earth, radioactive decay within the mantle accounts for half of the total heating.
The primordial heat and radioactive decay inside solid roaming planets can keep them warm for billions of years - perhaps long enough for the planets to remain volcanically active and to have enough energy to start life.
Rogue planets can also have dense, heat-trapping atmospheres. Compared to gas giants like Jupiter and Saturn, the Earth's atmosphere is thin and fragile as the heat and light from the Sun carries away light gases like hydrogen. Mercury is so close to the Sun that it has no atmosphere at all.
But roaming Earth-sized planets that are far from the influence of the native star may also have a primordial atmosphere. Stevenson calculated that the temperature and pressure on such a planet would be enough to keep water liquid on the surface even in the absence of any sunlight.
Moreover, rogue planets will not be subject to the fall of large meteorites, as the Earth once did. They can be thrown out of their home solar system even with their satellites on a leash, which will subsequently provide some heating due to tidal forces.
Even if such a planet does not have a dense atmosphere, it can still be habitable.
In 2011, planetary scientist Dorian Abbott and astrophysicist Eric Schwitzer of the University of Chicago calculated that planets three and a half times the size of Earth could be covered with thick ice entirely. Beneath it there will be an ocean of liquid water many kilometers below the surface, warmed by the bowels.
"The overall biological activity will be lower than on a planet like Earth, but you can still find something," Abbott said. He hopes that as space probes explore the subsurface oceans of Jupiter's icy moon in the coming decades, we will learn more about the possibility of life on ice planets.
Abbott and Schwitzer call these lost worlds the "Steppenwolf planets" because "any life on such worlds will be like a lone wolf roaming the galactic steppe." The habitability of life on such a planet could be up to 10 billion years or so, similar to that on Earth, Abbott says.
If he's right, there could be wandering planets in interstellar space outside of our solar system, and on them alien life. Finding them at such a distance, tiny and dark, will be very difficult. But if you're lucky, such a planet can pass at a distance of thousands of AU. e. (distance from the Earth to the Sun) and reflect a tiny amount of sunlight. We could try to see it with our modern telescopes.
If life can form and survive on the Steppenwolf interstellar planet, Abbott and Schwitzer say, the simple conclusion can be drawn: Life must be everywhere in the universe. Yes, life on them would be weird as hell. Imagine swimming in warm volcanic springs under an eternal night like Iceland in winter. But for those who don't know anything else, it will feel like home.