The terms "alien" and "extraterrestrial" are often associated with characters in science fiction works. However, although it is speculative, there is a branch of biology that investigates and considers the existence of extraterrestrial life: exobiology.
But how is it possible to study organisms whose existence is not proven? What and where should exobiologists look to understand if there is life in the universe?
Lto drake equation
In 1960 Frank Drake, an American astronomer, conducted the first investigation at the National Radio Astronomy Observatory, to try to detect radio signals from extraterrestrial civilizations. A year later, Drake formulated an equation still applied today in the field of exobiology, designed to estimate the number of extraterrestrial civilizations in our galaxy, indicated by the letter N.
The Drake equation takes several parameters into account and is formulated as follows:
The values of the Equation
The first value is R *, which is the rate of star formation in the Milky Way. After that, only stars linked to planetary systems should be taken into consideration; these must have the necessary conditions to host life, requirements that are not easy to satisfy and are respectively represented by f p y n e . F l corresponds to the fraction of planets where life is supposed to develop, while fi es the fraction of these where the life that develops is intelligent.
Not only does it have to be smart, but the variable f csays that these life forms must be able to develop technology that emits a radio signal into space. The last variable is L, the period of time in which the signals are supposed to be sent. As can be seen, the variables are many and it is difficult to establish exactly each individual value, so we talk about probability. However, there are estimates and results that can, at least theoretically, give a value to the variable N and answer the question.
Interpretations and solutions
Since the first formulation of the Equation, many scientists have tried to elaborate its result. From the 1960s to today, the scientific tools available for processing values have evolved, but the equation is, in fact, still a way of discussing the issue in scientific terms, rather than providing definitive answers.
The most recent estimates assume up to 23 extraterrestrial civilizations (exobiology)
But then why have we never had proof of its existence? This is precisely the dilemma known as the Fermi paradox, which took its name from the Italian physicist who first proposed it, Enrico Fermi. Since there is no certainty in this regard, scientists dealing with exobiology today have tried to focus their attention on the requirements that an organism must have in order to develop, without excluding the most hostile environments.
Exobiology: conditions for life to exist
When looking for life forms in space, it is assumed that they are found on planets with characteristics very similar to those of Earth: abundance of water, energy sources and other fundamental molecules.
According to exobiologists, these are the minimum requirements, but we must remember that we cannot establish with certainty that life is always based on the same identical molecules.
More generally, we are not even sure that it can be hypothesize the presence of life if all the ingredients that we tend to consider indispensable are present: a liquid solvent, a source of energy and the so-called basic components, that is, basic molecules, organic and inorganic , that combined with each other give rise to more complex structures. Other variable parameters are pH, temperature, pressure, salinity and radiation. Planets with characteristics similar to Earth's are more commonly called exoplanets.
However, thanks to organisms known as extremophiles, we know that life can thrive not only on exoplanets, but wherever the minimum conditions exist.
exoplanets and light year
What we call Exoplanets They are celestial bodies that are part of a solar system, in ours or in other galaxies. They revolve around their sun at a distance that allows the presence of liquid water or other solvents, one of the most important requirements for the development of life. These planets, like Earth, can have a multitude of environments in which the chemical and physical conditions are potentially good to support life. Unfortunately, most of them are several light years away from our solar system.
El light-year is the distance that light travels in one year. Light from the Sun reaches us in 8 and a half minutes, traveling a distance of 150 million km. The distance traveled by light in one year (light year) is approximately 63.000 times the distance traveled by the Sun to Earth. So 63 thousand times 150 million km.
Exobiology: Proxima B
The closest is Next b, is part of the Proxima Centauri system in our galaxy, the Milky Way. Proxima b is 4,2 light-years away and is the eighth most Earth-like planet according to the ESI index, a physical measurement scale used to compare other planets to Earth. The value of this index is between 0 (no similarity) and 1 (planet identical to Earth) and is calculated based on the radius, density, escape velocity and surface temperature. Proxima b has an ESI value of 0,87 and indicates that the planet is very similar to Earth. However, this data does not provide information about its habitability.
Windscreens
The search for life in space is not limited to exoplanets, but also affects their satellites, the moons. An example can be found just inside our solar system. It is believed that a moon of Saturn, Enceladus, and a moon of Jupiter, Europe, potentially harbor life.
The distance from the sun Enceladusit doesn't allow it to receive enough solar radiation to heat itself, so its surface temperatures range between -128°C and -240°C: definitely not a place where life would normally be sought. However, thanks to the Cassini probe it was possible to establish that water and organic molecules are present on this frozen moon. Analyzes have shown that nitrogen, carbon dioxide and methane are present in the water vapor jets emitted at the surface. For this reason, it is believed that under the frozen surface there is an abundant layer of water, in which various molecules are dissolved, responsible for the hydrothermal activity of the substrate and also for the geysers on the surface. It could be thought that this phenomenon is influenced by the hypothetical presence of methanogenic organisms.
In 2018, some researchers tried to reconstruct the conditions of Enceladus with an experiment, showing that the microorganism Methanothermococcus okinawensis it would have the ideal characteristics to live and produce methane in the underlying layer. The conclusion of this study tells us that similar organisms may be capable of doing this, and therefore actually be on Enceladus.
What bacteria could live on other planets?
Microorganisms with particular abilities are identified as extremophiles because they often live in conditions that are prohibitive for more complex organisms. It should be noted that these organisms normally live in these conditions, so it can be thought that they survive and are also found in more complex scenarios.
The most famous in the world of biology is surely the Thermus aquaticus, capable of growing at temperatures of 75°C; thanks to him it was possible to significantly improve the method of DNA amplification. There are many such microorganisms, each of which has adapted to one or more different conditions, thus becoming polyextremophilic.
Here are some fascinating examples:
- Picrophilus oshimae it lives in sulfate in very acidic pH conditions with a value of 0,6 out of 14, stronger than hydrochloric acid.
- Thermococcus piezophilus lives in the abyss at a pressure of 125 Mpa, which corresponds to approximately 1275 kg applied to an area of one centimeter. It has been verified that other microorganisms manage to remain metabolically active even under pressures of 2000 Mpa;
- Halarsenatibacter silvermanii lives in a highly alkaline lake where NaCl salt concentrations are 35% mg/L;
- Deinococcus radioduran s, to date considered the model microorganism for the study of resistance to radiation and vacuum, a polyextremophile capable of surviving the conditions of the planet Mars.
There is life in Mars?
Mars is the fourth most distant planet from our sun, before it is Earth. Many missions have been carried out in recent decades to explore it and conduct research. NASA's Perseverance is the newest, still active and expected to re-enter in 2033.
Soil data and conditions on Mars at the moment do not seem promising for exobiology. In 2003, a research team identified a match in terms of soil composition between the soil sample collected by the Viking mission and soil from a remote region of the Atacama Desert in Chile, and after several attempts determined that the soil was not It was not suitable for any type of organic cultivation. So where is it still possible to hope to find traces of life on Mars?
underground life
A 2022 discovery has energized exobiologists in their search for extraterrestrial life. These are small crystals present in rock inclusions in central Australia, dating back 830 million years. Within these small crystals, organic compounds and the presence of prokaryotic and eukaryotic cells that have lived preserved within this microenvironment have been identified. According to experts, these types of sediments, whether of Terrestrial or Extraterrestrial origin, should be considered as potential hosts for ancient microorganisms and organic compounds. This suggests a potential search and find site on other planets: the subsoil.
In addition, in the subsoil the phenomenon of serpentine. Chemical-physical reaction that takes place under alkaline pH conditions and that, thanks to the interaction between water and rocks, releases hydrogen, organic and inorganic carbon compounds. Serpentinization, according to exobiologists, is widespread in the celestial bodies of the solar system, including the Moons, and it is also thought that it may have played an important role on Earth, favoring the life of specific microorganisms.
Conclusions about exobiology
Research in exobiology is still ongoing, in October 2024 the NASA aerospace agency will launch a new mission: CLIPPER. The goal will be to search for traces of life from steam jets emitted by one of Jupiter's icy moons: Europe.
At the moment, extraterrestrial organisms have never been identified, but their possible existence in the cosmos cannot be ruled out. However, we must take into account that life could develop in conditions completely different from those on Earth, and that therefore it adapts and evolves in ways unknown to us. The discovery of extraterrestrial life forms would bring great attention from the scientific community to the branch of exobiology, opening paths hitherto completely unexplored.