Frequently we can read on the news newly discovered planets that could harbor extraterrestrial life. Often we have new information about Mars, other worlds with water and extremely resistant living beings, like tardigrades. But is life possible outside the Earth? What is life? What is needed to sustain life? Astrobiology tries to answer this questions. Do you want to find out more?
ASTROBIOLOGY AND EXOBIOLOGY
Astrobiology is a set of different scientific disciplines that studies the existence of life in the universe. To achieve this it combines knowledge of biology, physics, chemistry, astronomy, ecology, geography, geology, planetary science and molecular biology. Within astrobiology, exobiology studies the possibilities of life outside our planet. It should not be confused with ufology, a pseudoscience. Astrobiology tries to answer such exciting questions as:
– What is life?
– How did life appear on Earth?
– How does life evolve, and what is its adaptability?
– What is the future of life on Earth and other places?
– Is there life in other worlds?
WHAT IS LIFE?
Although it seems like a banal question, life is not easy to define. Apparently, we can recognize if something is alive or not if it can perform certain functions and has certain features. Living beings have vital functions:
- Nutrition: they can obtain energy from the environment to grow, survive and reproduce.
- Reproduction: they can create copies similar to themselves.
- Interaction: they can perceive what is going on the environment and inside themselves.
- Organization: living beings are formed by one or more cells
- Variation: variability between individuals allows species to evolve.
Problems begin when with beings that don’t have all the characteristics. The most classic example would be viruses: they are unable to reproduce on their own and lack cellular structure. Another example would be erythrocytes (red blood cells) of mammals, cells without genetic material or mitochondria.
WHAT IS NEEDED FOR LIFE TO EXIST?
We only know one type of life: the terrestrial one. This is why astrobiologists need to take it as a reference to know what to look for elsewhere. Could there be other forms of life different than terrestrial? Maybe, but it would be almost impossible to recognize them. If you do not know what you are looking for, you may find it but do not realize it.
It is considered that in order for life to appear and develop, it is necessary:
- A liquid where chemical reactions take place: on Earth, it is water.
- An element with ease to form stable compounds: on Earth, it is carbon.
- A source of energy: on Earth, it is the Sun.
We are looking for planets or satellites with these characteristics, although other possibilities such as liquid methane (in the case of Titan, a satellite of Saturn), ethane, sulfuric acid, ammonia or acetic acid as solvent are being considered. Life-based on other elements such as silicon, it is a recurring topic in science fiction stories.
WHAT IS NEEDED TO SUSTAIN LIFE?
The celestial body has to fulfill a series of characteristics so that life can be sustained:
- An abundance of chemical elements such as carbon, hydrogen, oxygen, and nitrogen to form organic compounds.
- The planet/satellite has to be within the habitability area of its star (orbiting at a distance that allows a temperature suitable for life).
- A source of energy enough to maintain the temperature and allow the formation of complex molecules.
- An appropriate gravity to keep an atmosphere and not crush the living beings of the planet.
- A magnetic field to divert the radiation incompatible with life.
In our Solar System, the candidates that possibly fulfill these characteristics are Mars, Europe and Ganymede (satellites of Jupiter), Enceladus and Titan (satellites of Saturn) and Triton (satellite of Neptune).
WHY CARBON?
Living beings are formed by cells, and if we reduce the scale, by molecules, and atoms (like all matter). Why is life-based on carbon?
In fact, in the constitution of organisms 26 elements are involved, but 95% of living matter consists of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P) and sulfur (S). We can imagine them as the “bricks of life”: by combining these building blocks, we can obtain complex organisms. These bricks can be joined to others by covalent bonds. Metaphorically, atoms can be imagined as spheres with hands which can be grasped by other hands. For example, the main energy source molecule for all living things is ATP (Adenosine triphosphate, C10H16N5O13P3).
The candidate element to sustain life would have to be an abundant element able to form a great amount of bonds with itself and with other elements. The 5 most abundant elements in the universe:
- Helium: does not form compounds
- Hydrogen and oxygen: they have 1 and 2 hands: they can only form very simple compounds
- Nitrogen: can bind to 3 atoms, but no chains of several nitrogen atoms are known.
- Carbon: it has 4 hands so it can be strongly bonded to other carbons with single, double, or triple bonds. This allows it to form long chains and three-dimensional structures and can still join to other atoms. This versatility allows constructing molecules chemically active and complex, just the complexity that makes life possible.
Could there be life in another place based on a different atom?
ALTERNATIVES TO CARBON
SILICON EXTRATERRESTRIALS
Since establishing 4 links is so useful, silicon is the first candidate for biologists and science fiction writers, even if it is not as abundant as carbon. Silicon (Si) can also form 4 bonds and is abundant on rocky planets like Earth, but …
- The Si-Si bond is quite weak. In an aqueous medium, life based on silicon would not be sustained for a long time as many compounds dissolve in it, although it could be possible in another medium, such as liquid nitrogen (Bains, W.).
- It is very reactive. Silane, for example (one silicon atom bonded to 4 hydrogens) spontaneously ignites at room temperature.
- It is solid at most temperatures. Although it can easily form structures with oxygen (silica or silicon dioxide), the result is almost always a mineral (quartz): too simple and only reacts molten at 1000ºC.
- It does not form chains or networks with itself, due to its greater size compared to carbon. Sometimes it forms long chains with oxygen (silicones), that perhaps could be joined to other groups to form complex molecules. The alien of the movie Alien has silicone tissues. The beings formed by silicones would be more resistant, which leads to speculate what kind of extreme conditions they could withstand.
NITROGEN AND PHOSPHORUS EXTRATERRESTRIALS
Let’s look at some characteristics of nitrogen and phosphorus:
- Nitrogen: can only form 3 bonds with other molecules and is poorly reactive.
- Phosphorus: its bonds are weak and multiple bonds uncommon, although it can form long chains. But it is too reactive.
By combining the two, stable molecules could be obtained, but the beings based on nitrogen and phosphorus would have other problems: the nitrogen compounds, from which they would have to feed, are not abundant in planets and the biological cycle would not be energetically favorable.
BORON, SULFUR AND ARSENIC EXTRATERRESTRIALS
The most unlikely biochemistries could be based on these elements:
- Boron: can form long chains and bind to other elements such as nitrogen, hydrogen or carbon
- Sulfur: can form long chains, but because of its size is highly reactive and unstable.
- Arsenic: is too large to form stable compounds, although its chemical properties are similar to those of phosphorus.
In 2010, the journal Science published a scientific research in which researchers claimed to have discovered a bacterium (GFAJ-1) capable of living only in arsenic, lethal to any living being. It broke the paradigm of biology by not using phosphorus (remember ATP and DNA structure) and opened up new study lines for astrobiology. In 2012, two independent investigations refuted the theory of researcher Felisa Wolfe-Simon and his team. Phosphorus remains essential for organisms to live and develop on Earth.
At the moment, these hypothetical biochemistries are nothing more than speculations, so astrobiologists are still looking for carbon-based life, although we already know that science never ceases to amaze us. Although we could identify life based on other elements if we ever find extraterrestrial life (or vice versa) the revolution will be so great that it won’t matter if they are carbon-based beings.
REFERENCES
- Mix, L.J. 2010. La vida en el espacio. Ed. Crítica
- Giménez A., Gómez, J., Rodríguez E. , Martín, D. (Coordinadores). Astrobiología: sobre el origen de la vida en el universo. 2011, CSIC.
- MOOC notes Astrobiology and search for extraterrestrial life. Edimburgh University.
- Superlife: el silicio orgánico
- Cover picture (Face hugger based in Alien movie)
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