We have been perplexed for centuries by the possibility of life on planets other than our own. We’ve looked at the skies and wondered if there are others like us out there or are we the exception. Stories about aliens, both friendly and malevolent, were being published even before we had any knowledge of the planets that lay beyond us. As science grew, and we came to know more about the planets, their moons and their environments, scientists started the search for life outside earth, giving rise to a new discipline – Astrobiology - which seeks to understand the origin, evolution, distribution and future of life in the universe. It integrates biology with planetary science, astronomy, cosmology and other physical sciences. The most important question that astrobiologists face is, if extraterrestrial life exists, what is it made up of and how does it perform the basic processes which keep it alive.
One of the main questions that we need to ponder over is whether the life beyond earth is built from the CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur) which can form a wide array of stable macromolecules like on Earth or if it has an alternative biochemistry. Scientists have been looking for molecules analogous to those on earth. In this alternative biochemistry, silicon may replace carbon and arsenic may replace phosphorus. However, since the CHNOPS elements are among the most abundant in the universe, their availability might not be a limiting factor to the cosmic distribution of life.
On Earth, all organisms have left-handed (L) amino acids and right-handed (D) carbohydrates rather than their mirror image stereoisomers. However, the selection of these particular stereoisomers has most probably been random and the existence of a “mirror biosphere” is quite possible.
On the other hand, unlike the CHNOPS elements, water is not available on many planets and moons. Water molecules are polar and can easily form hydrogen bonds. As a result, water readily dissolves other polar compounds such as carbohydrates and amino acids which can then react in aqueous solutions inside the cells. Not only this, the hydrophobicity of compounds and functional groups is also critical for biology as can be seen in the formation of lipid membranes. Scientists have been searching for alternative polar solvents such as ammonia which might be able to replace water. However, ammonia and most other polar solvents are stable liquids only at much colder temperatures than water which would result in very slow chemical reactions. Nevertheless, at extremely high pressures such as those on Jovian planets, ammonia could remain in its liquid state up to the critical temperature of 132 degrees Celsius. Alternatively, scientists are also looking into the possibility of a non-polar solvent which could replace water on places such as Titan where the surface conditions permit non-polar molecules such as methane and ethane to remain as liquids.
Another factor that scientists take into account while considering life elsewhere is the chirality of molecules. On earth, all the organisms have left-handed (L) amino acids and right-handed (D) carbohydrates rather than their mirror-image stereoisomers. This phenomenon, known as homochirality is possibly favored by natural selection as the polypeptides condensed from homochiral amino acids differ in folding and therefore function, from those condensed from heterochiral amino acids. However, the selection of these particular stereoisomers has most probably been random and a “mirror biosphere” is quite possible.
Life on earth mainly obtains its energy in two ways – by capturing light or by using chemical energy (either by respiration or fermentation). Scientists have now elucidated many pathways which use inorganic compounds through which microscopic organisms can produce energy without using oxygen. In extraterrestrial environments with little or no sunlight, similar pathways might be used. Alternatively, scientists have also been speculating over alternative biological pathways which might be able to capture energy from electromagnetic fields, kinetic energy, gravity or other similar sources.
As we learn more about space and ourselves, we move closer to uncovering the mysteries of the universe. Who knows we might get to meet our extraterrestrial counterparts in our lifetime…Fingers crossed!
References:
Mcmahon, S. & Centre, U. K. Astrobiology ( Overview ) What Is Astrobiology ? Habitability and the Limits of Life. (2021).
Hand, K. P., Chyba, C. F., Priscu, J. C., Carlson, R. W. & Nealson, K. H. Astrobiology and the Potential for Life on Europa. Europa 589–630 (2017) doi:10.2307/j.ctt1xp3wdw.32.
Chyba, C. F. & Hand, K. P. Astrobiology: The study of the living universe. Annu. Rev. Astron. Astrophys. 43, 31–74 (2005).
Comments