Between one and three planets in each planetary system lie within the ‘habitable zone’, so it is hard to imagine the true scale of the area we are searching. What if it turns out we have been searching for the wrong life in this huge area? All life we know of on Earth contains organic molecules based on carbon. Could it be possible that carbon-based life forms are not the only variation of life within the endless boundaries of space? Perhaps our Earth-centric idea of life has skewed our search strategies; we may have missed a whole spectrum of life that we didn’t know existed.
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Carbon, the fourth most abundant element within the universe, is regarded to be the basis of all life on this planet. The essential building blocks needed for life, including DNA, fats, tissues and proteins, all contain carbon. It also makes up other vastly different materials, such as graphite and diamonds.
So why has life on Earth evolved to use carbon within its cells? For one thing, carbon has a special property; it can form stable chains of atoms that are at an appropriate strength, which can be broken down or reformed by our cellular processes. Carbon can also combine with lots of other atoms in order to form a large variety of structures with lots of different properties. This variability, stability and manipulability are the main reasons we have evolved on the basis of carbon.
Silicon, like carbon, is found in group 14 of the periodic table, which means they share the same type of chemical properties. These properties are dependent on the fact they both can form four bonds with other atoms. These similarities make silicon a good contender for a non-carbon basis for life.
Silicon is able to make building blocks appropriate for life that are very similar to those made by carbon. It has even been suggested they would be more stable in more extreme environments, like those found on other planets. For example, silicon based sugars are soluble in liquid nitrogen, which is only liquid below -195.8oC. Nitrogen ‘glaciers’ forming liquid nitrogen lakes and rivers may have been found on the surface of Pluto by NASAs spacecraft New Horizons. However, silicon is more unstable and forms a smaller variety of bonds in comparison to carbon, and so may be less versatile when it comes to the complexity of forming life.
Boron has also been suggested as an alternative to carbon. The properties of boron mean that, like silicon, it is able to form similar building blocks to those made by carbon. It would also be incredibly stable in environments containing ammonia instead of water. However, the relative scarcity of boron compared to carbon in the universe makes it an unlikely candidate for non-carbon based life.
Inorganic chemical cells (iCHELLs) are an invention by Professor Lee Cronin at the University of Glasgow. These cells are made from lots of metals but no carbon. Cronin claims that these iCHELLs have selective outer membranes, compartments within their cells, and the ability to adapt to their environments. It has also been suggested that they are working on making these cells ‘photosynthesise’ like plants. This photosynthetic-like ability would mean that the cells would be able to power themselves. As of yet these iCHELLs do not divide, which as a microbiologist I think is a key concept for something that is living rather than merely surviving. But this experiment does prove a valid point that carbon may not be vital to life on another planet.
But who knows what is out there in the limitless expanse of space? It is quite possible that a variety of different life forms are waiting to be discovered, regardless of whether they are made up of carbon or something else entirely.