Chapter 1: The Role of Water in Life

Recently, I pondered a thought-provoking question: Scientists often assert that “if liquid water cannot exist on this planet, life is impossible here.” But doesn't this viewpoint reflect an anthropocentric bias? Why must extraterrestrial life mirror life on Earth? It’s conceivable that there are organisms capable of thriving in extreme temperatures without the need for liquid water.

Indeed, we shouldn't entirely rule out the chance that life elsewhere in the universe may be based on a distinct biochemistry, one that does not rely on water and carbon.

Source: pikabu.ru

In truth, scientists don’t entirely dismiss the idea of life forms emerging from other substances. It’s plausible that some planets have hosted organisms that evolved without liquid water. However, the likelihood of such life arising is considerably lower for two main reasons.

The first is that other substances generally cannot form complex compounds as readily as those based on carbon. The development and evolution of life on Earth, particularly complex multicellular organisms, were facilitated by the formation of intricate organic macromolecules.

The DNA molecule

Currently, we lack knowledge of any alternative substances capable of generating such complex molecules. Silicon is often considered a potential substitute for carbon, but the diversity and complexity of silicate compounds fall significantly short compared to organic compounds.

Silicon Life Form. A still from the Star Trek television series

Chemical groups made up of elements like hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc facilitate an extensive array of biochemical reactions necessary for metabolism and life processes. In contrast, silicon reacts with far fewer substances, resulting in less diverse silicate molecules when compared to the variety of organic macromolecules.

The second reason is that water and carbon rank among the most plentiful substances in the universe. Almost all proposed alternatives to carbon or water are significantly rarer in the cosmos.

The distribution of chemical elements in the Solar System shows carbon in fourth place. A similar table showing the elements' abundance in the universe was referenced in the original article, but an equivalent in English was not found. The elemental distribution in our solar system closely mirrors that of the wider universe.

For these reasons, it is logical to initially focus on searching for life that is based on water and carbon, as current knowledge suggests that this form of life is likely to be more prevalent than others.

To illustrate, imagine having 100 black boxes and 100 white boxes, with 10 keys hidden among them. If you know that 9 out of 10 keys are in the white boxes, wouldn’t it make more sense to start searching there?

The vast number of planets scattered throughout the universe far exceeds our capacity for exploration. Thus, it is pragmatic to prioritize planets that can support liquid water, and consequently carbon-based life, rather than wasting efforts on worlds where life is less probable.

Another point to consider is that until interstellar travel becomes feasible, our observations are limited to telescopes. At best, we can perform spectrographic analyses of exoplanetary atmospheres.

Artist's interpretation of the TRAPPIST-1 system, where several planets might possess conditions suitable for carbon-based life. Source: nasa.gov

If a planet has life with a water-carbon-based biochemistry, we can infer the presence of such life through specific biomarkers in the atmosphere. For instance, the detection of methane and monitoring its fluctuations over time could indicate life as we understand it.

However, consider a scenario where silicon-based life has evolved in oceans of liquid sulfur. The challenge is that we have no idea which atmospheric chemicals would signal the existence of such life.

While it's theoretically plausible for life to arise from an entirely different biochemistry, the practical likelihood is much lower. Even if we discover a planet harboring such life, determining its nature from atmospheric analysis would be exceedingly challenging. Therefore, it is most sensible to first seek out and investigate planets that are conducive to water-carbon life forms.

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