(ORDO NEWS) — A simple hydrocarbon may indicate the existence of life on an exoplanet, however, its traces are not so easy to fix.
To date, astronomers have discovered and confirmed the existence of almost five thousand exoplanets, and the same number of objects outside the solar system are waiting for confirmation of their exoplanetary status. Among this variety, there are both exoplanets, similar to the Earth, and very different from it. Of course, any extraterrestrial world is interesting, but especially the one in which life could potentially exist. But even if an exoplanet is similar to the Earth in its size and composition, orbits around a star similar to the Sun, all this cannot be considered sufficient conditions for the development of life on it. A good example of this is our planetary neighbor Mars, on the surface of which it is not the first rover that rovers in search of traces of something living. It would seem that the search for life is a disastrous business, especially on exoplanets located a million times farther from us than Mars.
Biomarkers can give a hint if anyone is alive on the exoplanet. This is how astrobiologists call chemical substances or their specific combinations in the atmosphere of an exoplanet, which, as an indicator, most likely indicate the occurrence of certain biochemical reactions. Biomarkers, like any other indicators, have a number of simple and logical requirements. For example, they should give as few false positive and false negative results as possible.
Take, for example, oxygen – by analogy with the Earth on exoplanets, this gas may indicate a photosynthesis reaction. Oxygen is a rather active gas, and without constant feeding in the form of the same bacteria, it is likely to react with other substances and disappear from the atmosphere. But if we do not “see” oxygen in the exoplanet’s atmosphere (this can be done by analyzing the spectral composition of light that has passed through its atmosphere), this does not mean at all that there is no life on the exoplanet. After all, on our Earth, there was no oxygen in the atmosphere for more than two billion years.
Another example is methane. This simplest hydrocarbon is secreted by many microorganisms, and it can also be a biomarker. But the source of methane in the atmosphere can be not only living organisms, but also volcanoes. Since there are many exoplanets, and they are different, the likelihood of seeing this or that biomarker increases. Therefore, astrobiologists are looking for new biomarkers to increase the chances of finding a habitable exoplanet. So researchers from the Massachusetts Institute of Technology proposed as a biomarker a molecule of one hydrocarbon – isoprene. The article was recently published in the journal Astrobiology, and its preprint is available at arxiv.org.
On Earth, isoprene is emitted by a variety of living organisms: from protozoa to mammals, although most isoprene (about 90%) is supplied to the air by forests. It can be produced by both aerobic and anaerobic organisms, which means that we can test for “living” and oxygen-free worlds. However, there is extremely little isoprene in the earth’s atmosphere, and this is despite the fact that about 500 million tons of this substance gets into the air every year. The whole point lies in its exceptional fragility. On average, isoprene “lives” in air for less than three hours: it is easily oxidized by oxygen, destroyed by ultraviolet radiation and enters into other chemical reactions. But if the exoplanet’s atmosphere is not as aggressive in this respect as the terrestrial one, then under certain conditions a measurable amount of isoprene can accumulate in it. Another benefit of isoprene as a biomarker, is that, unlike the same methane, it does not have significant geochemical sources. That is, if there is isoprene in the atmosphere, it means that it was formed with a very high probability as a result of biochemical processes.
But the isoprene biomarker also has its drawbacks. First, there must be quite a lot of it in the atmosphere of an exoplanet for it to be at least theoretically possible to detect it using modern equipment. If, for example, an exoplanet is located next to a star like our Sun, then with a high probability the ultraviolet flux will destroy isoprene in the upper atmosphere faster than it can accumulate there. Second, isoprene can be confused with other hydrocarbons that may not necessarily be of biochemical origin. And it will be possible to register the signal of isoprene in the spectrum of an exoplanet only with the help of telescopes that are just being created. For example, like the orbiting infrared telescope James Webb, which is planned to be launched into orbit this fall.
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