A complex life can be much rarer in the universe than we thought



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One of the most fundamental questions in the search for extra-terrestrial life is also the most fundamental. In the words of Enrico Fermi: "But where is everyone?" Scientists from many disciplines have examined the question of where and how a smart life could be born elsewhere in the universe. A new study suggests that extraterrestrial life complex – the kind of life we ​​might someday encounter and potentially interact with – could be present in many fewer worlds than previously thought, due to the toxic presence of so much CO2 on some planets that are otherwise in the habitable zone (HZ) of their respective stars.

The argument, in general terms, is that CO2 The levels required to hold a planet's liquid water toward the outer edge of its star's ZS may be too high to allow a complex life to survive on its surface.

Life on the primitive earth

I am not a biologist, but I am lucky to know him. Jessica Hall has agreed to leave the HE retreat and verify the facts.

Before diving into the flesh of this paper, there are some useful things to know about the emergence of life on Earth. It's incredibly old. The first indisputable evidence of life on Earth is three billion years old. Evidence may exist for life as far back as 4.2 billion years ago, but these findings are still disputed. In any case, life appeared on Earth relatively early in life.

For most of the life of our planet, however, the life she supported consisted of single, unicellular, coreless organisms. Multicellular life only appears in the fossil record from 600 million years ago. If we use point 3B (dates 3.5B and 3.8B have also been proposed as dates for the first life), this means that our planet has sustained life for about 66% of its own existence. For 80% of this time, life on Earth was strictly monocellular.

Discussions about why and how organisms on Earth have taken the leap from one to one to many are of great interest to scientists across multiple disciplines. In many cases, radical changes in the type of life on Earth are related to changes in its climate, atmosphere and active geological processes. The great event of oxygenation, in which oxygen has replaced methane, was such a transformational event that forever reshaped the kind of life that could live on this planet (and that has become a reality). elsewhere caused a glacial age known as the Huronian glaciation that would have almost frozen the planet.).

Microsoft Word - Schwieterman_etal_2019b_Complex_HZ_R1_resubmit.

The black lines represent the conventional space. The blue zone indicates the range at different CO2 pressure levels from 0.1 bar to 1 bar. At certain temperatures, photochemical effects "may extend the life of CO beyond the short-term allowable limits for humans" in the IHZ. This is shown in brown. Picture of iopscience.

In other words, the idea that atmospheric gas concentrations could affect the types of life on a planet is well supported by our own research on the Earth. What these researchers have particularly noticed is that virtually all life on Earth is spectacularly unsuitable for life with high levels of CO2 required in the atmosphere to make a planet habitable if it is at the edge of the habitable zone of its star. Other greenhouse gases in addition to CO2 could theoretically work but CO2 That's what you hope to achieve if you have an oxygen atmosphere in the first place – and oxygen, it turns out, is the gas we expected from almost the most complex life that's . Paper:

The metabolic oxidation of organic matter with O2 produces significantly more free energy than any other plausible respiratory process, and O2 is the only high potential oxidant stable enough for s & # 39; Accumulate in planetary atmospheres (Catling et al., 2005). As a result, it is likely that the centrality of molecular O2 in the emergence and expansion of a complex biosphere on Earth is a general phenomenon (Catling et al., 2005).

Researchers performed sophisticated calculations to estimate the probable relative abundance of CO2 on various planets detected around their host stars. In many cases, the levels of carbon dioxide on these planets would be several orders of magnitude higher than any known life on Earth can tolerate. The alternative chemistries proposed to them did not meet all the requirements currently considered necessary for the evolution of complex life.

The end result of this may be that the living area for complex living could evolve could be much smaller than previously thought. This would not prevent a simpler life from using alternative chemistry, and the authors readily admit that no, we will not know for sure until we have passed our current technologies. However, this suggests that the habitable area of ​​a star – usually defined as the temperature range allowing liquid water to be present on the surface – may not be a particularly useful measure. It may be that if a simple life can still survive on such worlds, more complex life forms require much rarer conditions in the universe.

Background image: ESO / M. Kornmesser CC BY 4.0

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