Scientists will have to revise their estimates of the number of planets outside our solar system, which can maintain the rich biodiversity that we are familiar with on Earth, since the accumulation of toxic gases in the atmospheres of most of the potentially inhabited exoplanets makes them unsuitable for complex life.
Traditionally, when searching for extraterrestrial life, attention was focused on the habitable zone, that is, the range of distances from the star, where there is enough heat for liquid water to exist on the surface of the planet. It is suitable for unicellular microbes, but not enough for complex organisms, from simple sponges to humans. We have shown that taking into account the predicted levels of some toxic gases narrows the safe zone for a complex life, at least by half, and in some cases completely eliminates it.
Today, thanks to the NASA “Kepler” space telescope, as well as other instruments, approximately 4,000 exoplanets have been confirmed, but we will not be able to visit any of them personally in the near future. They are just too far. The closest is Proxima b, but modern spacecraft will be able to reach it only in 54.5 thousand years. Therefore, the only way to determine the content of gases in the atmospheres of exoplanets, which will tell about the potential suitability for life of these worlds, are telescopes.
Imagine a habitable zone for a complex life, defined as safe for rich ecosystems that flourish today on Earth. Our results show that they cannot exist in most regions of the traditionally defined habitable zone.
In order to limit the extrasolar worlds that are worth paying close attention when searching for a developed extraterrestrial life, astronomers conducted computer simulations that revealed the atmospheric climate and photochemistry on various planets, while focusing on carbon dioxide. Any scuba diver knows that too much of this gas in the body can be deadly. However, planets that are too far from their star need it to keep their temperature above zero. Including the Earth.
To maintain liquid water on the outer edge of the usual habitable zone, the planet will need tens of thousands of times more carbon dioxide than we have on Earth today. This is far beyond the levels that are known to be toxic to humans and animals.
-Edward Schwyterman, lead author of the study at the University of California at Riverside (USA)
As a result, calculations have shown that carbon dioxide toxicity itself narrows the traditional habitable zone, reducing the area where chances for the development and survival of simple ecosystems are halved. And for people and other animals of a higher order – by two thirds.
Moreover, for some stars there is no safe zone at all, including the two closest neighbors of the Sun: Proxima Centauri and TRAPPIST-1. The type and intensity of ultraviolet radiation emitted by these cold, dim stars can lead to a high concentration of carbon monoxide on their planets, even a small amount of which can destroy the cells of a living organism. Fortunately, it cannot accumulate on Earth, because our hot, bright Sun provokes chemical reactions in the atmosphere that quickly destroy the deadly gas.
The conducted work provides one of the ways to choose which of this huge number of extrasolar planets we should observe in more detail. And, considering that apart from Earth, no planet in our solar system contains life that can be characterized at a distance, exoplanets are our best hope for finding inhabited worlds similar to ours.
Finding exoplanets with rich biodiversity
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