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Life on planets around our neighbor stars can only be caused by a high radiation dose possible

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Artistic representation of a "young earth" around a red "sun" (illu). Copyright: J. O & Malley-James, Carl Sagan Institute, Cornell University, Jeff Tyson

Ithaca (USA) – At the latest when Earth-like planets were discovered in the living-friendly zones of our closest neighbors, the hopes of local life were just as great as they were quickly destroyed because it became clear that the high levels of radiation that the central planets themselves bomb these worlds Ask the origin and development of our known life. However, a new study now comes to a very different assessment of the possibilities of extraterrestrial life around our neighbor stars.

Proxima-b, a planet on Earth circling the red dwarf, Proxima Centauri, just 4.2 light-years away, receives 250 times more X-rays than the Earth's sun and is thought to be exposed to deadly doses of ultraviolet radiation on it. surface. Against this background, many scientists embrace or even deny life on Proxima-b. Cornell University astronomers, on the other hand, say that life has survived this form of bright radiation before – and they even provide evidence: ourselves!

Such as Lisa Kaltenegger and Jack O & # 39; Malley-James of Cornell University currently include in the magazine Monthly announcements from the Royal Astronomical Society (DOI: 10.1093 / mnras / stz724) Finally, the entire life of today on earth has evolved from organisms that thrived during an even greater UV radiation attack than the one subjected to Proxima-b. The earth four billion years ago was therefore a "chaotic, irradiated, hot mess," the two authors. "And yet life somehow gained in importance and spread inexorably across the planet." So with some of our next exoplanets, this process could also take place at that time, so Kaltenegger and O & # 39; Malley-James.

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For their study, the two researchers modeled the surface UV environments of the four possibly habitable closest exoplanets: Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b. They all circle around small red dwarf stars, which, unlike our sun, are very active and often produce so-called flares (sun bursts) that dip their planets in high-energy UV radiation.

Although it is not currently known exactly what the surface conditions of these planets are, it is known that such outbreaks are biologically harmful and can also erode and damage the entire atmosphere of the planets exposed to them. High radiation doses also cause biological molecules such as nucleic acids to mutate or even switch off.

In their models, O & # 39; Malley-James and Kaltenegger have therefore investigated various possible atmospheric compositions. Starting with scenarios that are similar to the current earth, with "eroded" and "anoxic" atmospheres, ie planets with very thin atmospheres that do not or hardly block UV radiation, as well as those without ozone protection.

As expected, the simulations initially show that with a decreasing atmosphere and a low ozone layer, the higher-energy UV radiation reaches the ground. Then the two astrobiologists compared the models with the Earth from about four billion years ago to today: although the modeled planets today receive higher levels of ultraviolet radiation than our present time, this radiation is considerably lower than what the Earth was at the time of its creation Receive life.

"Given that the early earth was inhabited," the researchers write, "we show that UV radiation does not appear to be a limiting factor for the habitability of planets in orbit around M-stars (ie red dwarfs). Our closest neighbors worlds remain fascinating destinations for the search for life outside our solar system. "

On the other hand, another question is posed by planets around inactive M stars, that is, planets exposed to a low radiation current. In the face of our own earth, the question arises as to whether the evolution of life may need the high radiation levels of the early earth?

To assess the possible viability of worlds with different radiation irradiation, the researchers studied the mortality rates at different UV wavelengths of the extremophilic bacterium Deinococcus radiodurans, making it one of the most well-known radiation-resistant terrestrial organisms.

background
In fact, Deinococcus radiodurans (see Figure 1) is so resilient that scientists had already suggested in 2010 that the bacterium itself was originally transported from the earth with asteroids, meteorites and comets and could fertilize the young earth (GreWi reported).

Not all wavelengths of UV radiation are equally harmful to biological molecules: the researcher reports, for example: "a 360 nanometer UV radiation dose should be three orders of magnitude higher than a radiation dose at 260 nanometers to have comparable mortality rates in a population of those To create an organism. "

Many organisms on Earth use survival strategies – including protective pigments, biofluorescence, or retreating to the surface, in water, or under rocks – to cope with high levels of radiation. Such strategies could also mimic life on other worlds. This would make life on distant planets more difficult to find because even the atmospheric biosignatures of surface lives, such as those that can be detected with telescopes, are harder to find.

"The story of life on earth provides us with a wealth of information on how biology can tackle the challenges of environments that we consider today hostile to life," concludes O & # 39; Malley-James and Kaltenegger, "Our research shows see now that our next mail systems for finding life on other worlds are fascinating goals to explore. "

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