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New models built by Cornell University astronomers demonstrate that life could emerge in systems that are much harsher than Earth. The research of Lisa Kaltenegger and Jack O’Malley-James suggests that high ultraviolet (UV) radiation and prolonged bursts of X-ray radiation do necessarily exclude the possibility of life on the rocky exoplanets orbiting red dwarf scars
Their findings, published in Monthly Notices of the Royal Astronomical Society, also suggest that life could therefore be possible in wide variety of planetary atmospheres, not just those similar to Earth. For example, they said that an atmosphere that has eroded and is devoid of oxygen and the protective ozone can still foster life.
Astronomers refer to the range of orbits around a star within which a planetary surface can support liquid water as the habitable zone (HZ). The four rocky exoplanets that are orbiting closest in the HZ of their host stars are Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b. Proxima-b, a planet about 1.3 times the mass of Earth, is the closest to Earth and orbits the star Proxima Centauri, a red dwarf also known as the M-star – a star that is cooler, smaller, and longer-lived than the sun.
Because M-stars are cooler than larger stars, the HZ is relatively closer in, which means that the planets in the HZ are subjected to intense radiation emanating from the star. For example, Proxima-b receives 250 times more radiation from Proxima Centauri than the Earth does from the sun.
Studies in the past concluded two things: first, the severity of exposure to radiation would prevent any biotic processes; second, UV fluxes would likely boil off liquid water. In 2017, the same team of researchers assessed the chances of life existing of Trappist-1e. They concluded that the atmosphere deprived of ozone would result in “surface environments hostile even to highly UV tolerant terrestrial extremophiles.”
Now, however, the researchers are more optimistic, concluding that life is possible on all four planets. Astronomers and astrobiologists are severely limited by the sample size of one – that is, Earth and its life forms. However, Kaltenegger and O’Malley-James, look back at our planet’s history, pointing out that Earth’s atmosphere was quite different four billion years ago during an influx of radiation – and yet life emerged.
Based on this information, the researchers modeled worst-case scenarios for the possible atmospheric conditions on the target planets, and still came to the same conclusion. “While the anoxic atmosphere does result in a considerably more biologically harmful radiation environment compared to the present-day Earth, it is still approximately an order of magnitude less biologically harmful than early Earth’s,” they write. “Therefore, UV surface radiation levels should not rule out surface habitability for our closest potentially habitable planets or for planets orbiting in the HZ of active M stars in general.”
In scenarios with high ultraviolet radiation, which can last for billions of years due to the longevity of red dwarfs, an evolution of detectable traits may happen, say the researchers. “In a high UV surface environment, mechanisms that protect biota from such radiation can play a crucial role in maintaining surface habitability, especially on planets around active M stars with thin, eroded or anoxic atmospheres, where other UV-attenuating gases/particles are not present,” they write.
Earth’s biology helps the researchers make their case: they mention that some species of microorganisms and lichens have been taken to space and have survived radiation exposure, developing protective cells or pigments as UV screens. In addition, living beneath thin layers of sand or soil reduces UV exposure as well – and the shortest of the ultraviolet wavelength can be significantly reduced by as little as a micrometer of water.
Kaltenegger and O’Malley-James acknowledge that while it’s currently impossible to know the exact atmospheric conditions on the four planets, the radiation itself should not be a self-contained explanation used to discard the possibility of habitability.
“Even for planets with eroded or anoxic atmospheres orbiting active, flaring M stars the surface UV radiation in our models remains below that of the early Earth for all cases modelled,” they conclude. “Therefore, rather than ruling these worlds out in our search for life, they provide an intriguing environment for the search for life and even for searching for alternative biosignatures that could exist under high-UV surface conditions.”