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Proxima b, TRAPPIST-1e, Ross-128b and LHS-1140b – the potentially livable outermost planets – revolve around a different type of star from our Sun: M-type stars (red dwarfs) . Such stars can blaze frequently, bombarding planets with biologically harmful ultraviolet (UV) light, exposing their atmosphere to erosion risk and challenging the habitability of these worlds. However, a new study concludes that UV radiation should not limit the habitability of planets orbiting M-type stars and that the closest extraterrestrial worlds remain intriguing targets for the search for life beyond our solar system.
The impression of this artist shows a view of the surface of Proxima orbiting Proxima Centauri. Alpha Centauri AB also appears in the image at the top right of Proxima. Proxima b is a little more massive than the Earth and orbits in the habitable zone around its star, where the temperature is conducive to the presence of liquid water on its surface. Image credit: Mr Kornmesser / ESO.
All life on Earth has today evolved from creatures that developed during an ultraviolet attack even more intense than that of Proxima-b and other nearby exoplanets currently in place.
"The primitive Earth was a chaotic place, irradiated and burning. Even so, life has improved and expanded, "said Lisa Kaltenegger, professor of astobiologists at Cornell University, and Jack O'Malley-James.
"The same thing could happen right now on some of the closest exoplanets."
Astronomers have modeled the surface UV environments of Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b.
They modeled various atmospheric compositions, ranging from those similar to today's Earth to eroded and anoxic atmospheres – those with very fine atmospheres that do not block UV and non-ozone shielded, respectively.
The models show that as atmospheres become scarce and ozone levels decrease, more and more high-energy UV rays reach the ground.
The team compared these models to the history of the Earth, from nearly 4 billion years to the present day.
Although the modeled planets receive higher UV radiation than that emitted by our own Sun today, this radiation is much lower than what the Earth had received 3.9 billion years ago.
An opposite question arises for planets orbiting inactive M-type stars on which the radiation flux is particularly weak: does the evolution of life require the high levels of early radiation? of the earth?
To judge the potential habitability of worlds with varying rates of radiation influx, the researchers evaluated mortality rates at different UV wavelengths of the extremophile. Deinococcus radiodurans, one of the most known radiation resistant organisms.
"All wavelengths of UV radiation are not as damaging to biological molecules," the scientists said.
"For example, a dose of 360 nm UV radiation should be three orders of magnitude higher than a 260 nm radiation dose to produce similar mortality rates in a population of this organism."
"Many organisms on Earth use survival strategies to cope with high levels of radiation that could be imitated by life on other worlds. Underground life would be harder to find on distant planets without the kind of atmospheric biosignatures that telescopes can detect. "
"The story of life on Earth provides us with a wealth of information about how biology can overcome the challenges of environments we consider hostile," O'Malley-James said.
"Our research shows that in our quest for life on other worlds, our closest worlds are fascinating targets to explore," said Professor Kaltenegger.
The study was published in Monthly Notices from the Royal Astronomical Society.
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Jack T. O'Malley-James and L. Kaltenegger. 2019. Lessons Learned from the Beginnings of the Earth: Surface UV radiation should not limit the habitability of active M star systems. MNRAS 485 (4): 5598-5603; doi: 10.1093 / mnras / stz724
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