Enthusiasm for exoplanets soared when rocky Earth-like planets were discovered in orbit in the habitable zone of some of our closest stars, until life expectancies were frustrated by the high levels of radiation that bombarded those worlds.
Proxima-b, only 4.24 light-years away, receives 250 times more X-ray radiation than Earth and could experience deadly levels of ultraviolet radiation on its surface. How could life survive such a bombardment? Astronomers at Cornell University say that life has already survived this kind of fierce radiation, and they have proof: you.
Lisa Kaltenegger and Jack O & # 39; Malley-James present their case in a new article, published in Monthly notices from the Royal Astronomical Society. Kaltenegger is an badociate professor of astronomy and director of the Carl Sagan Institute at Cornell, where O & # 39; Malley-James is a research badociate.
Today, all life on Earth evolved from creatures that thrived during an attack of UV radiation even greater than Proxima-b, and other nearby exoplanets, currently enduring. The Earth of 4 billion years ago was a hot chaos, irradiated and chaotic. However, despite this, life somehow took hold and then expanded.
The same could be happening right now in some of the closest exoplanets, according to Kaltenegger and O & # 39; Malley-James. The researchers modeled the surface UV environments of the four exoplanets closest to Earth that are potentially habitable: Proxima-b, TRAPPIST-1e, Ross-128b, and LHS-1140b.
These planets orbit small red dwarf stars that, unlike our sun, shine frequently, bathing their planets with high-energy UV radiation. While it is not known exactly what conditions prevail on the surface of the planets orbiting these burning stars, it is known that such flares are biologically damaging and can cause erosion in planetary atmospheres. High levels of radiation cause biological molecules such as nucleic acids to mutate or even to turn off.
O & # 39; Malley-James and Kaltenegger modeled various atmospheric compositions, from those similar to those of today's Earth to "eroded" and "anoxic" atmospheres: those with very fine atmospheres that do not block UV radiation well and those that do not they have ozone protection, respectively. The models show that as atmospheres decrease and ozone levels decrease, more high-energy UV radiation reaches the ground. The researchers compared the models with the history of the Earth, from almost 4 billion years ago until today.
Although the modeled planets receive a UV radiation higher than that emitted by our own sun at present, this is significantly lower than what the Earth received 3.900 million years ago.
"Given that the primitive Earth was inhabited," the researchers wrote, "we show that UV radiation should not be a limiting factor for the habitability of planets that orbit M stars. Our nearest neighbor worlds remain interesting targets for the search for life beyond our solar energy system. "
An opposite question arises for planets orbiting inactive M stars in which the radiation flow is particularly low: Does the evolution of life require the high levels of radiation from the primitive Earth?
To judge the potential habitability of worlds with varying rates of radiation influx, the researchers evaluated the mortality rates at different UV wavelengths of the extremophile Deinococcus radiodurans, one of the most radiation resistant organisms known.
Not all wavelengths of UV radiation are equally harmful to biological molecules: for example, write to the researchers, "a dose of UV radiation in 360". [nanometers] it would have to be three orders of magnitude higher than a dose of radiation at 260 [nanometers] To produce similar mortality rates in a population of this organism. "
According to the researchers, many organisms on Earth employ survival strategies, including protective pigments, biofluorescence and life under the soil, water or rock, to cope with the high levels of radiation that could be imitated by life. in other worlds. Subsurface life would be harder to find on distant planets without the kind of atmospheric biosignatures that telescopes can detect.
"The history of life on Earth gives us a wealth of information about how biology can overcome the challenges of environments that we might consider hostile," said O & # 39; Malley-James.
Kaltenegger said: "Our research shows that in the search for life in other worlds, our closest worlds are fascinating targets to explore."
The hunt is on the planets closest to Earth.
Jack T O & Malley-James et al, Lessons from the primitive Earth: UV radiation from the surface should not limit the habitability of active M star systems, Monthly notices from the Royal Astronomical Society (2019). DOI: 10.1093 / mnras / stz724
Life could be evolving right now in the closest exoplanets (2019, April 9)
retrieved on April 9, 2019
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