Life may be evolving on rocky, Earth-like planets orbiting in the habitable zone of some of our closest stars which are bombarded by high levels of radiation, according to a study.
Proxima-b, only 4.24 light years away, receives 250 times more X-ray radiation than Earth and could experience deadly levels of ultraviolet (UV) radiation on its surface, said researchers from Cornell University in the US.
According to the study, published in the journal Monthly Notices of the Royal Astronomical Society, life already has survived this kind of fierce radiation on the Earth.
All of life on Earth today evolved from creatures that thrived during an even greater UV radiation assault than Proxima-b, and other nearby exoplanets, currently endure.
The Earth of four billion years ago was a chaotic, irradiated, hot mess. Yet in spite of this, life somehow gained a toehold and then expanded.
The same thing could be happening at this very moment on some of the nearest exoplanets, researchers said.
They modelled 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 which, unlike our Sun, flare frequently, bathing their planets in high-energy UV radiation.
While conditions prevail upon the surface of the planets orbiting these flaring stars, it is known that such flares are biologically damaging and can cause erosion in planetary atmospheres.
High levels of radiation cause biological molecules like nucleic acids to mutate or even shut down.
The researchers modelled various atmospheric compositions, from ones similar to present-day Earth to "eroded" and "anoxic" atmospheres -- those with very thin atmospheres that don't block UV radiation well and those without the protection of ozone, respectively.
The models show that as atmospheres thin and ozone levels decrease, more high-energy UV radiation reaches the ground. The researchers compared the models to Earth's history, from nearly four billion years ago to today.
Although the modelled planets receive higher UV radiation than that emitted by our own Sun today, this is significantly lower than what Earth received 3.9 billion years ago.
"Given that the early Earth was inhabited, we show that UV radiation should not be a limiting factor for the habitability of planets orbiting M stars," said researchers.
"Our closest neighbouring worlds remain intriguing targets for the search for life beyond our solar system," they said.
To judge the potential habitability of worlds with varying rates of radiation influx, the researchers assessed 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 damaging to biological molecules, researchers said.
A dosage of UV radiation at 360 nanometres would need to be three orders of magnitude higher than a dosage of radiation at 260 nanometres to produce similar mortality rates in a population of this organism," they said.
Many organisms on Earth employ survival strategies -- including protective pigments, biofluorescence, and living under soil, water or rock -- to cope with high levels of radiation that could be imitated by life on other worlds, the researchers note.
Subsurface life would be more difficult to find on distant planets without the kind of atmospheric biosignatures telescopes can detect.