Proxima b, TRAPPIST-1e, Ross-128b and LHS-1140b — the closest potentially habitable exoplanets — orbit a different kind of star than our Sun: M-type stars (red dwarfs). Such stars can flare frequently, bombarding the planets with biologically damaging ultraviolet (UV) radiation, placing their atmospheres at risk of erosion and bringing the habitability of these worlds into question. A new study, however, finds that UV radiation should not be a limiting factor for the habitability of planets orbiting M-type stars and that the closest alien worlds remain intriguing targets for the search for life beyond our Solar System.
This artist’s impression shows a view of the surface of Proxima b orbiting Proxima Centauri. Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around its star, where the temperature is suitable for liquid water to exist on its surface. Image credit: M. Kornmesser / ESO.
All 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 early Earth was a chaotic, irradiated, hot place. Yet in spite of this, life somehow gained a toehold and then expanded,” said Cornell University astobiologists Professor Lisa Kaltenegger and Jack O’Malley-James.
“The same thing could be happening at this very moment on some of the nearest exoplanets.”
The astronomers modeled the surface UV environments of Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b.
They modeled 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 team compared the models to Earth’s history, from nearly 4 billion years ago to today.
Although the modeled 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.
An opposite question arises for planets orbiting inactive M-type stars on which the radiation flux is particularly low: does the evolution of life require the high levels of radiation of early Earth?
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,” the scientists said.
“For example, a dosage of UV radiation at 360 nm would need to be three orders of magnitude higher than a dosage of radiation at 260 nm to produce similar mortality rates in a population of this organism.”
“Many organisms on Earth employ survival strategies to cope with high levels of radiation that could be imitated by life on other worlds. Subsurface life would be more difficult to find on distant planets without the kind of atmospheric biosignatures telescopes can detect.”
“The history of life on Earth provides us with a wealth of information about how biology can overcome the challenges of environments we would think of as hostile,” O’Malley-James said.
“Our research demonstrates that in the quest for life on other worlds, our closest worlds are fascinating targets to explore,” Professor Kaltenegger said.
The study was published in the Monthly Notices of the Royal Astronomical Society.
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Jack T. O’Malley-James & L. Kaltenegger. 2019. Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems. MNRAS 485 (4): 5598-5603; doi: 10.1093/mnras/stz724
