A new NASA study has poured another bucket of cold water on hopes of one day discovering life on the closest exoplanet ever discovered – the Earth-sized world Proxima b, which is thought to orbit in the habitable zone of the red dwarf Proxima Centauri. A computer model is now asserting that the atmosphere of the exoplanet could have long since been destroyed by the intense levels of radiation emitted by the parent star.
The new research sought to study whether the atmosphere of Proxima b was capable of surviving the onslaught of radiation and intense solar flares issuing from its star. Ordinarily the study of exoplanet atmospheres would lean heavily on observations made as the world passed between its parent star and Earth.
These transit observations let astronomers analyze the light from the star as it passes through the exoplanet's atmosphere, giving them clues as to its characteristics and composition. Unfortunately, Proxima b's orbit doesn't allow for such niceties, which is a serious problem considering that, when it comes to habitability, atmosphere is crucial.
Red dwarfs such as Proxima Centauri are known to give out intense levels of ultraviolet radiation, and to bombard orbiting exoplanets with intense and frequent flares. These attributes could strip an exoplanet's atmosphere away, leaving it brutally exposed to the predations of its star.
More specifically, upon striking the atmosphere, the high-energy ultraviolet radiation from a red dwarf ionizes gasses in an exoplanet's atmosphere, creating electrically charged particles capable of escaping the planet's gravity. This gives rise to a chain reaction that allows further particles to escape, essentially eroding the atmosphere.
Due to its tight orbit with Proxima Centauri, Proxima b absorbs hundreds of times the amount of radiation than the Earth does from the Sun. This onslaught is capable of stripping not only the lighter molecules – such as hydrogen – from the atmosphere, but also heavier elements like oxygen and nitrogen.
In the absence of a planetary transit, the scientists turned to computer modelling in an attempt to estimate how much of Proxima b's atmosphere is lost, and how fast.
Since our world is the only habitable planet discovered to date, the team decided to take some of Earth's characteristics, including its atmosphere, magnetic field, and gravity, and to create a simulated exoplanet that travels along the same orbit as Proxima b.
The team used data from NASA's Chandra X-Ray Observatory to estimate Proxima Centauri's radiation intensity and flare frequency, and observed how the virtual Earth's atmosphere fared.
According to the computer simulation, an Earth-like Proxima b could lose its atmosphere up to 10,000 times faster than the (real) Earth does.
"This was a simple calculation based on average activity from the host star," states NASA scientist and lead author of the study, Garcia-Sage. "It doesn't consider variations like extreme heating in the star's atmosphere or violent stellar disturbances to the exoplanet's magnetic field — things we'd expect provide even more ionizing radiation and atmospheric escape."
The team went on to consider other factors that could significantly impact atmospheric loss, including the size of Proxima b's polar caps, and the temperature of the planet's exosphere.
Taking both the computer model and potential exacerbating factors into account, the team estimated that, in a best case scenario, Proxima b would lose the equivalent of Earth's entire atmosphere in the space of two billion years. At worst, it could happen in 100 million years.
Considering that Proxima b is thought to be roughly four billion years old, the research paints a pretty bleak picture for Earth's closest neighboring exoplanet.
"Things can get interesting if an exoplanet holds on to its atmosphere, but Proxima b's atmospheric loss rates here are so high that habitability is implausible," said Jeremy Drake, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics and co-author of the study. "This questions the habitability of planets around such red dwarfs in general."
However, it is worth noting that while the research is certainly not supportive of the chances of life on Proxima b, it also does not rule it out. Certain phenomena, such as extreme volcanic activity or intense comet bombardments could potentially slow the atmospheric loss.
The paper detailing the research has been published in The Astrophysical Journal Letters.