Proxima b, an Earth-size planet located in the habitable zone of our nearest star outside the solar system, may not have the potential to host life as it is unable to keep a grip on its atmosphere, NASA scientists say. The lack of atmosphere leaves the surface exposed to harmful stellar radiation, reducing the planet’s potential for habitability.
At only four light-years away, Proxima b is our closest known extra-solar neighbour. However, since it has not been seen crossing in front of its host star, the exoplanet eludes the usual method for learning about its atmosphere.
Instead, scientists used computer models to understand whether the exoplanet is habitable. One such model considered what would happen if Earth orbited Proxima Centauri, our nearest stellar neighbour and Proxima b’s host star, at the same orbit as Proxima b. The study, published in The Astrophysical Journal Letters, suggests that the Earth’s atmosphere would not survive in close proximity to the violent red dwarf.
“We decided to take the only habitable planet we know of so far – Earth – and put it where Proxima b is,” said Katherine Garcia-Sage, a space scientist at NASA’s Goddard Space Flight Center in the US.
Just because Proxima b’s orbit is in the habitable zone, which is the distance from its host star where water could pool on a planet’s surface, does not mean it is habitable.
It does not take into account, for example, whether water actually exists on the planet, or whether an atmosphere could survive at that orbit.
Atmospheres are also essential for life as we know it: Having the right atmosphere allows for climate regulation, the maintenance of a water-friendly surface pressure, shielding from hazardous space weather, and the housing of life’s chemical building blocks.
The computer model used Earth’s atmosphere, magnetic field and gravity as proxies for Proxima b’s. They also calculated how much radiation Proxima Centauri produces on average, based on observations from NASA’s Chandra X-ray Observatory.
With these data, their model simulates how the host star’s intense radiation and frequent flaring affect the exoplanet’s atmosphere. An active red dwarf star like Proxima Centauri strips away atmosphere when high-energy extreme ultraviolet radiation ionises atmospheric gases, knocking off electrons and producing a swath of electrically charged particles.
In this process, the newly formed electrons gain enough energy that they can readily escape the planet’s gravity and race out of the atmosphere. Opposite charges attract, so as more negatively charged electrons leave the atmosphere, they create a powerful charge separation that pulls positively charged ions along with them, out into space.
In Proxima Centauri’s habitable zone, Proxima b encounters bouts of extreme ultraviolet radiation hundreds of times greater than Earth does from the Sun. That radiation generates enough energy to strip away not just the lightest molecules like hydrogen but also, over time, heavier elements such as oxygen and nitrogen.
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