NASA scientists have dashed hopes that Proxima Centauri b, an Earth-ish-like planet orbiting the closest star to the Sun, could be habitable.
Last year, it emerged there was a rocky exoplanet, dubbed Proxima Centauri b, within Proxima Centauri's crucial Goldilocks zone – meaning there was an alien world in the sweet spot around a star where it's possible to hold liquid water that's essential to life as we know it. Proxima Centauri is only about four light years away from our home planet. Everyone was excited.
Little is known about the star's Proxima b, which was dubbed a "second Earth". Boffins have, thus far, not been able to confirm the presence of liquid water nor an atmosphere on the exoplanet. Atmospheres are vital for life since it regulates its climate and temperature, preventing liquid water from freezing or evaporating, and shielding the necessary chemical building blocks from harmful radiation.
It’s difficult to determine if the planet does have an atmosphere since it has not been been seen crossing its star’s path, meaning scientists cannot use the traditional transit method to identify any potential elements in its skies.
Instead, a team of experts, led by folks at NASA’s Goddard Space Flight Center in the US, have turned to computer simulations to work out if Proxima Centauri b is habitable. In producing their model, they ask a hypothetical question: what would happen if Earth swapped places and orbited Proxima Centauri? What would happen to its atmosphere?
“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, lead author of the group's study and a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The results were published in The Astrophysical Journal last week, and show that in Proxima b's position, the Earth would be stripped of its atmosphere by its host star’s powerful radiation rays.
Proxima Centauri is an active red dwarf prone to bursts of extreme ultraviolet rays hundreds of times stronger than what the Earth receives from our Sun. According to the model, the high energy would ionize the particles in the world's gases, robbing them of electrons.
The extra kick of energy also means they would escape the planet’s gravity and flee into space. Since charges attract, as the stream of negatively charged electrons fly away, a pool of positively charged ions follows in its wake. Ultimately, as a result, the atmosphere would disappear over time.
“The question is, how much of the atmosphere is lost, and how quickly does that process occur?” said Ofer Cohen, coauthor of the paper and a space scientist at the University of Massachusetts, Lowell.
“If we estimate that time, we can calculate how long it takes the atmosphere to completely escape — and compare that to the planet’s lifetime.”
In one scenario, if Proxima b had the highest thermosphere temperature and a completely open magnetic field, it could lose an amount equal to the entirety of Earth’s atmosphere in 100 million years – a short time considering the planet has been around for about four billion years.
In more conservative calculations, assuming it had the lowest thermosphere temperature and a closed magnetic field, the same rate of loss is extended to over two billion years.
“This was a simple calculation based on average activity from the host star,” Garcia-Sage said. “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.”
It looks like the chances of life are slim for our closest exoplanet neighbor. “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, coauthor of the study and an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. “This questions the habitability of planets around such red dwarfs in general.” ®