If you were living on Proxima Centauri b, your weather forecast would sound something like this: partly sunny with supersonic winds and a chance of a coronal mass ejection, plus extreme radiation your sunblock would be useless against. Then why are astronomers so interested in it?
Before anyone answers “aliens”, this Earth-size exoplanet that orbits the M-dwarf star Proxima Centauri 4.2 light-years away is an ideal specimen to study for understanding low-mass stars and their planets on a deeper level. What makes Proxima B of special interest is that its orbit is right in the habitable zone of the Proxima Centauri star system, and the possibility of life sent waves of excitement through the astronomy community when it was discovered last year—but the weather is problematic.
M-dwarfs are much more hazardous to planets than their low luminosity lets on. Proxima b is constantly bathed in a corrosive cocktail of X-rays and UV rays so extreme that they actually evaporate the atmosphere. It seems ironic for a planet in the habitable zone of a star so much cooler than our sun, but Proxima Centauri’s is something of an oxymoron. Photoevaporation of the atmosphere on Proxima B is only worsened by magnetic activity from its star, which propels stellar winds and incites coronal mass ejections that wear away at whatever is left.
Because the phenomenon of photoevaporation has not been investigated enough, much about its impact is still unknown, which is why Proxima B is such an outstanding specimen. CfA astronomers Cecilia Garraffo, Jeremy Drake and Ofer Cohen recently launched a new study with a mission to understand the ravages of M-dwarfs on planets in their habitable zones by simulating stellar winds and magnetic field strength. “Inclement weather” is an understatement for what they found. If you think a blustery day on Earth is intense, try erratic stellar wind pressure anywhere from a thousand to ten thousand times higher. The atmosphere (or what remains of it) expands and contracts from the unpredictable pressure up to thrice a day throughout the course of Proxima B’s orbit.
So what about aliens? While Proxima B has a rocky surface and adequate temperatures to support life as we know it, hostile radiation could make mass-extinction events a daily occurrence. The thought of anything surviving where stellar flares result in the absence of an atmosphere is just too far-out (at least for now), but if something that conforms to a different set of biochemical rules can survive, who knows.