Planet Habitable Zone Cannot Be Too Windy
The “habitable zone” of a planet usually concerns its distance from the star such that it can support liquid water. But what if the star fries the surface with intense stellar winds?
“Deadly stellar winds could put a stop to life” reads the headline of an article on Astrobiology Magazine. Planet Earth has a protective magnetic field that shields its inhabitants from the solar wind. But the solar wind emitted by our star is mild compared to the intense storms of charged particles flung out by other stars.
Joe Llama from the University of St. Andrews looked at the winds that bombard one of the “hot Jupiters” (giant planets orbiting close to parent stars), named HD 189733. He considered what such a wind would do to an earth-like planet, even if it orbited in its habitable zone:
In the case of HD 189733b, this is not a huge problem as it and other hot Jupiters are already far too hostile for life to survive. But strong stellar winds could also strip away the atmospheres of potentially habitable planets further out, something that would have dire consequences for their habitability. Joe comments: “Imagine what the Earth would be like with its air stripped away, placed in a radiation bath. There could be numerous planets like this that in many ways resemble our world, but where life never stood a chance.”
The article did not quantify how many stars would irradiate their planets to death, but Llama thinks they “could be numerous”. There would be no point looking for life on planets in such systems. “For more than two decades we have been stepping up the search for other planets like the Earth,” he said. “Our new work will help refine this quest, enabling us to rule out the sites where dangerous activity on stars would kill off life from the start.”
Earlier this month on Astrobiology Magazine, Sara Seager of MIT presented a “revised” Drake Equation that leaves off Frank Drake’s final factors about intelligent life evolving. She’s just interested in any kind of life emerging on a habitable planet. Llama’s conclusions will surely affect one factor she calls FQ, the fraction of stars that are quiet. “Some stars are like our solar maximum all the time, with flares and other activity,” Seager said. “We don’t like those noisy stars.”
The “habitable zone” theory is is a classic case of dogmatic slumbers being jolted awake by physical reality. In Carl Sagan’s Cosmos days, it seemed so simple: billions upon billions of stars might have life, based on the idea that all you needed was a rocky planet at the right distance from its star. Over the past decade we have been adding complications to the “habitable zone” idea, narrowing down substantially the probability of finding those lucky stars. Last year about this time, our list of factors (reported from astrobiology literature) was at nine (see 9/08/12); we predicted additional factors would be thought of. Now we have #10: the Stellar Wind Habitable Zone. Earth continues to look mighty special in this vast universe. You can thank your lucky stars if your world view requires heavy doses of superstition. Others of us find positive evidence for intelligent design.