The first document examines how long an atmosphere could survive in Proxima Centauri b, which is only four light-years from Earth and was discovered in 2016. The second document badyzes a type of planet called the "water world" and questions how long oceans could survive. Because life takes billions to evolve, research has broad implications for understanding the habitability of the planet.
The researchers concluded that the Proxima Centauri b atmosphere would only be retained if the stellar wind pressure is low and the planet has a magnetic field to deflect the particles. Otherwise, they wrote, eventually the stellar wind would eliminate the planet's atmosphere, which would become so thin that water evaporated from the atmosphere could not fall like rain.
"Our results indicate that [Proxima Centauri b] and similar exoplanets are generally not able to withstand an atmosphere at sufficiently long time scales when the stellar wind pressure is high," said Dong.
Worse still is what happens to the type of star in which Proxima Centauri b orbits. The planet is near a red dwarf star, which is smaller and produces less energy than our sun. The habitable zone around these stars could change as the star evolves. Specifically, when a red dwarf is young, it is prone to produce high stellar winds. This means that a young and nearby planet could lose much of its atmosphere before life has a chance to develop.
"In addition, these nearby planets could also be blocked in a tidal way like our moon, with one side always exposed to the star," Dong said. "The resulting weak global magnetic field and the constant bombardment of the stellar wind would serve to intensify the loss of atmosphere on the side of the star."
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The second document asked how the oceans would fare in the water worlds under different stellar wind conditions. He considered three scenarios. The first was an environment similar to the winds that the Earth experiences. The second considers the strong stellar winds that are believed to be present in young stars. And the third evaluates what would happen in the case of a large solar storm called the "Carrington event", which refers to an incident in 1859 on Earth when auroras interrupted the telegraph service.
Unfortunately for life prospects, simulations showed that the stellar wind of the young stars would produce greater atmospheric loss in the young planets, causing the oceans to dry out. And Carrington-type events, which are believed to be frequent in these young stars, would cause even more damage.
The researchers said that these findings could alter Drake's famous equation, which calculates how many civilizations there might be in the Milky Way. One of the factors in the equation is the number of planets in a given star system with a suitable environment for life. But how that environment is determined to be suitable for life is still a matter of debate.
The authors say that more observations are needed, including the James Webb Space Telescope that NASA will launch in 2019, to better estimate how many planets could support life.
While the case of life looks pessimistic in Proxima Centauri b, scientists did note that another recent discovery could have a better chance of life. Ross 128b, announced for the first time in November, is only 11 light-years from Earth. It is rocky and in the habitable region of its star. But unlike Proxima Centauri b, the red dwarf star near Ross 128b seems to be quieter, with no eruptions that can interrupt life on its surface.
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