Free Astronomy Magazine January-February 2015

8 ASTROBIOLOGY Cohen's team has reached these conclu- sions using a three-dimensional numerical model (developed at the University of Mich- igan), thanks to which it was possible to simulate the evolution of the atmospheres and magnetospheres of three (real) planets located in orbit around a (simulated) mid- dle-aged red dwarf star. Even when attri- buting to the planets a magnetic field as strong as that of the Earth, the result of the processing inevitably indicates that over time the planets’ gaseous envelopes lose their water content and become biologi- cally sterile (without considering several other potentially negative factors). Just a couple of months later, in August 2014, Brad Hansen, a researcher of the Uni- versity of California at Los Angeles (UCLA), portrayed an entirely different scenario in which the number of habitable planets of red dwarfs was largely greater than that previously estimated. Also in this case the author had carried out computer simula- tions started with the intention of determin- ing the rate at which red dwarfs can host Earth-sized planets in their habitable zones. Hansen has modeled stars with mass equal to half that of the Sun, with protoplane- tary disks extending from 0.05 to 1 AU, and containing a mass of gas and dust equiva- lent to 6 times the Earth's mass. The plane- tary systems resulting from the simulations comprise, on average, from 4 to 6 planets within 0.5 AU, 1-2 of which appear inside the habitable zone (that in this case ranges from 0.23 to 0.44 AU). Moreover, Hansen found that these planets could have accu- mulated huge amounts of water, about 25 T his hellish land- scape could be the norm on the surface of Earth- type planets in orbit in the habit- able zones of red dwarfs. [Inga Niel- sen, Hamburg Obs.] On the next page video, we have an example of how a red dwarf can transform a planet before entering the main sequence. [Mcplanetearth]