Free Astronomy Magazine November-December 2019

46 NOVEMBER-DECEMBER 2019 EXOPLANETS A ccording to the astrobiol- ogist Ravi Kumar Kopparapu, here photographed during an inter- view with CBS, the Earth orbits almost in the inner edge of the habitable zone of the Sun. Below is shown the case of a planet with an orbit that only partially enters the habitable zone of its star. more efficiently generate a magnetic field than that produced by the Earth’s core, pro- tecting the surface from cosmic and stellar rays. This shielding, together with a foresee- ably thicker atmosphere, would allow for the less-threatened diffusion of life com- pared to the same proliferation that hap- pened on Earth. In this regard, however, we must point out that, ac- cording to some re- searchers, biodiversity on our planet has been promoted by mutations occurring in certain or- ganisms following peaks of space radia- tion. If things really went that way, a mag- netosphere and an at- mosphere that are too protective could hinder the sudden appearance of new species. Just as there is an upper limit to the size of a planet for it to be hab- itable, does a lower limit perhaps exist? Of course it does, and ac- cording to recent work by Constantin Arn- scheidt (Harvard University) and colleagues, that limit approaches just 3% of the Earth’s mass in the case where the atmosphere is subjected to a greenhouse effect that ex- pands it enough to balance heat absorption and radiation. It is clear that we are consid-