The Kepler space telescope, launched in 2009, has discovered several thousand exoplanet candidates. There is really no other scientific instrument that is giving wings to astrobiologists to think that life is, as the biochemist and Nobel Prize winner Christian De Duve said, a cosmic imperative . At least because of the abundance of planets, a sine qua non condition for the appearance of life. The truth is that seeing the millions of galaxies in the universe, each with more than a hundred billion stars, it is difficult to think that the Earth is the only planet with living beings on its surface, however unlikely it may be. emergence of life Now, this reasoning assumes that any planet around any star and in any galaxy is valid. It really is that?
Things are not so simple. Let us first think of galaxies: not all of them are suitable for life , even if it is microscopic. In active galaxies, such as the Seyferts, their cores emit high-energy radiation fluxes that are capable of sterilizing any attempt to fill a planet with life . In addition, it is not well known how the morphology of a galaxy -whether it is spiral, elliptical or irregular- affects the habitability of the planets that are in it. What is clear is that a galaxy with low metallicity -that is, with few elements such as iron, carbon, phosphorus, sodium…-, is unfeasible for the appearance of planets and life forms. This is the case with, for example, elliptical galaxies .
drawbacks for life
But suppose we have a suitable galaxy, like our own Milky Way; that does not mean that any place is appropriate for the appearance and maintenance of life. Thus, it is very likely that the regions near the core are not habitable . On the one hand, because the levels of radiation coming from the super black hole located in the center and from the neighboring regions are so high that they make it impossible to create complex molecules: in our case, the center of the Milky Way emits a quantity of gamma radiation (the most energetic of all existing) which is 250,000 times higher than that received by our planet . Another reason is that in the most central regions of the galaxy the stellar density is very high, which means that chance encounters between stars are much higher. When this happens, the orbits of their planets are strongly affected by gravitational pulls , which can lead to changes in their surface or, more likely, climatological. In the most extreme case, the encounter with another star can take them out of orbit, launching them into interstellar space. A third item to keep in mind is that the higher the density of stars in a region, the more likely it is that a sufficiently massive one will end its days as a supernova , the most dramatic end a star can have. Within two seconds, the star collapses and explodes, becoming as bright as all the stars in the galaxy put together: this cataclysmic endgame is capable of severely affecting any planet within 30 light-years.
All that has been said leads us to define a sterile sphere around the galactic center with a radius of at least 10,000 light-years . But this is not the only region hostile to life. Regions far away from the galactic center are not appropriate either, as they present a significant shortage of heavy elements -iron, carbon, nitrogen, nickel, magnesium…-, essential for the formation of rocky planets and, of course, the molecules of the life. And in the case of spiral galaxies, the arm region has a much higher density of stars than the interarm regions (which is where our Sun is), so we face the same problem of collisions between stars that we have in the galactic center, although on a smaller scale.
Not every star is valid
However, in this case we must take into account a detail that complicates the situation: the stars orbit around the center of the Milky Way. This means that a star that is now in a galactic arm does not mean that it has always been there. This is a peculiar situation because galaxies present what is called differential rotation, that is, the stars do not all rotate at the same speed around the center : there are places where the stars rotate faster than the spiral arms, and also vice versa . Because of this different speed we are going to have stars that enter the spiral arms periodically, which are regularly exposed to the inconvenience of being in areas of high stellar density.
The speed of stars and arms depends on their distance from the nucleus: those closer to the nucleus move faster than the arms, and those further away travel slower. Therefore, there is a corotation zone where the two speeds are equal: a star that is located in this zone and far from the arms is the most suitable so that, if it has a planetary system, life appears. Obviously this does not mean that in the rest of the stars it will not appear, but its periodic passage through the spiral arms, with the greater probability of inconvenient stellar encounters, makes life more exposed to global extinction.
Ward, P.D. & Brownlee, D. (2007) Rare Earth, Copernicus