Free Astronomy Magazine July-August 2020

47 JULY-AUGUST 2020 ASTROBIOLOGY rocky planets with an at- mosphere even less thick than that of the Earth would not have under- gone permanent steriliza- tion (our planet is located 8.2 kpc from the galactic center). According to Bal- bi and Tombesi, during the nearly 50 million years of strong activity of our galactic core, any Earth- like planet within the 7 kpc critical radius may have lost a significant part of its atmosphere, but probably not all. For large elliptical galaxies, the story is different, as their SMBHs are typically hun- dreds or thousands of times more massive than Sgr A* and are propor- tionally more intense − the radiation flux in the past that flooded potentially habitable planets would have been biologically lethal. Since the progenitors of the current ellipticals were more compact, the devas- tating action of quasars must have neces- sarily affected almost all of the planets in those galaxies. If billions of years ago the Earth had been about 8 kpc from the quasar of an elliptical galaxy, it would have lost much of its atmosphere and per- haps today would be sterile. To argue that technological species are more likely to exist in elliptical galaxies, therefore, does not seem realistic, espe- cially if we consider that the high metallic- ity of those galaxies, one of the strengths of Dayal and colleagues’ work, is also one of its weakest points. According to that same team, the relative number of gaseous planets in large ellip- tical galaxies is up to a thousand times that of the Milky Way. Hence, there would be a considerable disproportion between gaseous planets and terrestrial planets, caused by the high metallicity that, de- spite how one might associate the terms “metallicity” and “terrestrial,” favors the O n the right is M101, also known as the Pinwheel Galaxy. It is located in Ursa Major and is about 1.5 times the size of the Milky Way. Below, the barred spiral NGC 4236 in the constella- tion Draco. [Cima Rest Astronomical Observatory, Ma- gasa, Valvestino, Italy]