Free Astronomy Magazine July-August 2020

JULY-AUGUST 2020 ASTROBIOLOGY of years without showing a significant variability. One might assume that it doesn’t matter if the star and its plan- etary system are in a spiral galaxy or an elliptical galaxy, but this is not so. The typol- ogy of the host galaxy seems to be able to influence the habitability of planets. A well-known theoretical work in this area was conducted in 2015 by researchers coordi- nated by astronomer Pratika Dayal (University of Durham). The team modeled the num- ber of potentially habitable planets in the local uni- verse relative to the plan- ets within the Milky Way. The conclusion was that large elliptical galaxies, with at least double the mass of the Milky Way, should contain 100 to 10,000 times more terres- trial planets than galaxies similar to ours. Dayal and colleagues interpreted this high proportion as a conse- quence of three properties that typically distinguish large elliptical galaxies from spiral ones: the greater stellar mass, the very low supernovae rate, and the very high metallicity. Large elliptical galaxies have, in fact, already transformed almost all the gas available into stars, while the most massive stars have aged and died more or less rap- idly, enriching the interstel- T hree spiral galaxies simi- lar in size to the Milky Way or slightly larger: on the side is M61 in the constellation Virgo; below is NGC 2336 in Camelopardalis; to the lower left is M109 in Ursa Major. The latter galaxy is a barred spiral, morpho- logically very similar to ours. [Cima Rest Astro- nomical Observa- tory, Magasa, Valvestino, Italy]