Are Local Group Dwarf Spheroidal Galaxies the First Safe Planet-hosting Environments?

Abstract We explore whether Local Group dwarf spheroidal (dSph) galaxies might have hosted Earth-like planets dwelling unexposed for several billions of years to major galactic threats to life, such as supernovae and gamma-ray bursts. To this aim, we developed a novel semiempirical model that exploits the observed chemical abundances and star formation histories of a selected sample of local dSphs, to explore whether their stars may have (i) reached the minimum metallicity to trigger planet formation and (ii) avoided exposure to destructive events long enough to provide time for possible biological development. From our work two scenarios emerge. If planet formation is possible for [Fe/H] ≲ −1, then in all dSphs with 5 × 10 3 L ⊙ ≤ L V ≤ 2 × 10 7 L ⊙ a fraction ≈0.1%–10% of stars might have safely hosted terrestrial planets for more than 1 Gyr. In this scenario, ancient ultrafaint dwarf galaxies (UFDs, L V ≤ 10 5 L ⊙ ) would have been the first to reach this condition in the history of the Local Group. Conversely, if planets form for [Fe/H] ≥ −0.6 then they should not exist in UFDs, while only ≈0.001%–0.1% of stars in dSphs with L V ≥ 3 × 10 5 L ⊙ would host planets dwelling in safe conditions for long times. Interestingly, we find a “luminosity sweet spot” at L V ∼ 10 6 L ⊙ , where dSphs in our sample safely host terrestrial planets up to 4 Gyr and in any planet formation scenario explored. In conclusion, planet formation at low metallicity is key to understanding which types of galaxies might have formed Earth-like planets that dwelt unexposed to galactic threats over several billions of years, first in the history of the Local Group.