The best place and time to live in the Milky Way (2009.13539v2)

Abstract: Counted among the most powerful cosmic events, supernovae (SNe) and gamma-ray bursts (GRBs) can be highly disruptive for life: Their radiation can be harmful for biota or induce extinction by removing most of the protective atmospheric ozone layer from terrestrial planets (TPs). Nearby high-energy transient astrophysical events have been proposed as possible triggers of mass extinctions on Earth. We assess the habitability of the Milky Way (MW) throughout its cosmic history against potentially disruptive astrophysical transients with the aim of identifying the safest places and epochs within our Galaxy. We also test the hypothesis that one long GRB played a leading role in the late Ordovician mass-extinction event (~445 Myr ago). We characterised the habitability of the MW throughout its cosmic history as a function of galactocentric distance of TPs. We estimated the dangerous effects of transient astrophysical events (long and short GRBs and SNe) with a model that connects their rate to the specific star formation and metallicity evolution within the Galaxy throughout its cosmic history. Our model also accounts for the probability that TPs form around FGK and M stars. Until about six billion years ago, the outskirts of the Galaxy were the safest places to live, despite the relatively low density of TPs. In the last about four billion years, regions between 2 and 8 kpc from the center, which had a higher density of TPs, became the best places for a relatively safer biotic life growth. We confirm the hypothesis that one long GRB played a leading role in the late Ordovician mass-extinction event. In the last 500 Myr, the safest neighborhood in the Galaxy was a region at a distance of 2 to 8 kpc from the Galactic center, whereas the MW outskirts were sterilized by two to five long GRBs.

• The paper develops a habitability model linking terrestrial planet distribution with astrophysical threats like SNe and GRBs.
• It demonstrates that regions between 2 and 8 kpc have become safer over the past 4 billion years for potential life.
• The study validates a GRB’s impact on the late Ordovician mass extinction and informs future exoplanet search strategies.

The Best Place and Time to Live in the Milky Way

This paper proposes a comprehensive analysis regarding the habitability of the Milky Way (MW) in light of destructive astrophysical events, focusing particularly on supernovae (SNe) and gamma-ray bursts (GRBs). The researchers aim to identify the safest regions and time periods for life, integrating this within the context of galactic evolution.

Overview of Methodological Approach

The authors develop a model to assess the habitability of the MW throughout its cosmic history. This model incorporates the distribution of terrestrial planets (TPs) around different types of stars, while linking the rates of potentially harmful events—long and short GRBs and SNe—with both specific star formation rates (SFR) and the evolution of metallicity in the galaxy.

Key Findings

1. Temporal and Spatial Distribution: Until approximately six billion years ago, the outskirts of the Galaxy were the safest, albeit with a lower density of TPs. Over the recent four billion years, regions between 2 and 8 kpc from the galactic center emerged as favorable for life due to a higher density of TPs and a decreased threat from high-energy events.
2. GRB Impact on Mass Extinction: The paper confirms the hypothesis that a long-duration GRB likely contributed to the late Ordovician mass-extinction event about 445 million years ago.
3. Current Safe Zones: In the past 500 million years, regions between 2 and 8 kpc from the center of the MW are notably safer, whereas the outskirts have seen sterilization from multiple GRBs.

Implications for Galactic Habitability

The results have significant implications for the search for habitable extraterrestrial environments. The identified safe zones present promising regions for locating exoplanets capable of sustaining life. The findings stress the importance of considering both the aftermath of astrophysical events and the galactic metallicity gradient when evaluating planetary habitability.

Speculation on Future Developments

The paper indirectly hints at future directions in astrobiology and galactic studies, primarily through more detailed, simulation-based examinations of the MW's habitability. This includes refined models of planet distribution and survival likelihood in relation to evolving galactic dynamics. Additionally, advancements in observational astronomy tools could enhance our ability to detect such potentially habitable zones.

Conclusion

Overall, this paper provides a detailed theoretical framework for understanding the complex interplay between galactic environment and planetary habitability over time. The findings are integral in directing both observational efforts to locate habitable exoplanets and theoretical studies that further explore the conditions which foster life in our galaxy.