Cai, and three co-authors, reported in late 2020 that in the field of astrobiology, the precise location, prevalence and age of potential extraterrestrial intelligence (ETI) have not been explicitly explored. The authors address these inquiries using an empirical galactic simulation model to analyze the spatial-temporal variations and the prevalence of potential ETI within the Galaxy. This model estimates the occurrence of ETI, providing guidance on where to look for intelligent life in the Search for ETI (SETI) with a set of criteria, including well-established astrophysical properties of the Milky Way. Further, typically overlooked factors such as the process of abiogenesis, different evolutionary timescales and potential self-annihilation are incorporated to explore the growth propensity of ETI.
The authors examined three major parameters:
1) the likelihood rate of abiogenesis (λA);
2) evolutionary timescales (Tevo); and
3) probability of self-annihilation of complex life (Pann).
The authors found Pann to be the most influential parameter determining the quantity and age of galactic intelligent life. Their model simulation also identified a peak location for ETI at an annular region approximately 4 kpc from the Galactic center around 8 billion years (Gyrs), with complex life decreasing temporally and spatially from the peak point, asserting a high likelihood of intelligent life in the galactic inner disk. The simulated age distributions also suggest that most of the intelligent life in our galaxy are young, thus making observation or detection difficult.
The authors concluded that a galactic model with resolution of individual Earth-like planets has been developed to analyze where and when potential intelligent life is most abundant throughout time, and how old the majority of complex life are at the current time in the Milky Way. A precise analysis for the propensity of potential ETI is formed, with a set of criteria including: the formation of two distinct phases of the Galaxy, recent observational analysis on the prevalence of Earth-like planets, sterilizing SNe that threaten land-based life, Poisson process of abiogenesis, different evolutionary timescales and potential self-annihilation of life.
The authors utilized three parameters, λA, Tevo, Pann, to trace the origin, evolution and development of life in the Milky Way, and assess the spatial and
temporal variations for the quantity of ETI. Extending the time to 20 Gyrs, regardless of change in key parameters, resulted in the peak for the number of ETI across all galactic regions around 4 kpc, and time at 8 Gyrs. The number of ETI decreased spatially and temporally from the peak. Even if the authors varied the evolutionary timescale to be 2 Gyrs less or greater than what was required on Earth, and with different values of λA and extreme case scenarios of Pann (0 and 0.99), the authors' conclusion of the precise location and time for the peak is not altered.
Their results suggest that the quantity of intelligent life does not always increase with time; in fact, their model predicts that after the peak occurs, the number of ETI starts to decrease monotonically with time, and this propensity remains throughout the next 6.5 Gyrs. More, their results show that the level of ETI will eventually reach an equilibrium between birth and death of intelligent life at approximately 20 Gyrs.
Further investigation for the number of sterilizing SNe events suggests that the prevalence of ETI reaches its maximum despite having the highest frequency of sterilizing SNe events by the location of the peak. The authors also also concluded that at the current time of the study, most intelligent life in the Galaxy is younger than 0.5 Gyr, with values of probability parameter for self-annihilation between 0 - 0.01; with a relatively higher value of the annihilation parameter (≥ 0.1), most intelligent life is younger than 0.01 Gyr. As authors cannot assume a low probability of annihilation, it is possible that intelligent life elsewhere in the Galaxy is still too young to be observed by us. Therefore, their findings can imply that intelligent life may be common in the Galaxy but is still young, supporting the optimistic aspect for the practice of SETI. The authors' results also suggest that our location on Earth is not within the region where most intelligent life is settled, and SETI practices need to be closer to the inner Galaxy, preferably at the annulus 4 kpc from the Galactic Center.
Additionally, the authors found the potential self-annihilation to be highly influential in the quantity of galactic intelligent life, suggesting another possible answer to the Fermi Paradox; if intelligent life is likely to destroy themselves, it is not surprising that there is little or no intelligent life elsewhere. Further, the probability of abiogenesis was not found to influence the quantity of intelligent life within the range developed by previous works, which supports the optimistic perspective of life being common in the Galaxy.
The results of their model yield a range of possible quantities for intelligent life over time. They assert that the intelligent life will always be most abundant approximately 4 kpc from the galactic center, peaking at time around 8 Gyrs, decreasing monotonically from that peak point, and that a majority of potential intelligent life is still young. The exact number of the intelligent life estimated here is not the focus of the authors' work; rather, it is instead the development of a statistical, comprehensive galactic picture tracing the potential growth propensity of intelligent life over a course of ~20 billion years.
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