A Statistical Estimation of the Occurrence of Extraterrestrial Intelligence in the Milky Way Galaxy
Round 1
Reviewer 1 Report
Whether annihilation of extraterrestrial intelligence could depend on heavy deleterious impacts should be discussed, even briefly.
Ryder G (2003) Bombardment of the Hadean Earth: Wholesome or Deleterious? Astrobiology 3, 3.
Schmieder M, Krin DA (2020) Earth’s Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits. Astrobiology 20, 91.
A statistical approach explaining to general public why it is very improbable to detect a signal of extraterrestrial intelligence has been developed and could be quoted (Foucher F, Hickman-Lewis K, Westall F, Brack A (2017) A statistical approach to illustrate the challenge of astrobiology for public outreach. Life 7, 40.
Author Response
Pleasee the attached file
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Review of “A Statistical Estimation of the Occurrence of Extraterrestrial Intelligence in the Milky Way Galaxy”
Ref. Manuscript ID: galaxies-1055720
1) General comments
This paper presents a galactic simulation model to analyse the spatio-temporal variations and the prevalence of potential ETI within the Milky Way and the results obtained. Three parameters are varied: 1) the abiogenesis rate; 2) evolutionary timescales; and 3) the probability of self-annihilation.
The results are extremely interesting, showing a peak of ETI civilizations in an annulus at 4 kpc from the galactic centre and at 8 billion years after the Big Bang.
Unfortunately, certain hypotheses are quite restrictive and oversimplifying, i.e. (see L139-144), stars of mass comparable to the Sun’s mass, planets of radius, orbital radius and stellar irradiance comparable to the Earth’s ones. It is a pity that the authors do not address other potential habitability zones similar to those hypothesized in our Solar System, mainly on some moons of the giant planets Jupiter and Saturn, with energy coming from tidal energy created by the central giant planets. This would certainly increase the probability of finding ETI life around other stars by including Jupiter-like planets with moons.
Similarly, other values for stellar masses, planets radii, orbital radii and stellar irradiances should be investigated.
Furthermore, this Reviewer would like to offer another hypothesis, next to the self-annihilation hypothesis. If it is correct that ETI civilizations are to be found preferentially in the galactic zone around 4 kpc, i.e., that the spatial density of ETI civilizations would be relatively high, one should not forget that these civilizations could enter into wars among themselves for whatever reasons (supremacy, appropriation of resources, …), therefore increasing the probability of annihilation, in addition to self-annihilation. It would be interesting to model this new hypothesis in similar simulations.
2) Detailed comments
L33-35: « Hart (1975) suggested that no other intelligent life exists to address the question of the Fermi Paradox, which contradicts a lack of evidence and… ». It seems that this sentence contains a contradiction. Please rephrase appropriately.
L77: “Hair (2011)” instead of “Hairs (2011)”
L91: “Scharf and Cronin, 2016” instead of “Scharfa and Cronin, 2016”
L116-123: These lines should be removed from the final text.
L128-129: “Our simulation modeling approach to the ETI investigation is illustrated in the Appendix Figure 1.” The reference to an Appendix Figure 1 is not understood. An Appendix Figure could not be found in the paper. If it is referred to the Fig. 1 of page 8, why not bring this figure closer to this call in Lines 128-129? Furthermore, calling this figure is not sufficient to give a good idea of the simulation method used, it would require more explanations in this part of the text. Maybe a simpler sentence after the enumeration of the four points, something like “(refer to Fig. 1 for an overview)”, or simply put no reference at all to Figure 1.
L139-144: “Within our simulation, we defined Sun-like stars as stars with mass of 0.8 M⊙ ≤ M ≤ 1.2 M⊙,” This hypothesis is quite restrictive. Can’t we imagine that planetary habitability zones could still exist around larger or smaller stars? Same comment for the three further hypotheses on planets characteristics (radius, received stellar energy, orbital radius/period). Are these not also too restrictive?
Maybe a short justification of why these hypotheses were chosen would be appropriate here.
L154: The reference “Fuentes et al. (2017)” is not listed in the Reference list at the end of the paper. Ref. 47 is “Sans Fuentes et al. (2017)”. Please correct appropriately.
L171-173: It seems that there is a mix-up of parameters in the right part of eq. (2). The ‘Sum symbol’ with index ‘gas’ could be easily confused with the same symbol in eq. (1) while here it is the dimensionless gas surface density. So, either use here a different symbol or use the same symbol but divided by its unit “1 M⊙ pc-2” like in the original paper of Kennicutt (1998).
Furthermore, the parameter N should be put as an exponent to the dimensionless ‘Sum symbol’ with index ‘gas’.
L235: The reference “Miller and Branch (1990)” is not listed in the Reference list.
L237: The reference “Richardson et al. (2016)” is not listed in the Reference list. Ref. 38 is “Richardson et al. (2014)”. Please correct appropriately.
L246: See comment for L235.
L246: “may not be the most realistic” instead of “may not obtain the most realistic”
L249-250: “When a sterilizing event occurs on a life-bearing planet, the ozone layer of that planet is depleted and the land-based life within the planet is removed.” instead of “When a sterilizing event occurs on a life-bearing planet, the ozone layer of that planet will be depleted and the land-based life within the planet will be removed.” (Avoid mixing present and future tenses)
L251-252: Put the sentence at the present tense.
L255: “(Wesson, 2010)” instead of “(Wessen, 2010)”
L255 and L128: “modelling” or “modelling”, choose one spelling but be consistent in the whole text.
L266: “… range of (0.1 to 1) Gyr.” Why the parentheses? Remove if not necessary.
L284-285: “We focus on the parameter of 1 and 10–6 Myr –1,” This sentence could be rephrased, maybe as “We chose the range of values of the parameter to be 1 to 10–6 Myr –1,” or something similar.
L304: “itself” instead of “themselves”.
L307: The reference “Sagan and Shklovskii (1966)” is not listed in the Reference list. Ref. 44 is for “Shklovskii and Sagan (1966)”. Please correct appropriately.
L309: The references “Nick, 2002” and “Webb, 2011” are not listed in the Reference list.
L415: In the submitted PDF copy, the two Figures 3 and 4 overlap each other. Fig. 3 hides the Fig. 4 colour codes.
L430: The reference “Forgan et al., 2017” is not listed in the Reference list.
L478: See comment for L304.
L516: Ref. 12 (Fener…) should come before Ref. 10 (Forgan…) for references to be listed in alphabetical order.
L585: Ref. 49 (Tammann…) should come before Ref. 48 (Tarter…).
Author Response
Please see the attached file
Author Response File: Author Response.pdf
Reviewer 3 Report
Overall, this is a very good paper. The astrophysical model is adequate for the purpose and well explained. I see only two points that could be improved.
First, the concept of galactic bel of life predate Gonzalez et al (2001) by decades. It was first proposed be Balázs in the 80’s. It was based on the consideration of the co-rotation of stars and density wave which was affecting the sensitivity to SNII sterilising events. From the result of the paper, it is likely that this effect is not strong, and that this hypothesis should be discarded.
Second, they have been discussion about the impact of sustainability constrain on the lifetime of civilisation (Dutil & Dumas 2007; Haqq-Misra & Baum 2009). Those raise pretty strong constrains on the lifetime of technological civilisation.
References,
Balazs, B. (1988). The Galactic belt of intelligent life. In Bioastronomy—The Next Steps (pp. 61-66). Springer, Dordrecht.
Dutil, Y., & Dumas, S. (2007). Sustainability: a tedious path to galactic colonization. arXiv preprint arXiv:0711.1777.
Haqq-Misra, J. D., & Baum, S. D. (2009). The sustainability solution to the Fermi paradox. arXiv preprint arXiv:0906.0568.
Marochnik, L. S., & Mukhin, L. M. (1988). Belt of life in the galaxy. In Bioastronomy—The Next Steps (pp. 49-59). Springer, Dordrecht.
Author Response
Please see the attached file
Author Response File: Author Response.pdf
