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Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk.

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Title: Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk.
Authors: Scherf M; Space Research Institute, Austrian Academy of Sciences, Graz Austria.; IGAM/Institute of Physics, University of Graz, Graz, Austria., Lammer H; Space Research Institute, Austrian Academy of Sciences, Graz Austria., Spross L; Space Research Institute, Austrian Academy of Sciences, Graz Austria.; IGAM/Institute of Physics, University of Graz, Graz, Austria.
Source: Astrobiology [Astrobiology] 2024 Oct; Vol. 24 (10), pp. e916-e1061.
Publication Type: Journal Article
Language: English
Journal Info: Publisher: Mary Ann Liebert, Inc Country of Publication: United States NLM ID: 101088083 Publication Model: Print Cited Medium: Internet ISSN: 1557-8070 (Electronic) Linking ISSN: 15578070 NLM ISO Abbreviation: Astrobiology Subsets: MEDLINE
Imprint Name(s): Original Publication: Larchmont, N.Y. : Mary Ann Liebert, Inc., c2001-
MeSH Terms: Extraterrestrial Environment*/chemistry , Exobiology*/methods , Earth, Planet*, Atmosphere/chemistry ; Carbon Dioxide/analysis ; Carbon Dioxide/chemistry ; Ecosystem ; Models, Theoretical ; Galaxies ; Stars, Celestial ; Evolution, Planetary
Abstract: In Lammer et al. (2024), we defined Earth-like habitats (EHs) as rocky exoplanets within the habitable zone of complex life (HZCL) on which Earth-like N2-O2-dominated atmospheres with minor amounts of CO2 can exist, and derived a formulation for estimating the maximum number of EHs in the galaxy given realistic probabilistic requirements that have to be met for an EH to evolve. In this study, we apply this formulation to the galactic disk by considering only requirements that are already scientifically quantifiable. By implementing literature models for star formation rate, initial mass function, and the mass distribution of the Milky Way, we calculate the spatial distribution of disk stars as functions of stellar mass and birth age. For the stellar part of our formulation, we apply existing models for the galactic habitable zone and evaluate the thermal stability of nitrogen-dominated atmospheres with different CO2 mixing ratios inside the HZCL by implementing the newest stellar evolution and upper atmosphere models. For the planetary part, we include the frequency of rocky exoplanets, the availability of surface water and subaerial land, and the potential requirement of hosting a large moon by evaluating their importance and implementing these criteria from minima to maxima values as found in the scientific literature. We also discuss further factors that are not yet scientifically quantifiable but may be requirements for EHs to evolve. Based on such an approach, we find that EHs are relatively rare by obtaining plausible maximum numbers of INLINEMATH and INLINEMATH planets that can potentially host N2-O2-dominated atmospheres with maximum CO2 mixing ratios of 10% and 1%, respectively, implying that, on average, a minimum of INLINEMATH rocky exoplanets in the HZCL are needed for 1 EH to evolve. The actual number of EHs, however, may be substantially lower than our maximum ranges since several requirements with unknown occurrence rates are not included in our model (e.g., the origin of life, working carbon-silicate and nitrogen cycles); this also implies extraterrestrial intelligence (ETI) to be significantly rarer still. Our results illustrate that not every star can host EHs nor can each rocky exoplanet within the HZCL evolve such that it might be able to host complex animal-like life or even ETIs. The Copernican Principle of Mediocrity therefore cannot be applied to infer that such life will be common in the galaxy.
Contributed Indexing: Keywords: Eta-Earth—Earth-like habitats—habitability—astrobiology—complex life—atmospheric evolution
Substance Nomenclature: 142M471B3J (Carbon Dioxide)
Entry Date(s): Date Created: 20241031 Date Completed: 20241031 Latest Revision: 20250728
Update Code: 20260130
DOI: 10.1089/ast.2023.0076
PMID: 39481023
Database: MEDLINE
FullText Text:
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PubType: Academic Journal
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  Data: Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk.
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  Data: <searchLink fieldCode="AU" term="%22Scherf+M%22">Scherf M</searchLink>; Space Research Institute, Austrian Academy of Sciences, Graz Austria.; IGAM/Institute of Physics, University of Graz, Graz, Austria.<br /><searchLink fieldCode="AU" term="%22Lammer+H%22">Lammer H</searchLink>; Space Research Institute, Austrian Academy of Sciences, Graz Austria.<br /><searchLink fieldCode="AU" term="%22Spross+L%22">Spross L</searchLink>; Space Research Institute, Austrian Academy of Sciences, Graz Austria.; IGAM/Institute of Physics, University of Graz, Graz, Austria.
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  Data: <searchLink fieldCode="JN" term="%22101088083%22">Astrobiology</searchLink> [Astrobiology] 2024 Oct; Vol. 24 (10), pp. e916-e1061.
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  Data: In Lammer et al. (2024), we defined Earth-like habitats (EHs) as rocky exoplanets within the habitable zone of complex life (HZCL) on which Earth-like N<subscript>2</subscript>-O<subscript>2</subscript>-dominated atmospheres with minor amounts of CO<subscript>2</subscript> can exist, and derived a formulation for estimating the maximum number of EHs in the galaxy given realistic probabilistic requirements that have to be met for an EH to evolve. In this study, we apply this formulation to the galactic disk by considering only requirements that are already scientifically quantifiable. By implementing literature models for star formation rate, initial mass function, and the mass distribution of the Milky Way, we calculate the spatial distribution of disk stars as functions of stellar mass and birth age. For the stellar part of our formulation, we apply existing models for the galactic habitable zone and evaluate the thermal stability of nitrogen-dominated atmospheres with different CO<subscript>2</subscript> mixing ratios inside the HZCL by implementing the newest stellar evolution and upper atmosphere models. For the planetary part, we include the frequency of rocky exoplanets, the availability of surface water and subaerial land, and the potential requirement of hosting a large moon by evaluating their importance and implementing these criteria from minima to maxima values as found in the scientific literature. We also discuss further factors that are not yet scientifically quantifiable but may be requirements for EHs to evolve. Based on such an approach, we find that EHs are relatively rare by obtaining plausible maximum numbers of INLINEMATH and INLINEMATH planets that can potentially host N<subscript>2</subscript>-O<subscript>2</subscript>-dominated atmospheres with maximum CO<subscript>2</subscript> mixing ratios of 10% and 1%, respectively, implying that, on average, a minimum of INLINEMATH rocky exoplanets in the HZCL are needed for 1 EH to evolve. The actual number of EHs, however, may be substantially lower than our maximum ranges since several requirements with unknown occurrence rates are not included in our model (e.g., the origin of life, working carbon-silicate and nitrogen cycles); this also implies extraterrestrial intelligence (ETI) to be significantly rarer still. Our results illustrate that not every star can host EHs nor can each rocky exoplanet within the HZCL evolve such that it might be able to host complex animal-like life or even ETIs. The Copernican Principle of Mediocrity therefore cannot be applied to infer that such life will be common in the galaxy.
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