The Astrobiological Copernican Weak and Strong Limits for Extraterrestrial Intelligent Life

Abstract

We present a cosmic perspective on the search for life and examine the likely number of Communicating Extra-Terrestrial Intelligent civilizations (CETI) in our Galaxy by utilizing the latest astrophysical information. Our calculation involves Galactic star-formation histories, metallicity distributions, and the likelihood of stars hosting Earth-like planets in Habitable Zones, under specific assumptions which we describe as the Astrobiological Copernican Weak and Strong conditions. These assumptions are based on the one situation in which intelligent, communicative life is known to exist - on our own planet. This type of life has developed in a metal-rich environment and has taken roughly 5 Gyr to do so. We investigate the possible number of CETI based on different scenarios. At one extreme is the Weak Astrobiological Copernican principle - such that a planet forms intelligent life sometime after 5 Gyr, but not earlier. The other is the Strong Condition in which life must form between 4.5 to 5.5 Gyr, as on Earth. In the Strong Condition (a strict set of assumptions), there should be at least 36${}_{-32}^{+175}$ civilizations within our Galaxy: this is a lower limit, based on the assumption that the average life-time, L, of a communicating civilization is 100 years (based on our own at present). If spread uniformly throughout the Galaxy this would imply that the nearest CETI is at most 17000${}_{-10000}^{+33600}$ light-years away, and most likely hosted by a low-mass M-dwarf star, far surpassing our ability to detect it for the foreseeable future. Furthermore, the likelihood that the host stars for this life are solar-type stars is extremely small and most would have to be M-dwarfs, which may not be stable enough to host life over long timescales. We furthermore explore other scenarios and explain the likely number of CETI there are within our Galaxy based on variations of our assumptions.

arXiv commentAstrophysical Journal, in press, 47 pages

References in corpus (20)

• Galactic Stellar and Substellar Initial Mass Function
• Cosmic Star Formation History
• On the normalisation of the cosmic star formation history
• The Galaxy in Context: Structural, Kinematic and Integrated Properties
• The false positive rate of Kepler and the occurrence of planets
• Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk
• On the evolution of star forming galaxies
• The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations
• The Universe is Reionizing at z~7: Bayesian Inference of the IGM Neutral Fraction Using Ly$\alpha$ Emission from Galaxies
• The Milky Way's Stellar Disk
• An analytical model for the history of cosmic star formation
• Distant future of the Sun and Earth revisited
• Bayesian analysis of the astrobiological implications of life's early emergence on Earth
• Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler
• The First Planets: the Critical Metallicity for Planet Formation
• Probing the Faintest Stars in a Globular Star Cluster
• How Much SETI Has Been Done? Finding Needles in the n-Dimensional Cosmic Haystack
• The relationship between star formation rates, local density and stellar mass up to z ~ 3 in the GOODS NICMOS Survey
• Why do we find ourselves around a yellow star instead of a red star?
• Copernicanism and the Typicality in Time

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• The Number of Possible CETIs within Our Galaxy and the Communication Probability among These CETIs
• A Stationary Drake Equation Distribution as a Balance of Birth-Death Processes
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• A Quantitative Assessment of Communicating Extra-Terrestrial Intelligent Civilizations in the Galaxy and the Case of FRB-like Signals
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• Avoiding the "Great Filter": Extraterrestrial Life and Humanity's Future in the Universe
• The Inferred Abundance of Interstellar Objects of Technological Origin
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