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- A alternative to the Drake Equation for thinking about how likely intelligent life on other planets has been recently published.
An article in The Astrophysical Journal (not open access, here is an arXiv version): The Astrobiological Copernican Weak and Strong Limits for Intelligent Life proposes a different way to think about the possibility of intelligent life other places than earth.
Abstract:
Why Copernican?
This is based on the only case of intelligent life that we know of, on earth.
They are only considering goldilock zone planets and not potential sites of life formation that could be heated by gravitation of large nearby planets (like Europa and Enceledus).
What I find interesting about their argument:
I believe that this also addresses parts of the "where are they?" argument.
Abstract:
We present a cosmic perspective on the search for life and examine the likely number of Communicating Extra-Terrestrial Intelligent (CETI) civilizations 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 their 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 civilizations based on different scenarios. At one extreme is the Weak Astrobiological Copernican scenario—such that a planet forms intelligent life sometime after 5 Gyr, but not earlier. The other is the Strong Astrobiological Copernican scenario in which life must form between 4.5 and 5.5 Gyr, as on Earth. In the Strong scenario (under the strictest set of assumptions), we find there should be at leastcivilizations within our Galaxy: this is a lower limit, based on the assumption that the average lifetime, L, of a communicating civilization is 100 yr (since we know that our own civilization has had radio communications for this time). If spread uniformly throughout the Galaxy this would imply that the nearest CETI is at most
lt-yr away and most likely hosted by a low-mass M-dwarf star, likely far surpassing our ability to detect it for the foreseeable future, and making interstellar communication impossible. 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 the Galaxy based on variations of our assumptions.![]()
Why Copernican?
Our assumption is based on what we call the Principle of Mediocrity: there is no evidence to assert that the Earth should be treated as a special case, and therefore – according to the Copernican Principle – we propose that the likelihood of the development of life, and even intelligent life, should be broadly uniformly distributed amongst any suitable habitats.
This is based on the only case of intelligent life that we know of, on earth.
They are only considering goldilock zone planets and not potential sites of life formation that could be heated by gravitation of large nearby planets (like Europa and Enceledus).
What I find interesting about their argument:
- Life is assumed to be a result of planetary processes.
- They emphasis the presence of metals in star forming areas. This means at least second generation stars, after higher molecular weight atoms have been formed in stellar processes, which means life would not be expected to form rapidly after the big bang. Heavier elements are essential for life as we know it (life on earth).
- They take how long various stages of life took to evolve on Earth as rough guidelines for when life could evolve after planets form.
I believe that this also addresses parts of the "where are they?" argument.