Gazing up at the stars on a summer night devoid of any light pollution yields an astonishing array of stars inhabiting our tiny corner of the Milky Way galaxy. On such occasions, thoughts naturally turn to existential questions as to whether or not any of those pinpoints of light harbor planets capable of sustaining not only basic biological processes, but actual alien civilizations.
In an optimistic new paper published this week in the online forum, The Astronomical Journal, scientists Tom Westby and Christopher J. Conselice from the University of Nottingham have theorized that our neighboring star systems could actually be harboring some 36 diverse extraterrestrial races.
Labelled as Communicating Extra-Terrestrial Intelligent (CETI) civilizations, these remote targets were perceived using conclusions derived by deploying a different set of parameters for measuring potential advanced societies which employ the most up-to-date astrophysical data.
"There should be at least a few dozen active civilizations in our Galaxy under the assumption that it takes 5 billion years for intelligent life to form on other planets, as on Earth," Conselice explains. "The idea is looking at evolution, but on a cosmic scale. We call this calculation the Astrobiological Copernican Limit."
According to the study, their calculations involve a combination of galactic star formation histories, metallicity distributions, and the potential likelihood of candidate stars hosting Earth-like planets in their "just right" Goldilocks habitable zones, under defined assumptions they describe as the Astrobiological Copernican Weak and Strong conditions.
At one end of the spectrum, which they call the Weak Astrobiological Copernican scenario, they examined how life has evolved in a metal-rich environment like Earth, which has developed over the course of 5 billion years. This postulates that planets form intelligent life sometime after 5 billion years, but no earlier. The other extreme is defined as the Strong Astrobiological Copernican scenario, where life must have formed between 4.5 and 5.5 billion years ago, as it did on our Big Blue Marble.
Assuming the most strict conditions using the Strong scenario, Westby and Conselice believe there must technically be at least 36 civilizations within our Milky Way galaxy when considering an assumption that the average lifetime of a communicating civilization is 100 years, which is the period humanity has invented and used wireless radio communication.
Therefore, even if evenly distributed around the galaxy, this would infer that the closest CETI is approximately 17,000 light years away at the least, and probably protected by a low-mass M-dwarf star. Under those conditions, this would severely limit our ability to detect it right now using existing technology, making viable interstellar communication with such a habitable world almost impossible.
Currently, there are 4272 total confirmed exoplanets and 645 discovered around stars with solar masses of 0.95 to 1.05. While this optimistic model mirrors a positive outlook on potential alien life in our neck of the woods, it might not reflect other variables and assumptions on exactly how life on Earth began.
Variables such as whether or not our large Moon was a pivotal factor in stabilizing our axial tilt over vast spans of time, and if that provided a major filter crucial for our planet's long-term biological stability that only an infinitesimal number of potentially habitable exoplanets have. Also, what role do the existence of massive gas giants play to deflect or absorb debris collisions to habitable-zone planets that could inhibit biological evolution over millions of years?
"If we find that intelligent life is common, then this would reveal that our civilization could exist for much longer than a few hundred years; alternatively, if we find that there are no active civilizations in our galaxy, it is a bad sign for our own long-term existence. By searching for extraterrestrial intelligent life — even if we find nothing — we are discovering our own future and fate," Conselice noted.
Still, it's somewhat comforting to speculate over the idea that we're not alone out here in the inky void, even if our distant neighbors don't know it yet.