There could potentially be millions of habitable planets in the Milky Way. And according to a new study submitted to the arXiv preprint database, only four are the number of alien civilizations that would attack us and invade Earth if they could.
The new, yet peer-reviewed article begins with an odd question: What are the odds that humans will one day come into contact with a hostile alien civilization that may invade our planet?
Sole study author Alberto Caballero, a PhD student in conflict resolution at the University of Vigo in Spain, started by looking at human history before looking at the stars to answer this question. However, it is worth noting at this point that Caballero was not an astrophysicist. As
Caballero wrote in his work, “This paper attempts to provide an estimate of the prevalence of hostile extraterrestrial civilizations by estimating our probability as a human civilization to attack or invade an inhabited exoplanet.”
To arrive at his estimate, Caballero first determined the number of countries that occupied other countries between 1915 and 2022. He found that during that time, a total of 51 of the 195 nations on Earth had launched some form of invasion. The USA was at the top of the list with 14 invasions during this time. He then evaluated each country’s probability of launching an invasion, based on the percentage of that country’s global military spending. The US once again ranked first with 38% of global military spending.
Next, Caballero added the probability of each country launching an invasion, then divided the sum by the total number of countries on Earth and found what he defined as “the probability of humans invading an extraterrestrial civilization.”
According to this model, the probability of humans occupying another planet is 0.028%. However, Caballero noted that this possibility refers to the current state of human civilization, and humans are currently not capable of interstellar travel. If the current rate of technological development continues, interstellar travel will not be possible for another 259 years, as it turns out when Caballero categorizes how advanced a civilization is based on energy expenditure.
Assuming that the frequency of human invasions continued to decline at the rate that over the last 50 years (average minus 1.15% per year, according to Caballero’s article), during this time period, an interstellar or Type 1 civilization could potentially become an interstellar or Type 1 civilization 259 years from now. Once we become one, there will be a 0.0014% probability that the human race will invade another planet.
This probability may sound very low, and it remains small until you start multiplying that number by the millions of potentially habitable planets in the Milky Way. For his final calculation, Caballero used a 2012 paper published in the journal Mathematical SETI where researchers estimated that as many as 15,785 alien civilizations could theoretically share the galaxy with humans.
Caballero concluded that less than one of the Type 1 civilizations (0.22 to be exact) would be hostile to humans who came into contact. Caballero puts the number of malevolent neighbors at 4.42, taking into account civilizations that are not yet capable of interstellar travel, such as modern humans.
Four hostile alien races seem like nothing to worry about. Also, Caballero adds that the chances of humans coming into contact with one of these malevolent civilizations and then being invaded by them are next to non-existent.
Caballero’s article adds that planet-killing asteroids, such as the asteroids that sentenced the dinosaurs to death, are events that take place every 100 million years, adding that “the probability of an extraterrestrial invasion of a civilization whose planet we transmit … is about twice as likely as a planet-killing asteroid collision.” It says “low”.
While Caballero’s work provides an interesting thought experiment, the author acknowledges that his model has limitations. The possibility of invasion rests on a very narrow slice of human history and makes many assumptions about the future development of our species. Caballero also explains in his interview with Vice that the model also assumes that alien intelligence will have brain compositions, values, and feelings of empathy similar to humans, but that may not be the case.
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