Key Takeaways:

  1. A study estimates a very low chance (0.0014%) of humans invading another planet even after achieving interstellar travel.
  2. The study uses human invasion rates over the past century to estimate the likelihood of future human invasions and applies that logic to potential alien civilizations.
  3. The study considers potential threats within our own galaxy, estimating there might be 4 hostile alien civilizations we can’t yet reach.
  4. The study suggests it’s even less likely we’d be invaded by a hostile alien civilization due to the vast distances involved.
  5. The study admits its limitations, including using human history as a model for alien behavior and not accounting for completely different alien motivations.

There are millions of potentially habitable planets in the Milky Way, and a research published to the preprint database arXiv indicates that about four of them might be home to hostile alien civilizations that would invade Earth if given an opportunity.

The strange question posed by the recently published paper—which is still awaiting peer review—is this: What are the chances that we will ever make contact with an aggressive extraterrestrial society that is capable of destroying Earth?

Alberto Caballero, the only study author and a doctoral candidate in conflict resolution at the University of Vigo in Spain, started by examining human history before gazing up at the stars in order to provide an answer.

“This paper attempts to provide an estimation of the prevalence of hostile extraterrestrial civilizations through an extrapolation of the probability that we, as the human civilization, would attack or invade an inhabited exoplanet,” Caballero stated in the research.

(Caballero is not an astrophysicist, but he has an article in the peer-reviewed International Journal of Astrobiology about the infamous Wow! signal, which may be an indication of extraterrestrial life.)

Caballero first calculated the nations that invaded other nations between 1915 and 2022 in order to arrive at his estimate. He discovered that during that time, 51 out of the 195 countries in the world had carried out an invasion of some kind. (The U.S. sat at the top of the list, with 14 invasions tallied in that time.) Then, he weighted each country’s probability of launching an invasion based on that country’s percentage of the global military expenditure. (Once more, the United States topped the list with 38% of all military spending.)

The probability that each nation would launch an invasion was then added, and the total was divided by the number of nations on Earth to arrive at what Caballero refers to as “the current human probability of invasion of an extraterrestrial civilization.”

The probability of humans currently invading another inhabited planet is 0.028%, based on this model. However, Caballero wrote, that probability refers to the current state of human civilization — and humans aren’t currently capable of interstellar travel. Caballero estimated that interstellar travel would not be feasible for another 259 years if current rates of technological advancement continue. Caballero used the Kardashev scale, which categorizes civilizations according to how much energy they consume.

When humans become an interstellar, or Type 1, civilization in 259 years, there is a 0.0014% chance that we will invade another planet, assuming the frequency of human invasions declines over that time at the same rate that invasions have declined over the last 50 years (an average of minus 1.15% per year, according to Caballero’s paper).

When you start multiplying that by the millions of potentially habitable planets in the Milky Way, it does indeed seem like extremely slim odds. Caballero used a 2012 study that was published in the journal Mathematical SETI, in which scientists estimated that up to 15,785 extraterrestrial civilizations might potentially exist with humans in the galaxy, for his final calculation.

Caballero came to the conclusion that a smaller percentage of Type 1 civilizations—0.22, to be exact—would be hostile to contacting humans. But when you take into consideration civilizations that, like our own, are not yet able to travel across space, the number of hostile neighbors rises to 4.42, Caballero told Vice News.

“I don’t mention the 4.42 civilizations in my paper because 1) we don’t know whether all the civilizations in the galaxy are like us… and 2) a civilization like us would probably not pose a threat to another one since we don’t have the technology to travel to their planet,” Caballero told Vice.

That four alien powers are hostile doesn’t seem like something that matters. Furthermore, Caballero noted that there is a very unlikely chance that humans will come into contact with one of these hostile civilizations and subsequently be invaded by them.

“The probability of extraterrestrial invasion by a civilization whose planet we message is… around two orders of magnitude lower than the probability of a planet-killer asteroid collision,” he wrote in his paper — adding that planet-killing asteroids, like the one that doomed the dinosaurs, are 1-in-100-million-year events.

Despite the fascinating thought experiment presented by Caballero’s study, the author acknowledges the limitations of his model. The likelihood of an invasion depends on a relatively small portion of human history and numerous assumptions regarding the course of our species’ future evolution. Additionally, according to Caballero, Vice, the model assumes that alien intelligence will have human-like brain compositions, values, and empathy perceptions. This may not always be the case.

“I did the paper based only on life as we know it,” he stated. “We don’t know the mind of extraterrestrials.”

It appears that we won’t get there for at least a few hundred years.

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