There are millions of habitable planets in our milky way. Approximately four of them have hostile alien civilizations that could attack Earth, suggests new research by Alberto Caballero — a doctoral student in conflict resolution at the University of Vigo in Spain.

In his paper, Caballero asked: What are the odds that humans could one day contact a hostile alien civilization capable of invading our planet?

Caballero started by looking back at human history before looking out to the stars to find out the answer. Through his paper, he attempts to estimate the prevalence of hostile extraterrestrial civilizations by extrapolating the likelihood that human civilization will attack or invade an inhabited exoplanet.

Caballero arrived at his estimate by counting the number of countries that invaded others between 1915 and 2022. During that time, he discovered that 51 of the world’s 195 countries have undertaken some invasion. Then he calculated the chance of each country launching an invasion based on its percentage of global military spending.

Caballero then added each country’s probability of inciting an invasion by the total number of countries on Earth, arriving at “the current human probability of invasion of an extraterrestrial civilization,” as he describes it.

After calculating the weighted probabilities of invasion of each country, the estimated likelihood of invasion by a Type-0 civilization, as humanity is not capable of interstellar travel, is 0.026%.

Later, using the Kardashev scale, a method of measuring a civilization’s level of technological advancement, Caballero calculated current rates of technological advancement.

He wrote, *“In the last 50 years, the frequency of invasions has decreased at 1.15%. Meanwhile, world energy consumption has been increasing steadily at 2.24%.”*

*“We can extrapolate the data to determine whether a more advanced civilization could be more or less prone to invade our planet than humanity invading theirs. Humanity is expected to become a Type-1 civilization with average global consumption of 259.5 years. Such civilization has an estimated probability of 0.0014% invading another one.”*

Caballero told Vice.com, *“The probability of a hostile alien race invading Earth is low—very low. Therefore, 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.”*

*“0.22 Type-1 civilizations (capable of nearby interstellar travel), and 4.42 civilizations if all of them were like humanity (we aren’t a Type 0 yet). 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 (below Type-0), 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 (we will reach that technology once we become a Type-1).”*

*“I did the paper-based only on life as we know it. We don’t know the mind of extraterrestrials. An extraterrestrial civilization may have a brain with a different chemical composition, and they might not have empathy, or they might have more psychopathological behaviors. I found this way to do [the study], which has limitations because we don’t know the mind of what aliens would be like.”*

However, this is not the most sophisticated science. Still, Caballero hopes that by putting a number out there, he can start a conversation about whether it’s risky to send messages into space.

**Journal Reference:**

- Alberto Caballero, Estimating the prevalence of malicious extraterrestrial civilizations.
*arXiv*2022 DOI: 10.48550/ARXIV.2205.11618