A new study has a solution to the Fermi paradox, but (as usual) you probably aren't going to like it.
If you haven't heard of the Fermi Paradox, here it is in a nutshell: Given the high probability that alien life exists out there (bearing in mind the vastness of space and that we keep finding planets within habitable zones) why has nobody got in touch yet? If there are so many other civilizations out there – possibly at far more advanced stages than we are because of how long the universe has dragged on – surely at least one would send out probes, or is actively trying to inhabit the galaxy? If so, why haven't we come across them?
One take on the Fermi Paradox, cited in the new pre-print paper published on Arxiv, came from astrophysicist Michael Hart in 1974. In the paper, titled "An Explanation for Absence of Extraterrestrials on Earth", Hart argues that if a civilization tried, it would be able to colonize the galaxy in a relatively short time-span.
"Assume that we eventually send expeditions to each of the 100 nearest stars. (These are all within 20 light-years of the Sun.) Each of these colonies has the potential of eventually sending out their own expeditions, and their colonies in turn can colonize, and so forth," Hart writes. "If there were no pause between trips, the frontier of space exploration would then lie roughly on the surface of a sphere whose radius was increasing at a speed of 0.10c. At that rate, most of our Galaxy would be traversed within 650,000 years. If we assume that the time between voyages is of the same order as the length of a single voyage, then the time needed to span the Galaxy will be roughly doubled. We see that if there were other advanced civilizations in our Galaxy they would have had ample time to reach us, unless they commenced space exploration less than 2 million years ago."
If this is anywhere close to correct, and given that our galaxy is around 13.6 billion years old, it's puzzling that there is no evidence that we have found of extraterrestrial civilizations. In fact, the paper argues, the absence of aliens on Earth could be seen as evidence that there is no alien life out there. If there is, Hart said, it would require a sociological explanation for why they did not colonize the galaxy when they have had ample chance to do so. Unless a convincing one can be found, and until we have more information on planetary formation and other factors, Hart said that this is "strong evidence that we are the first civilization in the galaxy".
In a new paper, astrobiologist Jacob Haqq-Misra and space research scientist Thomas J. Fauchez propose a different solution: we have been overlooked because we have the wrong type of sun. The team acknowledges that, as Hart suggested, any solution to the Fermi Paradox must grapple with sociological explanations for why aliens have not colonized the galaxy, be it that they realize endless growth is unsustainable or that our part of the galaxy is unfashionable.
They note that aliens may want to expand into the galaxy for all sorts of reasons, including to study life as it evolves, and suggest that some of the most attractive places for aliens to settle would be places that can sustain life, possibly demonstrated by the fact that life is already there.
"Although it remains possible that long-lived technological civilizations do not expand, it also remains possible that such civilizations pursue galactic settlement in order to ensure their longevity," they write in their paper. "But why would an extraterrestrial civilization expand across the galaxy but not settle the solar system?"
"We suggest, following the hypothesis," they explain, "that an expanding civilization will preferentially settle on low-mass K- or M-dwarf systems, avoiding higher-mass stars, in order to maximize their longevity in the galaxy".
Our own sun is a G-dwarf star – one which generates energy primarily by the fusion of hydrogen into helium. K-dwarfs burn hydrogen and have been touted as having a larger "sweet spot" for hosting habitable planets. K-dwarfs, plus more abundant M-dwarfs, can provide a habitable environment for a lot longer than G-dwarfs like our own.
Any species looking for a place to settle down might prioritize longevity, given the distances involved in space travel and the resources it would take to colonize a new solar system.
The team looked at several scenarios for how long it would take an advanced civilization to settle in all K-dwarf systems; all K-dwarf and M-dwarfs; and all K-dwarf, M-dwarf, and G-dwarf systems. Including time to wait until suitable systems grow nearer, they estimate that it would take an advanced civilization (if there was only one of them) about 2 billion years to spread itself out over all low-mass stars (K and M dwarfs).
"We can exclude scenarios in which all G-dwarf stars would have been settled by now," the team adds, given that we have found no evidence of extraterrestrial life here on Earth, "but the possibility remains open that a Galactic Club exists across all K-dwarf or M-dwarf stars".
The team says that because of this, K-dwarfs may be the ideal target for searches for bio- and technosignatures in our search for extraterrestrial life. However, finding planets around these stars is more difficult due to their larger size.
The "galactic club" may be out there, thriving throughout the galaxy, and incredibly difficult to detect.