Past “Fermi’s Paradox” XIV: What’s the Aurora Speculation?


Welcome back to our Fermi Paradox series, in which we discuss possible solutions to Enrico Fermi's famous question "Where is everyone?" Today we are examining the possibility that the reason for the great silence is that we are “early to party”!

In 1950, the Italian-American physicist Enrico Fermi had lunch with some of his colleagues at Los Alamos National Laboratory, where he had worked on the Manhattan Project five years earlier. According to various reports, the conversation turned to aliens and the recent flood of UFOs. Fermi made a statement that would go down in the annals of history: "Where are all?"

This became the basis of the Fermi paradoxThis refers to the discrepancy between high probability estimates of the existence of extraterrestrial intelligence (EIT) and the apparent lack of evidence. Several resolutions have been proposed on his question since Fermi's time, including the Aurora hypothesis That is, just because planets are habitable doesn't mean intelligent life can settle there.

This hypothesis takes its name from a famous and relatively new science fiction novel (more on this below), although the seeds for it go back many decades. At its core, the hypothesis challenges the idea that an intelligent species would be able to colonize beyond its home star system, which effectively throws a wrench into one of the basic premises behind the Fermi Paradox.

This is nothing more than the foregone conclusion that an advanced civilization would create tangible signs of its existence in the form of space-based infrastructure as it spread across our galaxy (or other galaxies). This, of course, starts from the argument that, given the age of the universe and the sheer number of stars and planets, intelligent life must have evolved many times (and on many worlds) by now.

That damn guess!

As we discussed in Part II, the assumption that an advanced civilization (or more) should have colonized a significant portion of our galaxy did not come from Fermi himself. It wasn't until 1975 when astronomer Michael Hart wrote an article entitled “One Explanation for the Absence of Aliens on Earth, “wrote this idea was really linked to the Fermi Paradox.

The gist of Hart's argument was that if an ETI had formed in the Milky Way in the past, it would have had ample time to evolve interstellar travel and colonize nearby stars. These colonies would have led to similar colonization efforts over time and eventually resulted in civilization colonizing most of our galaxy. Since there is no evidence of such a civilization (Hart's "Fact A"), Hart argued that humanity was alone.

This argument was further elaborated in 1980 by the physicist and cosmologist Frank Tipler in his clearly titled study "Extraterrestrial intelligent beings do not exist". Here Tipler applied the cosmological principle and other points argued by SETI proponents, which essentially boil down to the idea that ETIs developed technologies as humanity based them on their understanding of the same scientific principles.

From this, he estimated that any advanced species that had appeared a billion years before mankind could colonize the Milky Way multiple times:

“In addition to rocket technology comparable to ours, it seems likely that a species involved in interstellar communications has some sophisticated, sophisticated computing technology. I therefore assume that such a species will eventually develop a self-replicating universal constructor with comparable intelligence on a human scale … and such a machine, combined with today's rocket technology, would make it possible to explore the galaxy in less than 300 million years and / or colonize. "

Tipler's contribution to this argument is why it bears both his and Hart's name. Subsequent theorists belonging to what the famous scientist David Brin called the "uniqueness hypothesis" have built their arguments on a similar basis. To this, Carl Sagan replied in a counter-essay (which he co-authored with earth scientist William Newman) that "the lack of evidence is not evidence of the absence".

All we know for certain is that no advanced civilization has colonized a significant portion of our galaxy. otherwise they would have been discovered by now. Aside from the conclusion that humanity is alone in the universe, which Sagan viewed as an "anthropocentric and self-congratulatory explanation," there must be another reason for fact A.

Interior view of a generation ship with alternating strips of habitable surface and "windows" to let in light. Photo credit: Rick Guidice / NASA Ames Research Center

Unfortunately, these arguments tend to attribute the “great silence” to sociological (exploration and expansion are not the norm) or biological (intelligent living is rare). Examples of this are the Rare Earth Hypothesis, the Ocean Worlds Hypothesis or arguments based on the principle of “non-convergent evolution”. However, there is also the possibility that biology is a factor in another sense.


The Aurora Hypothesis is named after the 2015 science fiction novel by Kim Stanley Robinson. The story revolves around the crew of an interstellar generation ship traveling at 10% speed of light to the Tau Ceti system to colonize a moon (Aurora) orbiting its Tau Ceti e. Shortly after arriving and trying to get used to the local environment, colonists begin to die from exposure to a primitive life form of prions.

Prions are misfolded proteins that can transfer their properties to other variants of the same protein. Because they were so small, once they settled on the planet, they could not escape the detection probes and crew of the Aurora. In response, the crew is being split between those who still wish to colonize the system and those who wish to return to Earth (which they now believe is the only human environment).

The hypothesis arose from a 2019 study entitled "The Fermi Paradox and the Aurora Effect: Settlement, Expansion, and Steady States in Exo-Civilization," by Adam Frank of Rochester University, Caleb Scharf, the Principle Investigator of the NASA Nexus, led exoplanetary systems science (NExSS) – and researchers from the Center for Exoplanets and Inhabitable Worlds at Penn State University and Columbia University.

Taking Hart's fact A into account, the researchers first examined the speed at which an advanced exo-civilization could settle across the galaxy. This included the caveat that not all systems have habitable planets and that populated systems would eventually send out their own probes and passenger ships, adding to a "settlement front" that spread across the galaxy over time.

In addition, they contained the possibility that the earth could have been colonized (or visited) in the distant past by an exo-civilization for which evidence no longer exists. What was new to their study, however, was the idea that a planet, while "habitable", may not be inherently "settable". Like you said:

“It is often assumed that each planet can be adapted to the specific needs of settlement civilization. The idea that the purpose of probes is to build habitable settlements and that all star systems are viable targets for such settlements goes to the agency of an exo-civilization. We therefore loosen this assumption in our work.

“In addition, some stars can host the life of indigenous forms, which can rule out settlement for practical or ethical reasons. This topic was explored in (spoiler alert) the novel Aurora by Kim Stanley Robinson (Robinson 2015), in which a world was formally habitable – it wasn't what we would call settable. Hence, we include the possibility that good worlds are hard to find – what we call the aurora effect. "

Planets everywhere. So where are all the aliens? Photo credit: ESO / M. Kornmesser

After incorporating all of this into a series of simulations, they came to several conclusions. First, they concluded that the time it would take for an exo-civilization to colonize the galaxy is less than (or comparable to) the current age of the Milky Way (13.5 billion years). However, if you include the “aurora effect” in the equation, a scenario arises in which only certain parts of the galaxy are inhabited.

Add to this the notion that the lifespan of civilizations is finite, and it appears that certain clusters of the galaxy are destined to be colonized and relocated while the surrounding areas become restless. If the earth is in a region of the galaxy that does not correspond to a "relocation cluster", it is very likely that we have not settled or visited for an extended period of time – up to 1 million years.

This hypothesis is reminiscent of the "percolation theory" argued by NASA scientist Geoffrey A. Landis in his famous 1993 study. According to Landis, astrophysics defines a maximum distance over which direct interstellar colonization is possible (e.g. communication time delays and time dilation). According to Landis, this would also lead to settlement clusters beyond which colonization will not take place.


What this means is the possibility that Earth was visited by the exo-civilization, but that the intervals that this is likely to be are quite large. From the point of view of Fermi Paradox and “fact A” this makes a lot of sense. As a species, humanity has only existed for about 200,000 years, and only the last 6,000 years have been recorded (inconsistently).

In addition, countless records and much older oral traditions have been lost over the years. If exo-civilizations were likely to show up only once in the eon, how would we find out about them today? When it comes down to it, human memories are short. Until we have existed as a species long enough, it is completely premature to say that the earth has never been visited by any other intelligent species.

In fact, the possibility that an intelligent species may have visited Earth was viewed as a serious possibility by Carl Sagan and Iosif Shklovsky in their book Intelligent Life in the Universe. In the chapter "Possible Consequences of Direct Contact" Sagan refers to an oral account of the first contact between the Tlingit in the Pacific Northwest and the French expedition led by La Perouse in 1786.

Although there was no written record of the event, the report was kept for over a century and at that point was shared with American anthropologist G.T. Emmons. While the story has been interpreted in the context of mythology and oral tradition of the Tlingit – e.g. The sailing ships were described as "huge black birds with white wings" – the type of encounter was faithfully preserved. As you have written:

“A blind old warrior had overcome his fears at the time of the encounter, boarded one of the French ships and exchanged goods with the Europeans. Despite his blindness, he argued that the ships' occupants were men. His interpretation led to an active trade between the expedition of La Perouse and the Tlingit. The oral reproduction contained sufficient information for later reconstructions of the true nature of the encounter, although the incidents were camouflaged in a mythological framework. "

This and other examples of folklore and mythology, so Sagan and Shklovskii, suggest that under certain circumstances a brief contact with an alien civilization could be recorded in a reconstructable way. Of course, Sagan and Shklovskii also emphasized that this should be treated with skepticism, in part because of the emergence of "ancient astronaut" theories in the 1970s that were completely unscientific.


In terms of weaknesses, the Aurora Hypothesis has the same low-data constraints as any other. In particular, the Hart-Tipler Conjecture and other versions of the “uniqueness hypothesis” have been criticized for being based on a fairly large assumption (“fact A”). However, it does so in a way that is still subject to assumptions, especially in the way arguments from Sagan and other "contact optimists" are taken into account.

Unfortunately, at the end of the day, astronomers don't have enough evidence to limit exoplanet habitability. While it is advisable to keep in mind that mere "Earth similarity" (or compatibility with the planet of origin of another species) does not mean that it could be colonized, nothing can be said definitively until exoplanets can be explored directly.

The Aurora Hypothesis, like its peers, is food for thought and extremely useful in this regard. As we continue to catalog "potentially habitable" planets, we cannot afford to become "colonization optimists". You know how protecting the planet emphasizes how human presence can threaten indigenous forms of life? Well that cuts both ways! Before we put boots on exoplanetary surfaces, we should be sure that it is safe to breathe the air.

We have written many interesting articles here at Universe Today on the Fermi Paradox, the Drake Equation, and the Quest for Extra Terrestrial Intelligence (SETI).

Are the aliens here? How the “big filter” could influence technical progress in space, why it would be bad to find someone else's life. The big filter, how could we find aliens? The Quest for Extra Terrestrial Intelligence (SETI) and Fraser and John Michael Godier discuss the Fermi Paradox.

Would you like to calculate the number of alien species in our galaxy? Go to the Alien Civilization Calculator!

Also check out the rest of our Beyond Fermi Paradox series:

Astronomy Cast has some interesting episodes on the subject. Here is episode 24: The Fermi Paradox: Where Are All The Aliens ?, Episode 110: The Search For Extra Terrestrial Intelligence, Episode 168: Enrico Fermi, Episode 273: Solutions To The Fermi Paradox.


  • Hart, M.H. "An explanation for the absence of aliens on Earth." Quarterly Journal of the Royal Astronomical Society, Vol. 16 (1975)
  • Tipler, F. J. "Extraterrestrial intelligent beings do not exist." Quarterly Journal of the Royal Astronomical Society, Vol. 21 (1980)
  • Sagan, C., Newman, W. I. "The solipsistic approach to extraterrestrial intelligence." Quarterly Journal of the Royal Astronomical Society, Vol. 24 (1983)
  • Brin, G. D. "The great silence – the controversy about the extraterrestrial intelligent life." Quarterly Journal of the Royal Astronomical Society, Vol. 24 (1983)
  • Jones, E. M. "Where Is Everyone?" A report on Fermi's question. Scientific and Technical Information Office Technical Reports (1985)
  • Landis, G.A. "The Fermi Paradox: An Approach Based on Percolation Theory." Journal of the British Interplanetary Society. 51, No. 5 (1993)
  • Gray, R. H. "The Fermi Paradox is neither Fermis nor a Paradox." Astrobiology. 15, No. 3 (2015)
  • Carroll-Nellenback, J. (et al.) "The Fermi Paradox and the Aurora Effect: Settlement, Expansion, and Steady States of Exo-Civilization." The Astronomical Journal, vol. 158, No. 3 (2020)

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