Beyond “Fermi’s Paradox” XIV: What is the Aurora Hypothesis?

Welcome back to our Fermi Paradox series, where we take a look at possible resolutions to Enrico Fermi’s famous question, “Where Is Everybody?” Today, we examine the possibility that the reason for the Great Silence is that we are “early to the party”!

In 1950, Italian-American physicist Enrico Fermi sat down to lunch with some of his colleagues at the Los Alamos National Laboratory, where he had worked five years prior as part of the Manhattan Project. According to various accounts, the conversation turned to aliens and the recent spate of UFOs. Into this, Fermi issued a statement that would go down in the annals of history: “Where is everybody?

This became the basis of the Fermi Paradox, which refers to the disparity between high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. Since Fermi’s time, there have been several proposed resolutions to his question, which includes the Aurora Hypothesis that states that just because planets are habitable doesn’t mean that intelligent life can colonize there.

This hypothesis takes its name from a famous and relatively recent science fiction novel (more on that below), though the seeds of it go back many decades. At its core, the hypothesis questions the idea that an intelligent species would be capable of colonizing beyond their home star system, which effectively throws a wrench into one of the fundamental premises behind the Fermi Paradox.

This is none other than the foregone conclusion that an advanced civilization would create noticeable signs of its existence in the form of space-based infrastructure as it spread across our galaxy (or other galaxies). This naturally extends from the argument that, given the age of the Universe and the sheer number of stars and planets out there, intelligent life must have evolved many times (and on many worlds) by now.

That Darn Conjecture!

As we addressed in part II, the assumption that an advanced civilization (or several) should have already colonized a significant portion of our galaxy did not originate with Fermi himself. It was not until 1975, when astronomer Michael Hart wrote a paper titled “An Explanation for the Absence of Extraterrestrials on Earth,” that this idea truly became associated with the Fermi Paradox.

The essence of Hart’s argument was that if an ETI arose in the Milky Way galaxy in the past, it would have had ample time to develop interstellar travel and colonize nearby stars. These colonies would have given rise to similar colonization efforts over time, eventually leading the civilization to colonize the majority of our galaxy. As there is no evidence of such a civilization (Hart’s “Fact A”), Hart argued that humanity was alone.

This argument was elucidated further in 1980 by physicist and cosmologist Frank Tipler in his unambiguously-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 notion that ETIs would have developed technologies just like humanity has based on their understanding of the same scientific principles.

From this, he estimated that any advanced species that had emerged even a billion years before humanity would have been able to colonize the Milky Way several times over:

“In addition to a rocket technology comparable to our own, it seems likely that a species engaging in interstellar communication would possess a failure sophisticated computer technology… I shall therefore assume that such a species will eventually develop a self-replicating universal constructor with intelligence comparable to the human level… and such a machine combined with present-day rocket technology would make it possible to explore and/or colonize the Galaxy in less than 300 million years.“

Tipler’s contribution to this argument is the reason it bears both his and Hart’s name. Subsequent theorists who belong to what famed scientist David Brin termed the “Uniqueness Hypothesis” have built their arguments on a similar foundation. To this, Carl Sagan famously replied in a rebuttal essay (which he co-wrote with earth scientist William Newman) that “absence of evidence is not the evidence of absence.”

All we know for certain is that no advanced civilization(s) have colonized a significant amount of our galaxy; otherwise they would have been detected by now. Beyond concluding from this that humanity is alone in the Universe, which Sagan considered an “anthropocentric and self-congratulatory pronouncement,” there must be another reason for Fact A.

Interior view of a generation ship with alternating strips of livable surface and “windows” to let light in. Credit: Rick Guidice/NASA Ames Research Center

Alas, these arguments tend to attribute the “Great Silence” to reasons that are sociological (exploration and expansion are not the norms) or biological in nature (intelligent life is rare). Examples include the Rare Earth Hypothesis, the Ocean Worlds Hypothesis, or arguments based on the principle of “non-convergent evolution.” However, there’s also the possibility the biology is a factor in another sense.


The Aurora Hypothesis is named after the 2015 science fiction novel by Kim Stanley Robinson. The story centers on the crew of an interstellar generation ship that is traveling at 10% the speed of light to the Tau Ceti system to colonize a moon (named Aurora) that orbits its Tau Ceti e. Shortly after they arrive and attempt to acclimate to the local environment, colonists begin to die due to exposure to a primitive lifeform of prions.

Prions are misfolded proteins that have the ability to transmit their properties onto other variants of the same protein. Being so tiny, they eluded detection by explorer probes and the crew of the Aurora once they set down on the planet. In response, the crew is divided between those who still want to colonize the system and those who want to return to Earth (which they now believe is the only environment suitable for humans.)

The hypothesis originated with a 2019 study titled “The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion, and Steady States,” which was led by Adam Frank of the University of Rochester, Caleb Scharf – the Principle Investigator of NASA’s Nexus for Exoplanetary Systems Science (NExSS) – and researchers from Center for Exoplanets and Habitable Worlds at Penn State University, and Columbia University.

Addressing Hart’s Fact A, the researchers began by examining 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 settled systems would eventually send out their own probes and passenger ships, contributing to a “settlement front” that spread across the galaxy over time.

On top of that, they included the possibility that Earth could have been settled (or visited) in the distant past by an exo-civilization, for which no evidence remains. What was novel to their study, however, was the consideration that though a planet might be “habitable,” it may not be inherently “settleable”. As they state:

“Often there is the assumption that any planet can be terraformed to the specific needs of the settling civilization. But the idea that the purpose of probes is to build habitable settlements and that all stellar systems are viable targets for such settlements goes to the agency of an exo-civilization; in our work we therefore relax this assumption.

“In addition, some stars may host indigenous forms life, which may preclude settlement for practical or ethical reasons… This theme was explored in (spoiler alert) the novel Aurora by Kim Stanley Robinson (Robinson 2015) in which even though a world was formally habitable it was not what we would call settleable. Thus 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? Credit: ESO/M. Kornmesser

After factoring all of this into a series of simulations, they reached several conclusions. First, they concluded that the time it would take for an exo-civilization to settle the galaxy is less than (or comparable to) the present age of the Milky Way (13.5 billion years). However, when one factors the “Aurora Effect” into the equation, it creates a scenario where only certain parts of the galaxy are settled.

Adding to that the notion that the lifetime of civilizations is finite, then it appears that certain clusters of the galaxy are destined to be settled and resettled while the surrounding areas will be unsettled. Last of all, if Earth is in a region of the galaxy that doesn’t correspond to a “resettlement cluster,” it’s entirely probable that we would not have been settled or visited for a long stretch of time – as much as 1 million years.

This hypothesis is reminiscent of “Percolation Theory,” which was argued by NASA scientist Geoffrey A. Landis in his famous 1993 study. According to Landis, astrophysics imposes a maximum distance over which direct interstellar colonization is feasible (such as communication time lags and time dilation.) This would also result in settlement clusters, said Landis, beyond which colonization will not occur.


What this amounts to is the possibility that Earth has been visited by exo-civilization, but that the intervals in which this is likely to occur are quite large. From a Fermi Paradox and “Fact A” standpoint, this makes a considerable amount of sense. As a species, humanity has only existed for about 200,000 years, and only the last 6,000 years have been (inconsistently) recorded.

On top of that, countless records and much older oral traditions have been lost over time. If exo-civilizations are only likely to come around once an eon, how would we know about it today? When it comes right down to it, human memories are short. Until we have existed long enough as a species, it’s entirely premature to say that Earth has never been visited by another intelligent species.

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

Though there were no written records of the event, the account was preserved for over a century, at which point it was shared with American anthropologist G.T. Emmons. While the story was interpreted in the context of the Tlingit people’s mythology and oral tradition – ex. the sailing ships were described as “immense black birds with white wings” – the nature of the encounter was faithfully preserved. As they wrote:

“One blind old warrior had mastered his fears at the time of the encounter, had boarded one of the French ships, and exchanged goods with the Europeans. Despite his blindness, he reasoned that the occupants of the vessels were men. His interpretation led to active trade between the expedition of La Perouse and the Tlingit. The oral rendition contained sufficient information for later reconstructions of the true nature of the encounter, although of the incidents were disguised in a mythological framework.”

This and other examples of folklore and mythology, said Sagan and Shklovskii, suggests that under certain circumstances, a brief contact with an alien civilization could be recorded in a reconstructible manner. Of course, Sagan and Shklovskii also emphasized that this was to be treated with skepticism, in part because of the emergence of “ancient astronaut” theories in the 1970s that were entirely unscientific.


In terms of weaknesses, the Aurora Hypothesis is subject to the same data-poor constraints as all the others. In particular, it criticizes the Hart-Tipler Conjecture and other versions of the “Uniqueness Hypothesis” for being based on a rather large assumption (“Fact A”). However, it does so in a way that is still reliant on assumptions, primarily in the way it incorporates arguments made by Sagan and other “Contact Optimists.”

Unfortunately, at the end of the day, astronomers do not have enough evidence to place constraints on exoplanet habitability. While it is wise to consider that simply being “Earth-like” (or compatible with another species’ planet of origin) does not mean it could be settled, nothing can be said definitively until exoplanets can be explored directly.

Like its peers, the Aurora Hypothesis is food for thought and highly useful in that respect. As we continue to catalog “potentially-habitable” planets, we cannot allow ourselves to become “colonization optimists.” You know how planetary protections emphasize how a human presence can threaten indigenous lifeforms? Well, that cuts both ways! Before we put boots on exoplanetary surfaces, we better be sure it’s safe to breathe the air.

We have written many interesting articles about the Fermi Paradox, the Drake Equation, and the Search for Extraterrestrial Intelligence (SETI) here at Universe Today.

Here’s Where Are The Aliens? How The ‘Great Filter’ Could Affect Tech Advances In Space, Why Finding Alien Life Would Be Bad. The Great Filter, How Could We Find Aliens? The Search for Extraterrestrial Intelligence (SETI), and Fraser and John Michael Godier Debate the Fermi Paradox.

Want to calculate the number of extraterrestrial species in our galaxy? Head on over to the Alien Civilization Calculator!


  • Hart, M.H. “An Explanation for the Absence of Extraterrestrials 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 Solipsist Approach to Extraterrestrial Intelligence.” Quarterly Journal of the Royal Astronomical Society, Vol. 24 (1983)
  • Brin, G.D. “The Great Silence – the Controversy Concerning Extraterrestrial Intelligent Life.” Quarterly Journal of the Royal Astronomical Society, Vol. 24 (1983)
  • Jones, E.M. “‘Where Is Everybody?’ An Account of Fermi’s Question.” Office of Scientific & Technical Information Technical Reports (1985)
  • Landis, G.A. “The Fermi paradox: an approach based on percolation theory.” Journal of the British Interplanetary Society, Vol. 51, No. 5 (1993)
  • Gray, R.H. “The Fermi Paradox is Neither Fermi’s Nor a Paradox.” Astrobiology, Vol. 15, No. 3 (2015)
  • Carroll-Nellenback, J. (et al.) “The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion and Steady States.” The Astronomical Journal, vol. 158, No. 3 (2020)


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