Beyond “Fermi’s Paradox” IV: What is the Rare Earth 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 planets capable of supporting life are simply too rare.

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 high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. Seventy years later, we still haven’t answered that question, which has led to many theories as to why the “Great Silence” endures. Today, we address another, which is the possibility that life-bearing planets like Earth are just very rare.

This is what is popularly known as the “Rare Earth Hypothesis,” which argues that the emergence of life and the evolution of complexity require a combination of astrophysical and geological conditions that are simply not common in our Universe. This contradicted previously-held notions by prominent scientists and SETI researchers, who were of the opinion that Earth was typical of rocky planets located throughout the Universe.

The “pale blue dot” of Earth captured by Voyager 1 on February 14th, 1990. Credit: NASA/JPL

Copernican Principle

The assumed prevalence of intelligent life is consistent with the idea of an isotropic Universe, meaning that it’s the same in all directions on a macroscopic scale. It’s also consistent with the Copernican Principle, which argues that something is randomly-sampled, it is likely to be representative of the majority. In the realm of astronomy and cosmology, this principle argues that Earth-like planets are common in our Universe.

But what if this is not the case? What if Earth is actually not representative of the whole and is in a class reserved for very few planets? What if Earth is an outlier? What if the “Pale Blue Dot” we all know and love is even more rare and precious than we give it credit for? Given the fact that we haven’t found any evidence of extraterrestrial intelligence (ETI) in the Universe yet, wouldn’t this seem like the more plausible scenario?


The term “Rare Earth” takes its name from the book Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), by Peter Ward and Donald E. Brownlee – professors of paleontology and astronomy at the University of Washington (respectively). Both are members of the UW Astrobiology Program, and Brownlee was even the principal investigator of NASA’s Stardust asteroid sample-return mission.

As the authors describe it, the Rare Earth argument comes down to two central hypotheses: One, microbial life is common in planetary systems; and two, advanced life (animals) is rare in the Universe. When combined, these two hypotheses lead to the inevitable conclusion that Earth-like planets evolve from a series of events and circumstances that are themselves quite rare, making Earth a very special place.

This argument was in response to the inherent assumptions and biases that the authors identified in the Drake Equation (the brainchild of astronomer Frank Drake and famed astronomer/science communicator Carl Sagan), which essentially asserts that intelligent life should be plentiful. Ward and Brownlee stated that this hypothesis is certainly breathtaking, but questioned its credibility:

“The solution to the Drake Equation includes hidden assumptions that need to be examined. Most important, it assumes that once life originates on a planet, it evolves toward ever higher complexity, culminating on many planets in the development of culture. That is certainly what happened on our Earth.

Life originated here about 4 billion years ago and then evolved from single-celled organisms to multicellular creatures with tissues and organs, climaxing in animals and higher plants. Is this particular history of life—one of increasing complexity to an animal grade of evolution—an inevitable result of evolution, or even a common one? Might it, in fact, be a very rare result?”

A Question of Probability

To recap, Francis Drake shared the equation that bears his name during a meeting at Green Bank facility in 1961. The subject of this meeting was the Search for Extraterrestrial Intelligence (SETI), which was an emerging field at the time. According to Drake, the equation resulted from his attempts to create an agenda and address everything SETI researchers needed to know.

The Blue Marble image of Earth from Apollo 17. Credit: NASA

Mathematically, the equation can be expressed as follows:

N = R* x fp x ne x fl x fi x fc x L

Where N is the number of civilizations in our galaxy, R* is the average rate of star formation, fp is the fraction of stars that have planets, ne is the number of planets that can support life, fl is the number that will develop life, fi is the number that will develop intelligent life, fc is the number advanced civilizations, and L is the length of time that these civilizations would have to transmit their signals into space.

While extensive research and surveys have helped astronomers to place tighter constraints on the Drake Equation, most of its variables are still subject to a lot of guesswork and uncertainty. For example, astronomers now estimate that there are between 250 and 500 billion stars in our galaxy, which forms new stars at a rate of about three Solar masses per year.

The discovery of over 4000 extrasolar planets in the past few decades has also allowed astronomers to get a much better sense of how many stars have planets, and the number of planets that are likely to be habitable. In fact, based on Kepler data, a study conducted in 2013 estimated that there could be as many 40 billion Earth-sized planets orbiting in the habitable zones of their stars, 11 billion of which would be orbiting Sun-like stars.

Nevertheless, there is still a great deal of uncertainty in the Drake Equation, especially when it comes to the emergence of life, the rate at which life will give rise to intelligent life, and everything that follows. Of course, the equation was meant to serve as a probabilistic argument and to illustrate the types of challenges SETI researchers faced, mainly by identifying the uncertain variables.

Rare Earth Equation

Because of this, Ward and Brownlee presented a revised version of the equation near the end of their book.

N = N* x ne x fg x fp x fpm x fi x fc x fl x fm x fj x fme
  • N* is the number of stars in the Milky Way
  • ne is the average number of planets in a star’s HZ
  • fg is the fraction of stars in the galactic HZ
  • fp is the fraction of stars in the Milky Way with planets
  • fpm is the fraction of planets that are rocky
  • fi is the fraction of habitable planets where microbial life arises
  • fc is the fraction of planets where complex life evolves
  • fl is the fraction of a planet’s lifespan where complex life is present
  • fm is the fraction of habitable planets with a large moon
  • fj is the fraction of systems with large gas giants
  • fme is the fraction of planets with a low number of extinction events

As you can imagine, many of these same parameters are also subject to guesswork. But using Earth as a template and employing the Copernican Principle, it’s easy to see how it would be difficult to find planets that meet all of the above-listed criteria. In addition, Ward and Brownlee list three other factors that were peculiar to Earth and are believed to have contributed to the emergence and evolution of life.

First, there’s the presence of plate tectonics, which have been fundamental to climate stability here on Earth. Thanks to an abundance of radioactive isotopes beneath the Earth’s crust, there is sufficient heat to keep the mantle in a viscous state and drive plate tectonics. This process is what allows for carbon sequestration (in the form of carbonate rocks) and the periodic release of CO2 through volcanic activity.
Artist’s impression of what a “Snowball Earth” might look like. Credit: NASA

This has ensured a relatively stable level of CO2 in our atmosphere over time, which has helped to ensure a degree of climate stability and that average temperatures have remained within tolerable ranges. Second, Ward and Brownlee cited geological evidence that indicated that twice in our planet’s history, the Earth was very cold and covered in ice.

These “Snowball Earth” epochs occurred roughly 2.2 billion and 635 million years ago, both of which coincided with key developments in terrestrial life. For the former, the glaciation coincided with the evolution of photosynthetic life, which drastically reduced greenhouse gases in the atmosphere by metabolizing and releasing oxygen – aka. the Great Oxygenation Event (ca. 2.4 to 2.0 billion years ago).

The latter Snowball period coincided with the Cambrian Explosion (ca. 570 and 530 million years ago) which was characterized by a burst of species diversification and the appearance of almost all animal lineages that exist today. In other words, two key events in the evolution of life on Earth appear to have followed (or been associated with) a Snowball Earth period.

Third, Ward and Brownlee argued the then-popular idea that bacterial life may have evolved on Mars prior to Earth, due to the fact that it cooled earlier. Since Mars also has lower gravity, ejecta produced by asteroid impacts could have reached Earth in the form of meteorites, thus seeding Earth with life. If true, a rocky planet that doesn’t have a Mars-like planet next door would be less likely to develop life.

Artist’s impression of life forms that existed during the Cambrian Era, a time of rapid change for terrestrial species. Credit: Smithsonian Natural Museum of History


While the Rare Earth Hypothesis is appealing in a number of ways, critics have pointed out a number of flaws. For starters, thousands of exoplanets have been discovered since Ward and Brownlee shared their theory, which has allowed astronomers to get a better understanding of what kinds of planets exist out there.

For instance, of the 4,197 exoplanets that have been confirmed in 3,109 star systems, 1,456 have been rocky – 1,296 Super-Earths and 160 Earth-sized. In the case of red dwarf stars, rocky planets appear to be very common. Examples include Proxima b, the closest exoplanet to our Solar System, and the seven rocky planets of TRAPPIST-1 (three of which orbit with the star’s habitable zone).

Second, the study of exoplanets and bodies within the Solar System have shown that Ward and Brownlee were incorrect in some of their assumptions regarding plate tectonics. For instance, they claimed that there was no evidence of similar activity on bodies within the Solar System, but the New Horizons mission revealed features on Pluto and Charon (its largest moon) that are indicative of icy tectonics.

Multiple lines of evidence also exist that indicate that Mars, thought largely geologically inactive today, experienced plate tectonics in the past. This evidence includes the “Martian dichotomy,” which refers to the sharp contrast in elevation between the northern and southern hemispheres. Moons like Europa have also been found to experience subduction and renewal in their icy surfaces.

Also, it is unclear whether or not plate tectonics are necessary for life to exist in the first place. While it has played a role in the evolution of life since it began 3 billion years ago, by which time photosynthetic organisms had already emerged. Similarly, recent research has found that planets that do not have plate tectonics (aka. “stagnant-lid” planets) could retain enough heat to be habitable.

Third, it is not clear whether or not the presence of a large Moon is necessary for life to emerge on a rocky planet. Moreover, recent research has shown that the impactor that created the Moon (consistent with the Giant Impact Hypothesis) could have formed in a stable orbit at the Earth’s Lagrange Point, which would mean the existence of large moons may not be as rare as previously thought.

Another key parameter, the existence of a Jupiter-size planet in an outer system, has also come under scrutiny. In the past, astronomers believed that Jupiter’s orbit prevented large-scale impactors from reaching Earth (thereby preventing extinction events). But more recent scholarship has shown that Jupiter’s gravitational influence may have actually caused more impacts than it prevented.

On top of all that, scientists have questioned the definition of “habitable zone” in recent years, with some suggesting it could be a lot narrower than previously thought. Other research has indicated that habitable planets could also be found in longer orbits, indicating that HZs are actually wider. It’s also possible that Earth does not represent the pinnacle of habitability and there may be a class of “superhabitable” worlds.

The chemicals that made life possible on Earth may have come from another planet that collided with Earth, forming the Moon. Image Credit: Rice University
The chemicals that made life possible on Earth may have come from another planet that collided with Earth, forming the Moon. Credit: Rice University

Considerable research has also been dedicated to how our very notion of habitability is based entirely on Earth’s current geological period. At many junctures in the past, atmospheric and climatic conditions were significantly different on Earth than they are today. And yet, these conditions are believed to have been essential to the evolution of life at different stages.


Like the Drake Equation, the Fermi Paradox, and all attempts to resolve them, the Rare Earth Hypothesis is subject to uncertainty. The reason for this is simple: humanity knows of only one planet where life exists (Earth). Having only this one template severely limits us when it comes to looking for life, which could exist in a range of environments and chemical conditions.

For starters, it is a foregone conclusion that life would need water to thrive, since that is the case here on Earth. However, the study of exoplanets (particularly those orbiting red dwarf stars) have indicated that these planets could have an overabundance of water. Similarly, the presence of oxygen gas is not a guarantee that a planet has life, especially since oxygen gas is toxic to many forms of life

Using Saturn’s moon Titan as a template, some scientists have argued that methanogenic life could exist in our Universe. Extremophiles, like those that live around hydrothermal vents on the ocean floor, also indicate that life can emerge and thrive in extreme environments. The many “Ocean Worlds” that exist in our Solar System could also be an indication that rocky planets may not be the best place to look for life.

In the end, we won’t know for certain if there is life out there (and under what conditions it can exists) until we start finding some! The beauty part is, we only need to find it once in order for the Fermi Paradox to be resolved. Beyond that, every lifeform and life-bearing environment we discover will either expand our definition of life, or serve to reinforce it.

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.

And be sure to check out the rest of our Beyond Fermi’s Paradox series:


2 Replies to “Beyond “Fermi’s Paradox” IV: What is the Rare Earth Hypothesis?”

  1. Another constraint may be the angular momentum or spin of the star. Sol is a relatively slow spinning star. Slow spinning stars tend to have less magnetic activity and less violent magnetic storms. Violent magnetic storms can result not only extinction level events, planetary sterilization events, including blasting the atmosphere of a planet away, to the point of the planet becoming unable to sustain life.

  2. It’s all unknown unknowns. Life happened on Earth at marginal regions: shallow water, thermal vents, and such. It seems some randomness is helpful to life, and a deep water planet may be sterile. A tidally locked planet around a red dwarf might have a huge marginal region all around the twilight zone. The star may never get much above the horizon, so this band will be partially protected from flares. Indeed, as the planet is not wholly fixed but will wobble, some of the zone will be in ‘night’.

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