What Is The Rare Earth Hypothesis?

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The Rare Earth Hypothesis argues that the development of complex life on Earth, not to mention intelligence, was an incredibly improbable thing in terms of the geological and astronomical variables involved, suggesting that the galaxy is not filled with other intelligent life forms waiting to be found.

From a very young age, whether through religious dogma, familial tradition, academic study, or other cultural norms, human beings are given the idea that we are intrinsically special. Societies around the world, across time and space, seem to share this self-appointed anthropological superiority of the human beings on Earth. Organized religion has promoted the idea of a higher power bestowing life on humanity in an otherwise empty universe.

However, there are many who don’t agree. Humans have been fascinated by the idea of aliens for thousands of years, and plenty of people would argue that we’ve already been visited by them! Additionally, as we have come to better understand the scale and scope of the universe—with hundreds of billions of galaxies, each with hundreds of billions of stars—it began to seem downright arrogant to think that we were the only form of intelligent life in the universe.

you think we're alone in the universe meme

The discussion of this idea—whether we are average or extraordinary—continues to this day, but to properly understand it, we should look at the history of the debate, which includes the Fermi Paradox, the Drake Equation, and the Rare Earth Hypothesis.

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Fermi Paradox

By the middle of the 20th century, humans had developed a deep appreciation for the vastness of the universe. We understood that there were billions of stars in our Milky Way alone, and that our galaxy was just one galaxy of billions flung light-years apart among an ever-expanding universe. We realized that our star was an average one, that many stars had rocky Earth-like planets, and that with the right combination of variables, out of hundreds of billions of options, life probably wasn’t all that unique. In response to that, Enrico Fermi posed a very obvious question—Where is everybody?

Enrico Fermi the renowned Italian physicist
Enrico Fermi the renowned Italian physicist (Photo Credit : public domain/Wikimedia Commons)

If we were so unremarkable, and the universe was teeming with alien life, then why had we not come across any solid evidence of their existence? This puzzle famously became known as the Fermi Paradox, and while many experts have proposed solutions or explanations for this apparent paradox, the lack of any demonstrable proof remains frustrating! Obviously, interstellar visitors landing on Earth and introducing themselves to world leaders would clear up any uncertainty, but UFO claims always seem to have terrestrial explanations.

Aside from little green men paying a visit, astronomers and other experts have turned the full weight of technological advancement toward the stars, both watching and listening for any evidence of intelligent life. The Search for Extraterrestrial Intelligence (SETI) is a catch-all term for the countless research projects and initiatives around the globe trying to find any sign that we’re not alone.

Also Read: Will There Be Any Humans Left When Aliens Invade?

Drake Equation

About a decade after Fermi posed his enigmatic paradox, Dr. Frank Drake designed an equation that could better estimate the likely number of active and communicative civilizations spread across our galaxy, or the universe at large. This equation seems sound in its scope and the factors it includes, but some of the variables are difficult, if not impossible, to determine accurately.

Drake Equation illustration
  • N = the number of technologically advanced and communicative civilizations in the galaxy.
  • R = the average rate of star formation in the Milky Way
  • fp = the fraction of those stars with planets
  • ne = the average number of planets per solar system that can potentially support life
  • fl = the fraction of planets that could support life that actually do develop life
  • fi = the fraction of planets with life that develop intelligent life
  • fc = the fraction of civilizations that release signs of their existence into space
  • L = the length of time such civilizations release such detectable signs into space

For those reading through that list of variables and scratching their heads… you’re not alone. The final four variables included in the Drake Equation are unknown, and can only be estimated. Whether you are a skeptic or an optimist, those estimates could be very different. That’s the ironic beauty of this equation—it is not meant to be solved, but instead functions as a framework for thought.

Depending on what values are used for those final four variables, estimates for the number of civilizations can range from less than 1 in our entire galaxy to more than 100 billion across the whole universe! Without knowing the raw probabilities of certain critical variables, despite increasing our accuracy of other variables, the Drake Equation remains a useful tool, but it simply has too much inherent subjectivity, which isn’t how science works!

What Is The Rare Earth Hypothesis?

The Rare Earth Hypothesis goes against the idea that Earth is an average planet orbiting an average star in an average spiral-armed galaxy, or that intelligent life is easily duplicable in other parts of the galaxy. Contrary to the beliefs of Carl Sagan and Frank Drake, the Rare Earth Hypothesis was put forward by Peter Ward and Donald Brownlee in their book Rare Earth: Why Complex Life is Uncommon in the Universe, published in the year 2000.

In this book, they suggested that the unique characteristics and unlikely phenomena that led to Earth’s formation and the subsequent development of intelligent life are, in fact, very rare in the cosmos. They argue that the vast majority of the galaxy is an empty place, barren of life, because the majority of planets are orbiting stars that lack the appropriate temperature, stability or longevity to support and sustain life.

They believe that there are far more variables involved in the development and maintenance of intelligent life than the Drake Equation would have us believe. For that very reason, they developed an adjusted version, referred to as the Rare Earth Equation, which is defined as follows:

N = N* × fp × fpm × ne × ng × fi × fc × fl × fm × fj × fme


N = the number of communicative, active intelligent civilizations in our galaxy

N* = the number of stars in the Milky Way (estimated at 500 billion)

fp = the fraction of those stars with planets (somewhat unknown variable)

fpm = the fraction of planets that are metal-rich (unknown variable, thought to be small)

ne = the average number of planets in the star’s habitable zone (the small “Goldilocks Zone”)

ng = the number of stars in the galactic habitable zone (again, a narrow range of stars)

fi = the fraction of habitable planets where life manages to develop (this could be a large number)

fc = the fraction of planets where complex metazoans develop (small, as a unique combination of events is required, per our planet’s history)

fl = the fraction of the planet’s lifetime when complex metazoans are present (unknown, could be very small)

fm = the fraction of planets with a large moon (our moon is rare, but is incredibly important for stability and the success of life)

fj = the fraction of solar systems with planets as large or larger than Jupiter (large planets deflect asteroids with their gravitational pull, greatly decreasing the likelihood of extinction-level impacts)

fme = the fraction of planets with relatively few extinction events (a stable and somewhat isolated solar system is required)

When you begin to look at the specificity of these variables, and how small many of these fractions could likely be, the Rare Earth hypothesis begins to make a bit more sense. Based on these variables, you can see how incredibly ideal—and potentially rare—the situation on our planet happens to be.

life finds a way meme

Our metal-rich planet is located at precisely the right distance from our stable star, which also happens to be an ideal distance from the galactic center. After less than a billion years, life arose, and it took another 3 billion years for metazoans to develop. Our moon formed shortly after the planet itself did, which has stabilized the tilt of the Earth, and thus our climatic fluctuations, giving life a chance to survive. The convenient presence of Jupiter has decreased our likelihood of extinction impacts by a factor of 10,000.

While Ward and Brownlee had to make educated guesses on some of these variables, their general conclusion is that life on Earth is an incredibly precious and rare thing, something potentially shared by only a handful of other civilizations in the whole galaxy! Considering that the Milky Way may contain 500 billion stars spread across more than 100,000 light-years of space, finding those other intelligent civilizations would be the ultimate search for a needle in a haystack. In other words, the Rare Earth Hypothesis states that we haven’t encountered intelligent life because it is extremely uncommon.

Also Read: Is There A Planet More Habitable Than The Earth?

A Final Word

The question of whether we are alone in the universe is far from being answered, but there are thousands of brilliant minds dedicated to the hunt. Whether they are designing new equations to plot the likelihood of life or listening for distant radio waves from the stars, our desire to answer this eternal question is undeniable. However, until we have a breakthrough, with undeniable proof that we aren’t alone in the universe, Sagan’s “pale blue dot” is the only bastion of life in an otherwise cold and impersonal universe.

Let’s try not to screw it up.

Also Read: Most Important Astronomical Discoveries To Date

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References (click to expand)
  1. Rare Earth Hypothesis - Planetary Sciences, Inc.. planetary-science.org
  2. Are we alone in the universe? Revisiting the Drake equation. The National Aeronautics and Space Administration
  3. Forgan, D. H., & Rice, K. (2010, February 24). Numerical testing of the Rare Earth Hypothesis using Monte Carlo realization techniques. International Journal of Astrobiology. Cambridge University Press (CUP).
  4. Bounama, C., von Bloh, W., & Franck, S. (2007, October). How Rare Is Complex Life in the Milky Way?. Astrobiology. Mary Ann Liebert Inc.
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About the Author

John Staughton is a traveling writer, editor, publisher and photographer who earned his English and Integrative Biology degrees from the University of Illinois. He is the co-founder of a literary journal, Sheriff Nottingham, and the Content Director for Stain’d Arts, an arts nonprofit based in Denver. On a perpetual journey towards the idea of home, he uses words to educate, inspire, uplift and evolve.