Editor’s Note: With the release last week of Christopher Nolan’s blockbuster Interstellar, dealing with the prospect of colonizing worlds beyond our planet, we are pleased to present a new series at ENV, "Exoplanets." Daniel Bakken is an engineer who teaches astronomy at the college level, and an entrepreneur in compound semiconductor crystal growth. In a series of articles he will critically examine recent claims about exoplanets beyond our solar system, asking whether our own planet Earth is a rarity, or common, in the cosmos.

Since there are so many planets in the galaxy and universe, it’s natural to assume that there must be other intelligent life forms out there. Enrico Fermi was an outstanding nuclear physicist well known for his work on the Manhattan Project. Fermi’s Paradox states that if extraterrestrial civilizations are common in our galaxy, then we should have been visited, colonized, or at least heard from them by now. His paradox is based on the astronomical timescales of the galaxy, versus the spread through the galaxy of an alien civilization, even at modest rates, once technology allows it.¹ Even though the universe and our Milky Way galaxy are vast, and even if space travel takes place at relatively slow speeds, every habitable star system in our galaxy should have been colonized by an alien civilization within about 50 million years.² Colonization times as short as 3.75 million years have even been proposed for the Milky Way.³

One of the obvious potential answers to Fermi’s Paradox is that technological races are exceedingly rare. The astronomical community has been encouraged to look for life, or at least habitable planets, in the universe for a long time, but only in recent years has the technology been gradually made available for us to start to answer these questions directly. SETI, the Search for Extra-Terrestrial Intelligence, has been operating since 1960, and has not found any reproducible signal.⁴ Although it has been characterized as difficult, nearly fifty years of searching allows us to begin to bring the upper limit down on the number of communicating alien civilizations in the galaxy. All solar type stars within 150 light years have been studied by SETI researchers and have come up dry.⁵ Yet in these searches, science isn’t the only method utilized. Where there is little to no data, conjecture and bias color the researchers’ conclusions.

Most researchers bring to their work the philosophical bias of methodological naturalism, which has become synonymous with science. They assume that the origin of life happens whenever conditions allow, and that evolutionary processes will inevitably lead to intelligence, and technology. The principle of mediocrity, which has helped science in the past, may be giving us rose-colored glasses in this search. The principle of mediocrity says that when dealing with a single sample, you must assume that it is the common case; therefore there is "nothing special about humans or the Earth."⁶ So it is assumed that Earth-like planets, life, and technological civilizations are common in the rest of the universe. The principle of mediocrity is an extension of what has become known as the Copernican Principle. Copernicus placed the Sun at the center of the solar system, instead of the Earth, apparently demoting it.

It is ironic that the Earth was not considered by the ancients as the privileged and important center of the universe, as popularly assumed. Rather, they thought the Earth was at the center because it was heavy, corruptible, and changeable. The sublunar domain was the sump, or low point of the universe, and the rest of the universe was seen as perfect. So Copernicus did not "demote" the Earth in their eyes at all, but instead made one of the perfect heavenly bodies lower, namely the Sun.⁷

An example of use of the principle of mediocrity is that most researchers assume that since life appeared on Earth very early in its history, life appears whenever conditions allow it. Despite the fact that the early Earth was very hostile, still settling down from its formation, the availability of liquid water and other chemicals from the crust of the planet and from comets and asteroids are assumed to provide enough conditions for life to occur. The extreme difficulty the origin-of-life research community has had in coming up with a robust theory explaining how this actually occurred⁸ is swept away by the researchers’ philosophical commitments to naturalism, and the principle of mediocrity. Since no non-natural forces are allowed in science, life must have arisen easily no matter what the difficulties, they say. We just don’t understand how, yet.

The new research field of astrobiology is drastically reshaping our understanding of potential extraterrestrial life, especially complex, intelligent life. This interdisciplinary field includes the questions we’re considering here, namely the search for habitable environments in space, and the origins of life on Earth. Astrobiology is starting to point to the idea that advanced life may be uncommon in the universe. If, then, the origin of life is extremely rare, it may have been totally by chance that life arose on Earth so early and so quickly. There is even the possibility that non-natural agencies are responsible, since life has the hallmarks of top-down design, if we think outside the limitations of naturalism.⁹ A recent article in PNAS (Proceedings of the National Academy of Sciences) shows that without evidence of life arising independently from Earth, we can’t assume that life is common in the universe because of its early appearance here.¹⁰

Despite the new research, the desire to discover that we aren’t alone is very strong. In both the popular and the scientific media, anything on the search for alien life is a sure attention-getter. Yet there are a growing number in the science community who are questioning this conclusion. Rare Earth: Why Complex Life Is Uncommon in the Universe, by Peter Ward and Donald Brownlee, published in 2000, was the first major book by scientists in the popular literature to genuinely question the perceived wisdom of advanced life being common in the universe.¹¹ These authors hold to the belief that life is inevitable given the appropriate conditions, yet seriously question the paradigm that technological life is abundant in the galaxy.

Observer bias is difficult to overcome because we are so used to assuming most examples will be of the common case. But we as the observer may not, in fact, be the common case. If you pick up a rock on the ground at random, it is a safe bet that it is an average kind of rock. Yet it may in fact be a large gold nugget, or a diamond to rival the Hope Diamond. So it is with the Earth, its life, and human civilization. We are immersed in a culture of science fiction, where aliens abound in the galaxy. In the majority of science-fiction media, alien technologies far surpass our own. Yet as Enrico Fermi famously asked, "Where are they?" It is a testament to the mediocrity principle that most researchers brush off this query so easily.

Another researcher who breaks the bias barrier is the author of Lucky Planet: Why Earth is Exceptional-and What That Means for Life in the Universe, astrobiologist David Waltham, who states, "Because, by definition, we conduct our research from a habitable planet, we cannot generalize our experience assuming that it is universal."¹² Waltham blames his colleagues’ disagreement with him on this point on observational bias. "Our view of what is really there has been misled by the accident of what we’re able to see."¹³ He uses the example of the night sky to illustrate this point. The vast majority of stars in the galaxy are invisible to our naked eye. Only the very close or giant stars are bright enough to see. But our view of the night sky is misleading, since we don’t see the real population of stars, only the visible bright ones. The Earth is our only example of a habitable planet, and due to the principle of mediocrity, we assume it is an average case. Waltham only hints at the quantifiable rarity of the Earth when he posits that Earth might be one in a trillion¹⁴, and says he doesn’t think there is any evidence to suggest it should be higher odds than that. This makes Earth, according to him, unique in at least the Milky Way, considering that there are only up to 200 billion planets in the galaxy.¹⁵ This probably means we are effectively alone, as intergalactic travel, and even communication, seems unlikely.

Another recent book presents the case that Earth is very rare. Alone In the Universe: Why Our Planet Is Unique, by astrophysicist and science writer John Gribbin, gives much support to the idea that Earthlike planets, at least those with technological intelligent aliens on them, are exceedingly rare.¹⁶ His reasons go beyond just the habitability factors a planet must meet. He also shows that there are many contingent events in the Earth’s history, the history of life, and in the development of technological civilization, that had to happen to bring us to this point in our existence. These rare chance events, such as the lunar formation event, the Cambrian explosion of animal life forms, and the development of science, all contribute to where we are today. It is far from probable that we and our technological civilization should be here.¹⁷ To expect Earth to be typical in the universe is overly optimistic. As Gribbin writes in the preface, we are the result of "a chain of coincidences that led to the emergence of intelligent life on Earth. And that chain has so many weak links that it may mean that, for all the proliferation of stars and planets in the Universe, as an intelligent species we may be unique."¹⁸

Recent astrobiological research now points to the realization that environments, intergalactic, galactic, planetary system, stellar, and planetary, which are stable enough to allow life to arise and develop over several billion years as it has on Earth, are so rare that few if any civilizations will actually make it to the point where they can communicate beyond their planet. New discoveries about our sun and solar system’s unique history have significant bearing on the issue of habitability. In addition, the Moon, the event that formed it, and its present existence also have important consequences for the issue of Earth’s habitability.

Next up: Let’s examine some of the conditions required for a planet to be habitable.

References Cited:

(1) Stephen Webb, If The Universe Is Teeming With Aliens…Where Is Everybody? Fifty Solutions to the Fermi Paradox And The Problem Of Extraterrestrial Life (New York: Praxis Publishing, 2002), 3.

(2) Ian Crawford, "Where Are They?," Scientific American (July 2000): 41.

(3) Webb, If The Universe Is Teeming With Aliens…Where Is Everybody?, 74.

(4) Crawford, "Where Are They?," 39.

(5) Richard A. Kerr, "E.T. Search: No Din Of Alien Chatter In Our Neighborhood," Science 303 (20 Feb 2004): 1133.

(6) David Waltham, Lucky Planet: Why Earth is Exceptional — And What That Means for Life in the Universe (New York: Basic Books, 2014), 10-11.

(7) Guillermo Gonzalez and Jay W. Richards, The Privileged Planet: How Our Place in the Cosmos is Designed for Discovery (Washington DC: Regnery Publishing, 2004), 222-227.

(8) Paul Davies, The Fifth Miracle: The Search for the Origin and Meaning of Life (New York: Simon and Schuster, 1999).

(9) Stephen C. Meyer, Signature in the Cell: DNA and the Evidence for Intelligent Design (New York: HarperOne, 2009).

(10) David S. Spiegel and Edwin L. Turner, "Bayesian analysis of the astrobiological implications of life’s early emergence on Earth" PNAS 109 No. 2 (January 10, 2012):395-400, accessed March 24, 2014, doi:10.1073/pnas.1111694108.

(11) Peter Ward and Donald Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe (New York: Copernicus, 2000).

(12) Forget, "On the Probability of Habitable Planets," 11.

(13) Waltham, Lucky Planet, 3.

(14) Ibid., 163.

(15) Swift et al, "Characterizing The Cool Kois Iv," 118.

(16) Gribbin, Alone in the Universe.

(17) Ibid.

(18) Ibid., xiii.

Image: Surface of Venus, photo taken by Soviet probe.