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Interstellar: Is the Earth Rare, or Common?

Daniel Bakken


Editor’s Note: With the release this week of Christopher Nolan’s blockbuster Interstellar, dealing with the prospect of colonizing worlds beyond our planet, we are pleased to inaugurate a new series at ENV, and welcome a new contributor. 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 in the cosmos. Or as Carl Sagan and others have insisted, is the Earth common, raising expectations that intelligent life and alien civilizations may be widespread in the Milky Way and beyond?

exoplanet2.jpgWe live in an exciting age when we can begin to answer one of the most profound questions of the ages. Are we alone in the universe, or are there alien technological civilizations inhabiting other Earths out there? Now that we can detect planets beyond our solar system, we can begin to satisfy our curiosity on these matters with some solid evidence. These recently discovered planets are referred to as exoplanets. Spacecraft designed for this purpose have generated a plethora of potential exoplanets, and the continuation of ground-based data gathering will continue to give precision to these answers. Although there are many other planets in the universe, the commonly held assumption, that there are many other Earths out there with technological alien civilizations on them, appears increasingly remote as we examine the current state of research. In particular, there are enormous challenges to finding a planet with an environment and climate stable enough to support advanced life over the required billions of years.

What percentage of exoplanets are suitable for life to begin and develop a technologically advanced civilization? We are going to place particular emphasis on long-term habitability, which is probably necessary for life to first modify its environment so we can detect it remotely, and second, to evolve to reach a technological level allowing the use of radio signal which could be detected from Earth, as estimated in the Drake equation. This equation was developed by astrophysicist Frank Drake in the 1960s to estimate how many other technological civilizations may be inhabiting the Milky Way galaxy.1 With only our planet as a viable example, addressing this question requires enormous scientific extrapolations and some trust in purely theoretical studies. Yet even when it was developed, Drake and others began trying to estimate the solution to this equation. The Drake equation is:

N = N* x fs x fp x ne x fi x fc x fl

N is the number of civilizations beyond Earth that might be able to communicate with in the Milky Way today. N* is the number of stars in the Milky Way galaxy, fs is the fraction of stars like the Sun, fp, the fraction of those stars that have planets, ne, the number of those stars that lie in their parent stars’ habitable zone, fi, the incidence of those planets on which life arises, fc, the fraction of those in which civilization develops, and fi, the lifetime of those civilizations.2 It is easy to see that even more than fifty years after this equation was formulated, the values of many of these factors continue to be little better than guesses, and would be greatly affected by one’s philosophical assumptions. An enthusiastic supporter of extra-terrestrial life, Carl Sagan in the 1970s estimated the number of technological civilizations in our galaxy at one million.1

The last factor, the lifetime of technological civilizations, is of course unknown, yet John Gribbin, author of Alone in the Universe,4 relates an interesting point. He discusses Michael Shermer’s realization that most people view human technological civilization as a homogenous whole. Yet Shermer shows that there have been many fits and starts to our technological civilization, with only one Earth-based civilization achieving the ability to build rockets and radios. Shermer estimates that about sixty civilizations rose and fell before developing sophisticated technology, such as the Sumerians, Babylonians, Egyptians, and the Greek and Roman empires. He calculates an average effective lifetime for each of these civilizations of only 420.6 years, the more recent ones dropping to 304.5 years.5 Yet the kind of technological civilization predicted by the Drake equation has only arisen once on our planet. Shermer, in his calculation for N, using otherwise very optimistic values for the other factors, concludes there would only be at most three concurrent radio-transmitting civilizations in the Milky Way.6

There is another important question to answer in conjunction with the issue we’re addressing. What makes Earth capable of supporting life? This is more complex than it may seem at first. Many factors contribute to Earth’s habitability, even more so if we are contemplating more complex forms of life such as humans. This goes beyond mere intelligence to the environmental conditions that allow the development of a technologically advanced civilization. These are, after all, what most think of when we ask whether we are alone.

To be able to answer the question "Is there anybody out there" at least without a direct message, we need to explore many different disciplines to ascertain the likelihood of alien life, potential habitat, and environmental constraints on the ability to develop technology. The question that can potentially be answered becomes "What is the probability that there are other planets with the capacity to harbor complex intelligent life that can also develop technology?" These questions cut across many disciplines, including biology, astronomy, and statistics. We look carefully at our own solar system, study other planetary systems, and estimate statistically how common or rare the circumstances that led to technological civilization on Earth are.

Planets, especially those small enough to have a solid or liquid surface, should be abundant in the galaxy. Researchers in 2010 claimed that "23 percent of stars harbor a close-in Earth-mass planet (ranging from 0.5 to 2.0 Earth masses.)"7 A newer study in 2013 calculated that 50 percent or more of M dwarf stars should have terrestrial planets (>1-10 Earth masses and orbital periods between 10-100 days).8 Kepler satellite telescope researcher William Borucki and his team used Kepler data to estimate that the frequency of stars with planets with a diameter <2 times Earth’s diameter and with an orbital period less than 50 days is 13 percent, possibly significantly higher.9 There may be as many as 100-200 billion planets in the Milky Way, according to recent research.10

Next up for consideration: Fermi’s Paradox.

References Cited:

(1) John Gribbin, Alone in the Universe: Why our Planet is Unique (New Jersey: John Wiley and Sons, 2011), 35.

(2) Gribbin, Alone in the Universe, 34-35.

(3) James Kasting, How To Find A Habitable Planet (Princeton: Princeton University Press, 2010), 10.

(4) Gribbin, Alone in the Universe.

(5) Ibid., 35-36.

(6) Ibid.

(7) Francois Forget, "On the Probability of Habitable Planets," International Journal of Astrobiology 12, no. 3 (July 2013): 179, accessed March 12, 2014,

(8) Xavier Bonfils et al, "Prized results from HARPS. Low-mass/Habitable/Transiting Planets Orbiting M Dwarfts," Hot Planets and Cool Stars, Garching, Germany, EPJ Web of Conferences, 47, id.05004 (April 2013), accessed March 14, 2014, DOI: 10.1051/epjconf/201347050042.

(9) Forget, "On The Probability of Habitable Planets," 2.

(10) Jonathan Swift et al, "Characterizing the Cool Kois Iv: Kepler-32 As A Prototype For The Formation Of Compact Planetary Systems Throughout The Galaxy," Astrophysical Journal 764, No. 1 (Feb 2013):105-119.

Image credit: NASA/JPL-Caltech

Daniel Bakken



Continuing SeriesExoplanetsScience