In a new consideration of Earth-likeness, or Earth-equivalency in astrobiologist David Grinspoon’s words, the separation between the search for life and the search for Earth-like planets is crucial. Departing from the passionate interest that we have vested in extra-terrestrial life, I move to instead focus on the how the exoplanets that we consider Earth-like compare to Earth’s other qualities. In addition to lying in their stars’ habitable zone, how do they compare in terms of mass and what kind of stars they orbit? What sorts of planets share their solar systems? The more solar systems we encounter, the more we learn how much our other planets are less like Earth. There seems to be an infinite number of ways to form a solar system.[i] After each new discovery, we re-learn that the circumstances of the Earth’s existence are rare. Most stars are significantly dimmer than our sun, and many of them orbit in binary star systems. The more information we find about the conditions of other solar systems, the more I’m convinced that conditions that surround Earth, will be found uncommon.
Even in our own solar system, potentially habitable planets fall outside the limits of our own habitable zone. Jupiter’s moons, Europa and Io, are thought to have volcanic activity and water, respectively.[ii] If we limit our points of interest to just those that fall within the habitable zone we will miss potentially habitable places. The continuously habitable zone is another obstacle to finding life and habitable planets. [iii]Over time, a star becomes more luminous, and this change in luminosity causes a change in the location of the habitable zone.[iv] Another factor is that the atmosphere of a planet can greatly affect how hot its surface is. An atmosphere that contains a high level of greenhouse gases could retain more of its parent stars’ heat even outside of the supposed habitable zone. Astronomers and astrobiologist have grappled with this problem of what planets to consider habitable. We have the choice of looking at everything because we don’t have enough data to narrow things down, or to only look for life that we would be able to recognize and measure i.e. Earth-like life.
The problem here is the language. Earth-like is a loaded term, tethered to our understanding of our planet. For us, the Earth is alive, either through the life that is on the planet, or the planet itself. [v] When we deem a planet to be Earth-like, since Earth is synonymous with life, we surround these mysterious exoplanets with all the baggage that comes with Earth. The term encourages us to imagine Earth with its richness transplanted to a far corner of the universe. It comes with a specific meaning. While scientists crafted the term for its generality (the planets are of Earth size and terrestrial), it’s interpretation is all too narrow. Once we recognize that there is unintended connotation between the intended meaning of Earth-like and the understanding of the term, we can move toward the precise language that is needed in science. Settling for Earth-sized, while a little more exact, still pushes us towards an association with Earth. When we find an Earth-sized planet, the conversation immediately goes to whether they will be habitable.
Is the Earth Earth-like? The answer should be an emphatic yes. It’s almost too obvious of a question. The question really is “Why is the Earth Earth-like?” Clearly its more than the size of our planet that determines the its qualities. But in our rush to separate the planets of interest from the terribly uninteresting exoplanets, we have brought along our Earth-bound thinking. In a field where we should be as objective as possible, we must struggle to release the weight of our subjectivity. Bayesian analysis takes our subjective knowledge and directs it towards objectivity, if we don’t pretend that we’re certain of anything. Hopefully even if our initial guess lies somewhere off center of the truth, with more information we will converge towards truth.[vi]
Water worlds, worlds that are entirely covered in a planetary-ocean don’t exist in our solar system, yet we know they can exist. However, they still capture associations with Earth, because we were also a water world, at one point in our development.[vii] When we claim associations with Earth, it gives us claims to legitimacy. It also makes the non-Earth-like planets available to be explored. In a game of six degrees of separation on a universal scale, we feel like we know these planets. While this intimate look at planets is good for making intangible objects feel more real, it also has a much graver effect. Much how we feel connected to someone who is a friend of a friend, we feel some kinship with these alien planets that have siblings here in our own solar system. This kinship makes it easier to claim ownership of these planets. Reflecting humanity’s drive to push and do, we feel like every no stone can remained unturned.
We view these alien planets as extensions of our own. A seemingly irrelevant quote that talks about the Mayan 2012 apocalyptic prediction “The 2012 commotion, Restall and Solari argue, is a testament to our continuing inability to stop viewing other societies as extensions of ourselves.” The end of the Mayan calendar in December 2012, did not mean the end of the world, instead it was a projection of the Western preoccupation with apocalyptic end times, driven by the apocalyptic books of the Christian Bible.[viii] Like always, our Earth activities don’t end when we look skywards. In our categorization of other exoplanets and solar systems, they become are a projection of our solar systems and of earth. Colonizers of the “New World” had to juggle experiencing an entirely new culture, while connecting it to familiar places. It seems that we are doing the same thing to the literal New Worlds. We can extend our touch to our cousin worlds and sister worlds out in the universe. Our attitudes directly impact how we feel entitled to other worlds. We’ve already feel a sense of ownership to the rest of the solar system, we allow ourselves to send probes to other planets, and even feel free to litter space and other planets with our debris. Do we have the right to do this?
Moving forward as technology develops, and more of these strange worlds are accessible to us, finding how earth-like these planets really are would be the true test of our language and terms. Even more is finding whether it is “fair” to impose our limited understanding onto the hypothetical conditions of our planets. What happens when we arrive at these planets, like NASA’s fictional tourists, and we see that they are only Earth-like by the loosest definitions. Imparting our conception of a “good” terrestrial planet is limiting in both our categorical knowledge and our interpretation of the data. From our limited perspective is it fair that all planets must align with the planets that happen to be in our solar-system? I mean I’m not one to insist on each exoplanet being a unique snowflake, but that imposing our view onto the planets also forces them to view them in a certain way. Seeing the exoplanets as associated with earth gives us a sense of ownership to the planet. They are free to explore without fear. Our solar system was formed under unique circumstances. When we looked towards exo-solar systems we expected them to find others like our own. Since we haven’t found a solar system like ours yet, maybe we can also hold off on trying to make the planets fit into our molds. We should ask not only is how Earth is Earth like, but is Jupiter Jupiter-like? Or Neptune, Neptune like? Moreover, can we expect all planets to fall in variations of our own nine groups. Are we lucky enough to exist in a solar system from which all the variations of planets stem? Clearly not.
The Copernican Principle allows for estimations of planets not only in our galaxy, but in the known universe. The principle was used to approximate the number of stars in the galaxy. A 2012 study of planets found using the microlensing technique, was used to roughly estimate nearly 10 billion terrestrial planets across our galaxy. In a similar manner, NASA estimated the number of stars. While it is wonderful to get an idea of the almost incomprehensible size of the universe, there’s no way to verify our results. If we accept the Copernican Principle as fact, we don’t need to think about our findings probabilistically. We think about the existence of individual objects as subject to a source of error, but not about what these things mean.
Acknowledging the limitations of our scope, we use the Copernican Principle to extrapolate our results to the rest of the universe. Nicolaus Copernicus created a heliocentric model of the solar system in 1543, which challenged the then popular Earth-centered planetary system[ix]. Maybe popular isn’t the right word; the geocentric planetary system had the backing of the powerful and influential Catholic Church. Giordano Bruno, a monk with a fiery disposition, was literally burned at the stake for claiming that there were many planets that had intelligent life. A geocentric model explained many aspects of the motion of the stars, moon, and Sun, and fed into our anthropocentric ideals (a mere coincidence that egocentric and geocentric differ by a letter switch). Copernicus shifted the Earth and humanity from the center to the sidelines. Geocentrism was convenient theologically, because humans are in a unique position to acknowledge and worship God. Why would God create us not in the starring role, when we are focus of the whole religion? And then why were the other planets not in our religious texts? (In the Catholic church, and additional snag occurs because now they would have to figure out if Jesus’ sacrifice counts for the aliens.)[x] We cannot pretend that the changing significance of Earth only affects the objective scientific world, the understanding of ourselves must be adjusted as well.
We were just like all the other planets, circling the real center of our solar system, the Sun. The departure from the special to the mundane framed a change in our understanding of the universe. In Neil deGrasse Tyson’s words, “not only is Earth not in the center of the solar system, but the solar system is not in the center of the Milky Way galaxy, and the Milky Way galaxy is not in the center of the universe.”[xi] The Copernican principle implies that no part of the universe is special or in the center. There is nothing inherently exceptional about us. Our extrapolations rest upon the assumption that Earth us not unique; scientist call this the principle of mediocrity. We infer that the areas of space we study, through being average, are representative of the greater universe. If exoplanets are common in our search of the small region of the Milky Way, then, through the Copernican Principle, they should be common in other galaxies. This is a clever way to go around the need for more complete sampling, since any sample is a perfect representation of a uniformly distributed universe.
When I first read Heavenly Errors, and Neil F. Comins described how scientists estimated of the number of stars in the universe I was amazed.[xii] Humanity, in our relentlessly creative way, had the gall to try to answer one our biggest questions. Although, the results are mind-blowing, they don’t leave much room for argument. While I am not wishing for a world of antiquity, it is undeniable that in the past, astronomical models were updated to accommodate unexpected results. When the Greek models of seven celestial spheres that revolved around Earth couldn’t predict retrograde planetary motion, we moved to planets that existed in space, but in a geocentric model. And when that wasn’t accurate enough, we adopted Ptolemaic epicycles. These models turned out to be inaccurate representations of our universe. Maybe the constancy our model signals that we’ve struck astronomical modeling gold. After all, a model that is right, is as simple as possible and won’t require massaging to make the outliers fit.
We make a big show of being concerned about the ethical considerations of contaminating another planet with Earth organizations, but we don’t have the same concern for the planet as it exists. We own the Earth, although that may rub some people the wrong way, humanity’s actions can change the planet. We have the power to shape the Earth in profound and long lasting ways, and as the ability visit other planets we must ask ourselves if this control extend to the universe. Because we control the Earth, any connections to earth we put on another planet allow us to exert the same sort of ownership to these earth like planets. And since is earth is a living planet and a living state is better (in a sense that it is worth considerations) than a nonliving one, this gives us access to all the non-living planets. If a planet is non-living no ethical consideration needs to be made. We went to the Moon and even left a human souvenir: an American flag. Now that we’ve been there done that with our lunar companion, we set our sights on Mars. NASA, and Tesla CEO, Elon Musk have hopes of getting men on Mars within a few decades, since we are convinced of the planets dead-ness it is available for our exploitation.[xiii] Earth Bias has primed our brains that Earth-like planets or planets with life are the only ones that we need to ethically consider. Is there not something worth preserving in the planets existence as it is?
This subjectivity leeching into our process of discovery and interaction into the greater world at large is just another iteration of our species extrapolating our experiences to the life that we hope is out there. There is a reason that most alien encounters in science-fiction are violent wars, they reflect the often-violent clash of cultures here on our planet.[xiv] It’s the same reason why sci-fi aliens tend to look like life here on earth.
Astrology and astronomy have often been confused, much to chagrin of professional astronomers, but I think they have and more in common than traditionally thought. Astrology assigns the positions of the planets and celestial objects special significance in determining the characteristics of humans specifically. The positon of the constellations and the planets can reveal someone’s basic personality traits. The stars have long been thought to determine the conditions of human affairs (see star-crossed lovers, “it was written in the stars”), but now we are at a point in history where the influence can be exerted in the opposite direction.
On Earth, we have unprecedented connectedness across the planet (Not only can we exert influence over the stars but we have influence across the planet). We can talk to people across the globe at any given moment. Even where technology hasn’t bridged the physical gaps, human travel, has brought animals and plants far away from their native homes. Globalization in the non-economic sense, but in terms of life, has homogenized the plant. The constant interaction of life from different places, imitated by Christopher Columbus, has homogenized things here on Earth creating the Homogocene era, where invasive “generalist” species take over the local plant and animal life all over Earth.[xv] In addition to the ecological impacts of globalization, we have cultural implications where people fight to maintain their local culture while participating in the new, usually Western culture.[xvi] How can they preserve local traditions and ecosystems while sacrificing some of their individuality to get some of the goods/luxuries/conveniences that the “globalizing” culture offers? Globalizing has his difficulties (immigration, migration, urbanization, invasive species) and right now we are figuring out what that means to the development of both our future cultural and ecological landscape.
We certainly haven’t figured that all out, which is not bad, but we are also on the verge of a new sort of globalization that will bring even more difficulties. The new globalization is a literal globalization, utilizing the synonym of globe: planets. As we move to leave Earth for other worlds, we risk repeating the same process here on Earth. But instead of having the colonized fighting against the colonizer, we have nothing. No one to ensure the preservation of the planet before we were there. In the absence of life to act in its defense, we must play the role of both parties. Although there is a human curiosity about the worlds around us that can only be satisfied by doing, we have to balance our desires with the respect to the sovereignty/dignity of the planet, regardless of Earth-likeness or not, or whether life is there or not. We do not own the other planets, like we do the Earth. The ramifications of globalizing the universe are something even more serious than the challenges that we face here on Earth.
We both extoll and mourn the successes of globalization. We enjoy having imported clothes and exotic foods, but mourn the agro-industrial complex, dead languages, and deforestation.[xvii] Understanding the challenges on creating a global civilization here at home, should make us attuned to the effects of broadening humanity’s span. But it hasn’t.
We have the unique chance here to mitigate the damage. Instead of exercising control in an outward direction but reaching towards these sister worlds, we can turn inwards and control our own behavior. Our Earth bias affects how we will treat these worlds, limited in a sense that we will never be able to understand. Everything about these alien worlds will be compared to something on Earth, and any attempts to understand them will cheapen their unique reality. We can do right by these planets, in a way that we haven’t done our own. Human history is intertwined with Earth history and before we run out to our neighbors, we have a duty to take care of own home. While Earth-bias may need adjusting when we eye other planets, it is perfectly tuned for our own planet. At the risk of sounding like an Earth nationalist, we have a duty to use the view that the planet has cultivated in us to help it. Moving in an ever onward and upward trajectory, it makes it easier to find home in Earth-adjacent territory instead of the “real” thing.
Whoever chooses to be the first Mars colonizers, will make it clear to us what planetary homesickness feels like. In what may be a common phenomenon, people will complain of feeling the sun on their un-space suited skin, the sight/sounds/ of rain, the greenery of our forests, being at home in a place where you were meant to be. Other planets may be Earth-like, but they will never be Earth.
[i] Rare Earth
[ii] Dartnell, Lewis. Astrobiology :Exploring Life in the Universe. 1st ed. New York, New York: Rosen Publishing Group, 2011.
[iii] Kasting, James F., and David Catling. “Evolution of a Habitable Planet.” Annual Review of Astronomy and Astrophysics 41, no. 1 (September 1, 2003): 429–63. doi:10.1146/annurev.astro.41.071601.170049.
[iv] “The Habitable Zone Lecture.” Accessed September 15, 2016. https://www.astro.umd.edu/~miller/teaching/astr380f09/lecture14.pdf.
[v] Grinspoon, David. Lonely Planets: The Natural Philosophy of Alien Life. First Edition. HarperCollins Publishers Inc., 2003.
[vi] Silver,Nate
[vii] Rare Earth
[viii] 1491
[ix] Scharf, Caleb
[x] Lonely Planets
[xi] https://www.nasa.gov/vision/universe/starsgalaxies/search_life_I.html
[xii] Comins, Neil F. Heavenly Errors: Misconceptions about the Real Nature of the Universe. Columbia University Press. 2003
[xiii] http://www.dailygalaxy.com/my_weblog/2017/01/destination-mars-elon-musk-and-trump-partnership-signal-nasa-shift-humans-a-multi-planet-species.html
[xiv] http://www.popsci.com/science/article/2010-04/hawking-aliens-are-out-there-and-want-our-resources%20
[xv] http://www.hcn.org/issues/341/16862
[xvi] 1493
[xvii] 1493
