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1 Introduction

Humans have lived with objects of the natural world for hundreds of thousands of years, and their understandings of these objects have changed as new worldviews and knowledge systems have become dominant. Few objects have changed as dramatically as the planets, which not only transformed from wandering lights in the night sky into other worlds, but also found themselves enlisted in arguments about the material substance of the universe, the history and evolution of the Earth and solar system, the origin and extent of life, and the possible futures of humankind. The history of human understanding of the planets, though discontinuous across time and geography, offers a window into changes and variations in how people in different cultures and time periods have viewed themselves and their relationship to their own world. This essay offers a brief history of human ideas about the planets. The author considers archeological speculations about the long prehistory of planetary observation, presents some examples that illustrate the prevalence in the ancient world of astronomical-astrological links between the planets and governance, appreciates the long medieval Islamic and Christian intellectual preoccupation with the Aristotelian-Ptolemaic world system, follows the slow transformation of the Earth into a planet and the planets into worlds in the seventeenth and eighteenth centuries, examines the natural histories and cosmic epics that gave the planets a common origin and evolutionary path, and concludes with our present era of robotic exploration of our own solar system and the perhaps countless planetary systems beyond it. In each section, the author also discusses relevant changes in historical approaches to planetary observation.

2 Prehistoric Humans and the Planets

We know nothing at all about the first human observations of the planets, let alone how our prehistoric ancestors understood their motions. If the planets can be said to have been “discovered,” then their moment of discovery is buried irrecoverably deep in the past. Archaeology tells us that Homo sapiens have existed for at least 300,000 years, and we don’t know when, where, or at how many times and in however many places they may have noticed the planets moving in the night sky first over the African continent, and later around the world. For that matter, we don’t know if the various other human species who coexisted with Homo sapiens, or who preceded them for two million years, noticed the planets moving – the present wisdom is that archaic humans such as Neanderthals were intelligent and lived socially complex lives from very early on (Harari 2018; Graeber and Wengrow 2021). For all we know, Homo sapiens were recipients as well as originators of prehistoric planetary knowledge. Since we only have written records representing the past 5500 years, this leaves a vast amount of time during which humans not so different from the ones who have written this book might have been observing and developing ideas about the planets and passing these ideas on via oral tradition and even artifacts that mostly no longer exist. It therefore makes little sense to talk about the “discovery” of the planets (at least those that can be seen with the unaided eye), but instead to draw attention to their ubiquity and endurance in human cultures around the world.

We can’t say much about the practices or purposes of early humans regarding planetary observation. We can say that their reasons for observing the planets and the questions they brought to these observations were almost certainly very different from ours. Early human communities may have observed the motions of the planets, in addition to the Sun and Moon, as ways of tracking seasonal changes, predicting animal migration patterns, and perhaps to determine the times of year when multiple groups would gather together for communal events. Archaeologists have found calendars of lunar cycles carved into bone that date back at least 34,000 years (Aveni 1995, 23). Archaeoastronomers believe that the architects of Stonehenge in England (~3000 BCE) and Nabta Playa in Southern Egypt (5000–4000 BCE) designed these megalithic structures to unite the social lives of community members with celestial events. Earlier literature attempted erroneously to find elements of modern astronomical knowledge or practice in these structures, describing them as “ancient observatories” used to predict eclipses or the movements of the planets. More recent work insists that prehistoric peoples gathered, socialized, and perhaps worshipped in these spaces designed to celebrate celestial motions and alignments. Anthony Aveni suggests that these were “sacred” observatories: “consecrated space[s] for watching the sky” in which “cosmic encounters were celebrated because they served to call people together to conduct rites to their gods” (Aveni 1997, 91). If these were spaces of religious, social, and perhaps even political significance, facilitating the maintenance of communal ties and collective identity, their removal from the world of objects and relationships in which they once participated makes them now seem mysterious wonders of a bygone world.

For these reasons, we don’t find much history of science literature treating prehistoric astronomy. We do, however, find a wealth of literature from archaeoastronomers who have attempted to reconstruct a history from what scant archaeological materials do exist, combined with computer technologies that allow them to replay cosmic occurrences in different locations at different times. One area that is ripe for historical analysis is the history of these contemporary practices and their knowledge claims about the past. In many instances, the answers we find in the deep past reflect more the questions and methods researchers bring to them than the prehistoric peoples we want to interrogate (Villalobos and Barnhart 2014).

3 The Planets in Ancient Cultures

By the time the first literate cultures emerged, knowledge about the night sky, its changes, and the connection of celestial events to natural and social occurrences was probably well established. We find this knowledge in creation stories as well as the astronomy-astrology practiced by cultures around the world. For example, the Popol Vuh of the ancient Maya not only recounts the creation of the “sky-earth” (universe) and the exploits of the hero twins Hunahpu and Xbalanque, but also tells how these and other gods and demigods came to reside in the night sky and how the motions of the Sun, Moon, and planets reenact their exploits (Tedlock 1996; Bazzett 2018). Monuments throughout the ancient Mayan world record the dates on which communities reenacted portions of the Popol Vuh in annual rituals that legitimated the divine rule of Mayan kings (Freidel et al. 2001; Aldana 2010), and archaeoastronomers believe that these reenactments were timed and city architecture planned so that planetary motions could participate in the renewal of the city’s religious charter. It is primarily for this reason, argues historian of science Gerardo Aldana, that royal families patronized the production of astronomical knowledge (Aldana 2010). The few Mayan codices that survive (of the thousands that existed precontact) speak to a well-developed practice of calendar-making that was useful not only in timing ritual events, but also as an agricultural almanac (Schele and Miller 1986; Aveni et al. 2003). Aldana reminds us to consider the context in which the Maya scribes worked rather than impose our own understandings and assumptions about the value of astronomical knowledge to agrarian societies in any attempt to reconstruct their intellectual project (Aldana 2022).

Royal patronage of Mayan astronomy created an expert class of astronomer-astrologer scribes. The knowledge that these scribes produced was meant to be understood within the context of an animate universe maintained by the actions of humans and gods. It was not esoteric knowledge for understanding the world, but for participating in it. These qualities of ancient astronomy-astrology are shared by many cultures around the world, even if the specific beliefs about the universe and the place of humans within it differ. In many ancient cultures, planetary motions and other celestial events were understood to yield information vital for good governance. In ancient China, for example, a creation story explained how the heavens and Earth became separate from each other (Major 1993). Only a heavenly monarch (thearch) attuned to the natural order of the universe could maintain harmony between these now separate parts of the world (Tseng 2011). A legitimate emperor acted as mediator between human society and the universe (Xiaochun 2000). An elite class of scribes practicing tian wen (heavenly writing) provided astrological guidance to the emperor that allowed him to rule in accordance with the universe (Sivin 2009; Cullen 2017). As with the Maya, the necessity of astrological information in the service of the state led to the patronage of astronomy-astrology.

The ancient culture that has long fascinated historians of science interested in the origins of astronomy is that of the Babylonian empire. Here again we see a culture who believed they lived in a cosmic state, in which their social world was connected to the celestial world, and in which knowledge of the heavens was necessary for good governance. The Babylonians believed that the appearance and motions of the planets delivered messages from their gods. Again, an elite class of astronomer-astrologer priests supported by royal patronage observed the heavens in search of good and ill omens of events to come. This information was necessary for state planning, and also for determining what ritual practices were necessary to communicate back to the gods (Rochberg 2004). Unlike later understandings of astrology, in which planetary influence causes earthly occurrences, Babylonian astrology understood planetary omens as warnings of what would happen if appropriate actions were not taken (Rochberg 2010).

Historians of science such as Noel Swerdlow have drawn attention to the mathematical instruments the Babylonian priests developed for predicting planetary motion and in them seen how “in the belief in omens lies the birth of science” (Swerdlow 1998, 5). The historian of astrology Nicolas Campion likewise insists that what happened in ancient Mesopotamia was “a small scientific revolution” (Campion 2008, 75). Historians and anthropologists have deciphered the roughly 7000 omens found in the 70 tablets of the Enuma Anu Enlil, as well as the reports the astrologer scribes sent to their royal patrons to piece together the complex system the scribes used to divine and predict. While the mathematics they employed does resemble science in many ways, it was employed for purposes that most today would consider incompatible with science. However, both the argument that this was science and the modern prejudice against it are indicative of the problem of demarcating science. On the one hand, searching for evidence of science in earlier eras rather than understanding what practitioners and their patrons understood themselves to be doing yields a skewed representation of ancient knowledge production. However, it is equally limiting to attempt to uphold a modern definition of science and to dismiss or diminish anything from the past (or present) that does not resemble it (Pigliucci and Boudry 2013). As one of the recognized founders of the history of science, George Sarton wrote in his own treatise on ancient science, “It is better to leave out for the nonce the consideration of science as science, and to consider only definite problems and their solutions” (Sarton 2011). Following the planets through time and geography does not help us to determine at what point science began, but it does help us to see how and in what contexts many of the practices we now associate with different forms of science emerged.

4 Greek Philosophers Shape the Cosmos

Practices of observing, recording, and attempting to predict the motions of the planets seem to have emerged in the context of cosmic states. So far, at least in the examples given above, this did not equate to any investigation of the structure or nature of the planets or the universe in which they moved. This was not a failure of these ancient projects, it simply wasn’t the goal. One of the earliest places where we see a culture engaged with the structure and materiality of the universe is ancient Greece. In the emerging tradition of Greek natural philosophy, the planets were understood not as signs or omens, but as physical objects that affected specific events and both collective and individual fates. This idea emerged in the writings of Greek philosophers still well known today (partly because of their influence over the following centuries, and partly because of the enduring interest of modern historians). Plato’s Timaeus imagined a new creation story based on the observed nature of the universe. He posited a living universe animated by a soul, constructed by a divine craftsman. Its movements are perfect and orderly. Humans, made from the imperfect residue of the universe, can observe the perfect heavens and aspire to restore themselves more nearly to perfection. The works of Plato’s student Aristotle established the four corruptible sublunar elements – Earth, water, air, and fire – and proposed a fifth element with a separate physics, the aether, that comprised the heavens. Plato and Aristotle introduced a new geometrical structure to the universe. The planets and the fixed stars all moved around the Earth in perfect circular motion (Lindberg 2008).

Plato and Aristotle provided the philosophical underpinnings of an understanding of the planets that would endure for centuries, but it was the Roman astronomer, Claudius Ptolemy, in the work that became known as the Almagest, who gathered together the astronomical and astrological tools that would be employed by those who followed (Gingerich 1993; Ptolemy 1998; Neugebauer 1975). Ptolemy’s work was no mere synthesis of earlier Greek ideas and practices; he benefited from increased contact with other cultures thanks to Alexander the Great’s imperial ambitions. Ptolemy wrote his works from the Roman Egyptian city of Alexandria, where he enjoyed the city’s museum and libraries and the Greek, Egyptian, and Babylonian knowledge they contained. Ptolemy’s system, while not perfect, was mathematically more sophisticated than previous approaches and did succeed in making planetary astronomy predictive while preserving the quality of uniform circular motion. Thus, his mathematical model could be justified through existing arguments from Greek natural philosophy (Evans 1998). Ptolemy’s other great contribution to the history of planetary astronomy was a four-book compendium on astrology, traditionally called the Tetrabiblos. These texts not only presented those aspects today associated with astrology – the influence of planets and zodiac signs on individuals (i.e., horoscopes) – but also addressed how the planets affected all aspects of the world, including the people, animals, and things that would be found in different regions of the world, only a small portion of which was believed to be inhabited. The Almagest and the Tetrabiblos had profound effects on understandings of the planets for centuries.

5 Planets and the Medieval Monotheist World

To the medieval natural philosopher, the planets – their positions, qualities, and movements through the zodiac – were of paramount importance. The planets participated in an intricate “machina mundi” (world machine) that structured the medieval European worldview. The Muslim, Jewish, or Christian God was the ultimate cause of everything within this world machine, and God’s will was enacted through the movements of the heavens. All of creation was connected. To some extent, these connections can be understood mechanically – in a world built from nested spheres, the movement of one sphere affected that of the others. But in other respects, the connections through which one thing affected another were occult; as much as the universe was a machine, it was also a tapestry of interwoven influences – relationships that were based on immaterial connections, attractions, and similarities. Natural philosophers adopted an Aristotelian-Ptolemaic worldview, based on the physics and elements of Aristotle and the methods, observations, and astrology of Ptolemy. They understood all change in the sublunar world to be due to the effects of the movements of the stars and planets. As the fourteenth-century Parisian scholar Themon Judaeus wrote, “every natural power of this inferior sensible world is governed by the heavens” (Grant 1997, 162). Left to their own devices, without the planets to induce birth, growth, maturation, degeneration, death, and decay, the elements would separate and all complex forms would cease to be. As the Islamic natural philosopher Ibn Rushd (Averroes) observed, “if celestial motion were destroyed, the motion of all inferior beings would be destroyed and so also would the world” (Grant 1994, 588).

Each planet had different effects on the world, and these effects varied depending on in which sign it was observed and what other planets were nearby. Why were the planets all different? Certainly, they each behaved differently. Mercury and Venus seemed tied to the Sun in their motions, with Venus shining bright sometimes in the early morning and other times in the evening. Mars could change in both brightness and color, from the color of wheat to that of blood, and these changes often occurred right before the planet seemed briefly to reverse direction in its path among the fixed stars. Jupiter and Saturn moved more slowly and steadily in the sky, but even they occasionally seemed to change direction. These visible differences no doubt informed speculation about the differences of the planets, although how they inspired early thinkers to associate different qualities with them is lost to time. But medieval natural philosophers also followed Ptolemy in inferring different effects based on what they knew of the Sun and Moon, both of which were also considered to be planets. The Sun was the example par excellence of planetary influence. While the Earth was the center of the universe, the Sun was the center of the heavens, and the most noble planet. It supplied heat and light. Its movement through the ecliptic brought the seasons. The Moon not only appeared different to the Sun, but it also caused different changes – its movements, phases, and appearance brought the tides, marked the times for planting, harvest, and religious ceremonies. It followed that the other five planets would also have differing qualities and influences (Grant 1994).

With regard to the history of the earth sciences, planetary knowledge was essential to understanding the characteristics of peoples around the world, as well as the nature of the lands they inhabited. These influences were understood to reach deep into the Earth’s interior, producing different metals and precious stones, or bringing forth vapors that could corrupt the air (in this way they could be blamed for bringing forth diseases and plague) (Wey Gómez 2008; Horrox 1994). The planets were also understood to affect the airs and waters directly, producing different weather events and defining regional climates. A tradition of astrological weather forecasting developed during this time period and persisted, in altered form, in modern almanacs (Fleming 2005; Lawrence-Mathers 2019).

In addition to affecting change in the sublunar world, the planets were also understood to affect the human body. Planetary knowledge belonged as much to the physicians as it did to the astronomers. Just as the Moon influenced the tides, it was also believed to influence the fluid humors of the body, as defined by the ancient Greek physician Hippocrates and more recent Islamic physicians such as Ibn Sina (Avicenna). The four humors corresponded to the four sublunar elements, and the body was conceptualized as a microcosm of the universe. Each planet had its own associations with the different senses and body parts. Planetary positions could be used to diagnose illness, and to determine when and what remedy to apply. They could also enhance or diminish the efficacy of various cures, as well as the medical competence of the physician (Isserles 2017; Geller 2014).

6 Finding Progress in Medieval Planetary Astronomy

Historians of modern science once mistakenly accused medieval natural philosophers of blindly following ancient authorities rather than embarking on their own investigations of nature. The Middle Ages were presented as a time of intellectual stagnation, in which nothing new was achieved. The period was presented as a foil to the “Scientific Revolution” that swept away dogmatic commitment to the Aristotelian-Ptolemaic worldview (Kuhn 1957, 1996; Lindberg and Westman 1990). More recent scholarship, however, has considered medieval scholarship on its own terms and has found it to be contiguous with the rise of modern science (Grant 1996; Falk 2020). The ancient texts coveted by scholars in the Islamic and Christian worlds during the medieval period were the subjects of intense scrutiny, criticism, and improvement, and this work lies at the center of at least two great renaissances of interest in the structure and purpose of the heavens. First in the Islamic world, the Abbasid dynasty promoted the cultivation of different types of learning, including astronomy (Kraemer 1986). A. I. Sabra has described this period as a “cultural explosion” featuring the translation, assimilation, and naturalization of ancient science and philosophy (Sabra 1987, 1996). This included the construction of libraries and centers of learning, and an ambitious project that sought to acquire manuscripts from around the known world, and employed Muslim, Persian, Jewish, and Christian scholars to translate and make sense of the “sciences of the ancients” (Goldstein 1986, 2001).

This work also involved empirical observation and the development of mathematical tools and instruments, such as refining the African astrolabe and developing the equatorium (Goldstein 1986; Morrison 2013). Historians have only barely addressed the importance of the Maragheh observatory and the associated “Maragheh school” in which the founder Nasir al-Din al-Tusi and other prominent astronomers of medieval Islam worked to improve Ptolemaic planetary theory through the compilation of more extensive astronomical tables (zij) and new mathematical models of planetary motion. They made important new theoretical insights in planetary theory in their studies of the discrepancies found in Ptolemy’s Almagest and his Planetary Hypotheses. The contributions of these scholars, as well as earlier work by al-Battani and Ibn Sina would continue to influence astronomy, astrology, and medicine through the seventeenth century. Interest in the Maragheh astronomers has primarily been for their place in the history of mathematical astronomy and its influence on the development of the Copernican system (Neugebauer 1975; Swerdlow and Neugebauer 1984; Morrison 2014). Islamic natural philosophers have otherwise been relatively neglected compared to their European peers and successors, even as Islamic scholars like Ibn Sina took the first steps toward bringing the geocentric model of the cosmos in line with the new monotheism, considering the planetary influences as a natural means through which God controlled occurrences on Earth (Morrison 2013). New scholarship promises to bring renewed attention to medieval Islamic science and medicine (Küçük 2019; Ragab 2018; al-Musawi 2015). The reception of these Greek and Arabic texts in Christian Europe beginning in Spain at the end of the eleventh century initiated a second great renaissance of learning, at the center of which were planetary astronomy and astrology (Ryan 2011).

7 Decentering the Earth

We are now almost up to the point where most histories of modern planetary physics begin. This period, long known as the Scientific Revolution, is described as the time during which science became more empirical, experimental, mathematical, and untethered to ancient or religious authority. At the center of this revolution is Copernicus’s heliocentric universe, set forward in his 1543 De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), which removed the Earth from the center and made it move like a planet in orbit around the Sun (Kuhn 1957). This work did not immediately unite terrestrial and planetary physics; the heavens were still spherical, and planetary motion was still perfectly circular. Nor did it do away with the idea of planetary influence, and scholars have in fact suggested that Copernicus’s work should be understood as engaged in the defense of astrology (Westman 2011, 2016). Copernicus’s work did speak to a generally shared consensus among astronomers that the Ptolemaic system with all its modifications was becoming increasingly cumbersome. Tycho Brahe celebrated Copernicus’s mathematical approach to planetary motion, but did not believe that the Earth could move – it was too heavy, and he had never seen any evidence in the relative positions of the fixed stars that indicated that the Earth was moving, or that the sphere of the fixed stars could be far enough away that no parallax would be observed (Blair 1990). Brahe proposed a compromise that combined elements of the geocentric and heliocentric models and preserved Aristotle’s distinction between the heavens and the sublunar world – the planets orbited the Sun in a heaven that was fluid, not crystalline, while the Sun, Moon, and fixed stars orbited the central Earth. What became known as the Tychonic system coexisted with the Ptolemaic and Copernican conceptions of the universe. For many astronomers, such as Christopher Clavius, neither Ptolemy nor his rivals knew the true structure of the universe (Lattis 1994).

Our view of this period as revolutionary is influenced not only by the historical literature’s focus on prominent “Copernicans” such as Galileo Galilei and Johannes Kepler, but also on the heroic treatment these figures received. While once depicted almost as modern scientists ahead of their time, more recent treatments of these figures have placed them in their proper contexts (Biagioli 1994; Heilbron 2012; Voelkel 2001). Galileo did introduce the telescope to astronomy, his observations of the Moon and planets did imply that the planets were similar to the Earth, and he did argue in his famous Dialogue Concerning the Two Chief World Systems that the planets were “world globes” perhaps subject to the same changes experienced by the Earth. But Galileo struggled to explain how such worlds would orbit a central Sun. His correspondence demonstrates that he did try to come up with an explanation as early as 1615. It was an explanation that drew upon astrological notions of the Sun’s occult influence, passages from the Bible, and his observations of sunspots: The movement of the sunspots demonstrated that the Sun rotated; perhaps they were also evidence that the Sun was consuming a power bestowed upon it from the fixed stars; perhaps this power then emanated from the Sun in a “spirited, tenuous, and fast substance” and penetrated everything in the universe, causing the celestial objects to rotate around the center; and perhaps this entire system was what was spoken of in Psalm 19 (Finocchiaro 1989, 65). This was by no means the type of modern explanation we today associate with Galileo and modern science, but one appropriate to an astronomer educated in the scholastic tradition. Only later would Galileo’s planetary observations and Kepler’s laws of planetary motion be divorced from Aristotelean logic and neo-Platonic geometry and incorporated into a new physical explanation of the planets. The transformation of planets into worlds and the Earth into a planet was a slow one.

8 A Family Resemblance

New philosophies and ultimately new mathematics emerged in the Enlightenment that supported a family relationship between the planets. Rene Descartes put forward one of the earliest fully mechanical descriptions of the universe that described how the planets, including the Earth, might all be formed from the same matter. Descartes’s universe was a plenum filled with matter, some so small as to be imperceptible, and held the solar system together in a vortex of matter in motion. God no longer enacted his will through the motion of the planets, but instead created the universe and acted only as the first cause of its perpetual motion (Westfall 1977). Such a model lent itself to imagining other planetary systems orbiting other stars, each a vortex. And it also lent itself to speculation about life on other planets throughout a larger universe. Bernard Le Bovier de Fontenelle in France and Christiaan Huygens in the Netherlands – early champions in the extraterrestrial life debate – both suggested that the Cartesian vortices implied innumerable such worlds, and gave us no reason to doubt their inhabitation (Dick 1996; Crowe 2012).

It was the English physicist, mathematician, and alchemist Isaac Newton who provided the mechanical philosophy that ultimately came to define modern science. To matter and motion, Newton proposed adding a third category – natural forces. The mathematical description of the universal force of gravity in his 1687 Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) was Newton’s landmark achievement. Newton’s forces acted at a distance, without direct contact, which eliminated the need for a plenum but also roused the skepticism of mechanists who saw this as a return to the influences and occult forces of Renaissance natural philosophy (Henry 2020). In Newton’s own view, he had replaced the plenum with God’s sensorium, in which His laws could act universally without interruption across time and space (Brooke 1991). God had likewise bestowed the qualities of matter (including gravity) at the time of creation, a belief historians have tied as much to Newton’s alchemy as to his Christianity (Westfall 1993; Henry 2020). Brahe had objected to the Earth’s motion in part due to its weight. Newton’s universal law of gravity allowed the Earth and other planets to move precisely because of their mass and their resulting gravitational relationship to the Sun.

Enlightenment geologists in search of a comprehensive “theory of the Earth” began to take seriously its history as a planet and its relationship to the other planets in the solar system. The Earth was a member of a family of planets, and it was susceptible to universal natural forces. It had originated and developed over time based on these forces. Benoît de Maillet’s posthumously published Telliamed (1748) was one of the first such theories. Descartes had proposed that the Earth had once been a global ocean whose waters had slowly been stripped by the motion of the vortex. Maillet saw evidence for this in the sedimentary rocks containing fossilized marine shells he had seen on mountains and deserts. This gradual drying of the Earth through natural planetary processes had created a long and ongoing sequence of change in which surface features were first deposited and then uncovered as the sea level lowered (Taylor 1998).

Georges-Louis Leclerc, Comte de Buffon published his own theory of the Earth during this same period. Newton’s theory of gravity, unlike Descartes’s plenum and vortices, was agnostic about whether planetary systems would be common or rare. Buffon proposed that the Earth and planets might have originated not due to beneficent design, but by cosmic accident. A comet might have grazed the Sun, its collision producing hot globules of solar matter that then congealed into planets. While Buffon did not believe that this was the true history of the solar system, he did believe that his theory demonstrated that the uniformities of the solar system could be accounted for by invoking a single natural cause (Numbers 1977). Buffon’s hypothetical story of solar system formation made all the planets the results of the same event, and all subject to the same directional process. Each planet had to cool from its initially molten state. After the crust solidified, great deluges created an ocean so deep that all the Earth’s surface was under water, with the global ocean eventually retreating to reveal the continents. As with Maillet, this global ocean explained the presence of sedimentary rocks on mountain tops. Buffon was also interested in fossils, and the Earth’s long period of cooling and drying out was his way of explaining why fossils resembling the bones of tropical animals could be found in northern latitudes (Bowler 2009).

9 Nebular Hypotheses and New Planets

Other thinkers elaborated on Buffon’s idea of a single original cause for the regular motions of the solar system. Pierre-Simon, marquis de Laplace imagined that the planets all orbited in the same direction and in roughly the same plane because they had all originally been part of a single rotating body. The Sun’s atmosphere might once have been hotter, extending as far as the distant orbit of Saturn, and rotating along with the Sun. As the atmosphere cooled, it contracted and began to rotate faster, throwing off rings that then condensed into planets. As in Buffon’s theory, the planets began as hot molten spheres before a cooling process shaped and transformed them over time (Brush 1987, 1996b).

The work of the German-born British astronomer William Herschel and his sister Caroline introduced a new idea to solar system formation. The Herschels set out to identify and classify the different types of nebulae they observed to understand how a gravity-driven process condensed clouds of nebular fluid into stars, which then attracted additional nebular material to produce planets, and then ultimately exploded to provide the material for new stars and planetary systems (Schaffer 1980a, b). William Herschel also became an authority on the planets due to his telescopic discovery of the planet Uranus in 1781. Herschel’s new planet was a surprise discovery but it conformed to predictions about the distances between planetary orbits – a mathematical expression known as the Titius-Bode law. This seeming confirmation of the law set astronomers looking for a planet between Mars and Jupiter, where a planet was predicted but none observed. In 1801 they discovered Ceres and what became known as the asteroid belt. When Neptune was discovered in the mid-nineteenth century, it did not conform to the law’s prediction.

10 Cosmic Epics and Dying Worlds

While laws governing the formation and distance between planets were proving unreliable, nineteenth-century British intellectuals nonetheless celebrated the progress of astronomy, along with all other evidence of progress they could identify in their ascendance to world domination. Historians Bernard Lightman and Nasser Zakariya have pointed out how directional cosmic narratives and “evolutionary epics” that traced the evolution of the physical and biological world did so in service of the Victorian ideal of progress, and remain a staple of popular science into the present (Lightman 2007; Zakariya 2017). One of the most famous of the Victorian epics, and also the best historically studied, is Robert Chambers’s 1844 Vestiges of the Natural History of Creation (Chambers 1994; Secord 2000). In fact, the book was a natural history of progress that wove together often racist and classist ideas about human intellect, culture, and civilization into a story of the natural unfolding of the universe one could read as a natural argument for the ascendance of the British Empire. The initial chapters of the book cover the formation of the planets, which was based on Laplace’s nebular hypothesis. Chambers described the planets as “bound up in one chain – interwoven in one web of mutual relation and harmonious agreement” (Chambers 1994, 12).

All of the planets were understood to have a common beginning, were acted upon by the same set of natural forces, and would all ultimately come to the same end. The American astronomer Percival Lowell, whose outspoken advocacy of canals and intelligent canal builders on Mars has been extensively treated by historians of science, saw on Mars the future of the Earth (Hoyt 1996; Dick 1996; Markley 2005; Lane 2011; Nall 2019). Lowell drew upon his own version of the nebular hypothesis as it applied to the evolution of the planets. Every planet in Lowell’s model went through six stages: first was the sun stage, in which the planet was hot enough to radiate light; second was the molten stage, in which the planet was still incredibly hot but beginning to cool; third was the solidifying stage, in which the planet developed a solid surface; fourth was the terraqueous stage, where there was liquid water at the surface; fifth was the terrestrial stage, when the oceans receded; and sixth was the dead stage, with neither water nor atmosphere. Mars, because it was smaller than the Earth, had already reached the terrestrial stage. The Moon, smaller than Mars, was what the red planet would become once its water and atmosphere were fully depleted. Mars – a barely habitable desert world on which survival required global feats of engineering – was thus an example of the next step in Earth’s own evolution, as natural forces caused deserts to increase and multiply.

The nineteenth century also stands out as the period in which geographical practices were first systematically applied to the planets. One can hardly imagine the Martian canal controversy without considering the authority of maps and globes. Despite their shortcomings, geographer K. Maria D. Lane argues that topographical maps of Mars became valuable cartographic icons in staking and communicating scientific knowledge claims during this period (Lane 2005, 2011). The process of mapping, and the use of a terrestrial vocabulary of seas, channels, islands, and continents (not to mention canals), reinforced the idea that Mars was Earth-like. While astronomers disagreed publicly in their interpretations of the features they saw on Mars, nearly all took for granted the idea that Mars was physically Earth-like, and most agreed that it was a living world, even if this life might be limited to vegetation. The maps connected planetary astronomy to the imperial practice of exploration, the context in which nineteenth-century geographical practice was rooted.

11 Twentieth Century

The nebular hypothesis was challenged in the twentieth century by ideas reminiscent of Buffon’s chance one-off occurrence. Perhaps the solar system had formed when the Sun interacted with another star passing nearby. Tidal forces might have torn material from the Sun and this material might have congealed into the planets. Many of Herschel’s nebulae were revealed to be very large galaxies, making his natural history of stars and planetary systems untenable. But spectroscopic evidence of the composition of the stars and planets (accumulating since the mid-1800s) provided ammunition for arguments that planetary formation could still result from a preexisting solar nebula (Brush 1996a, c; DeVorkin 2000). The discovery of radioactivity and the utilization of radioactive decay and isotope ratios in new geochemical and geophysical methods – techniques accelerated by World War II and Cold War projects in atomic and nuclear science – put limits on the age of the Earth and solar system, and yielded insights into the deep history of the Earth and solar system (Brush 1996b; Doel 1996; Shindell 2014, 2019).

In the USA, the earth and planetary sciences (the latter hardly existed before the Cold War) benefited from increased military and national patronage after World War II. This patronage had dramatic effects on the direction of these fields, even as new technologies and methods yielded new evidence about planetary structure, dynamics, and geomagnetism, and new understandings of global planetary systems (Doel 2003; Oreskes 1999, 2021). This included the founding of the National Aeronautics and Space Administration in the context of the Space Race between the USA and the Soviet Union. The rise of the rocket and the prospect of studying the planets from outside of Earth’s atmosphere, whether in orbit around Earth or in missions to fly by, orbit, or even land on another world attracted the interest of scientists from multiple disciplines. While within their home disciplines interest in the planets may have in some cases been marginalized, these scientists soon found a home in the burgeoning interdisciplinary field of planetary science (Tatarewicz 1990; Doel 1996).

The direction of US planetary science was shaped by technologies and funding opportunities made available by the state. NASA built a space science community through funding to universities for research and graduate fellowships – “a constituency that would help support the agency and its goals” (Lambright 2014). Scientists who wished to ask questions about the planets had to learn how to ask them in the new technical language of spaceflight. They worked with NASA engineers, mission planners, and the same aerospace contractors engaged in solving problems for the military and CIA to design instruments for planetary exploration. They worked within the constraints and timelines of missions that were often set with little regard for science. David DeVorkin has argued that many of the scientists who participated in the early years of rocket-powered science “placed themselves in the dual role of contributors to science and inventors of a new way to conduct science,” and developed “a tool-building technical culture” (DeVorkin 1992). Peter Westwick’s work has illuminated a trend of interdependency between civilian and military contracts at the NASA center behind most of the robotic science and exploration, Caltech’s Jet Propulsion Laboratory (JPL). Before JPL first began its work to support lunar and planetary exploration in the late 1950s, most of the the lab’s work was classified. Civilian contracts during the space race brought that percentage down to almost negligible levels, as government funds poured in for robotic planetary exploration. But even if JPL was mostly invested in developing civilian space technologies during these years, the lab nonetheless existed within a larger aerospace community in and around Los Angeles, California, that military contracts shaped into a “gun belt metropolis” (Westwick 2007).

Historians have approached twentieth-century planetary science and exploration in a variety of ways. Some, such as Schorn, have placed planetary exploration within a longer history of planetary astronomy or, like Sheehan, into an even longer history of planetary observation (Schorn 1998; Sheehan 1992). Histories funded by the NASA History Office have tended to focus on technical and administrative mission histories, including early pre-Apollo robotic missions to the Moon, the Viking program, and the Galileo project (Hall 1977; Ezell and Ezell 1984; Meltzer 2012). The History Office has also produced a very useful reference book on the history of planetary spacecraft (Siddiqi 2018). More recently, NASA published an edited volume commemorating 50 years of robotic exploration (Billings 2021). Other missions, such as the Voyager grand tour of the outer planets, have been chronicled without NASA support (Pyne 2010; Bell 2015). Some historians such as Koppes and Westwick have focused on the Jet Propulsion Laboratory as the institution at the center of robotic planetary exploration, and the interface between the national space agency and the academic community (Koppes 1982; Westwick 2007). Launius and McCurdy have treated a broader history of robots as space explorers (Launius and McCurdy 2008).

Mars has been more thoroughly explored than any other planet in our solar system (aside from Earth, of course). And it has also received the most attention from historians and popular authors. Erik Conway has explored the history of JPL in respect to engineering and operating missions to explore Mars (Conway 2015). Crossley, Markley, and Shindell have worked to place our present fascination with Mars – manifested not only in robotic exploration but also in imagined futures of human expeditions or long-term habitation – in a longer, literary, and intellectual history of imagining Mars and its connections to Earth (Markley 2005; Crossley 2011; Shindell 2023). Meanwhile, sociologists and anthropologists have made Mars exploration a site of ethnographic analysis and participant observation, providing perspectives on both the culture of planetary exploration and the role of culture in planetary practices (Vertesi 2015, 2020; Messeri 2016; Mirmalek 2020).

12 The Future of the Planets

Some of these most recent works speak to early twenty-first-century planetary science and exploration. It is yet unclear how our present moment should be understood, and no great work of the history of twenty-first century planetary physics has yet been written. Perhaps the meaning and significance of post-Cold War exploration will be determined by what comes next in the exploration of the planets. The US, Chinese, and Indian space programs all have plans of returning humans to the Moon in programs that emphasize scientific exploration as well as technology development. In most of these plans, Mars is considered the next logical destination after the Moon for human exploration (as it was for some in the Apollo era) (Logsdon 2015; Lambright 2014). If humans do return to the Moon and set foot on other planets and moons in the decades to come, how will this affect our view of the planets? No doubt our knowledge of them will be enhanced as it was with the return of samples from the Moon during Apollo, as well as the robotic return of samples from comets and asteroids. As for the potential significance of these findings, let alone the cultural changes that might result from becoming a multiplanet species, we can only speculate based on plans and ambitions.

We can already speak to the significance of the narratives produced by scientific and speculative visions of the planets. We live on a planet that seems to be unique in our own solar system in its ability to support life, and more alive with dynamic processes. There is reason to believe that life may have emerged on Mars or elsewhere in the solar system (though we’ve yet to find it). The notion that another planet could be made habitable likewise seems reasonable (though perhaps overly ambitious, at least with present technologies). The discovery of thousands of planetary systems, and even Earth-sized planets around other stars, has shown us that our situation may not be as unique as once believed, and that the formation of planets around stars is a regular occurrence (Impey 2023). The arrangements of these systems, many with gas giants larger than Jupiter orbiting close to their star, have changed our ideas about the order in which our own planets may have formed and the gravitational relationships that might have reordered them and placed Earth in the so-called “habitable zone.” We can find this story, more or less, in the recently released decadal survey for planetary science and astrobiology (National Academies of Sciences 2022). New space-based tools bring these exoplanet worlds into greater resolution, and yet they remain well out of reach.

Our ability to tell versions of this story, to imagine the historical relationship between the parts of our solar system, and to speculate about our future in this solar system and others is all based on the use of powerful technologies that mediate our relationship to the planets. Propulsion technologies lift rockets and spacecraft beyond Earth. Computer technologies guide these spacecraft and control their operations in space. Robotic technologies combined with instruments of seeing allow us to collect data remotely, and even to move from site to site on planetary surfaces. And yet this is still a very human enterprise. For every robot at work in the solar system, there are hundreds if not thousands of humans on Earth who worked to make the mission possible or continue to work to support and operate it, and teams of scientists working to interpret received data and formulate new questions. Bureaucratic funding structures and politics at the local and federal levels still define what questions these experts ultimately get to pursue in space. Publications like the decadal survey allow this expert community to exert their influence upon the priority setting process (Asner and Garber 2019). And, when it comes to imagining the broader significance of planetary science and exploration, we continue as a culture to transpose ideas of who we are, our utopian and dystopian visions of our future, and our understandings (celebratory and critical) of the past that shaped our present.

13 Conclusion

This present era of robotic exploration of our solar system began little more than half a century ago; the blink of an eye in the face of thousands of years of observing the sky. Advances in robotics, computing, and imaging have given us unprecedented access to the planets. Humans and robots have even brought pieces of the Moon, comets, and asteroids home to Earth for analysis. It is tempting to believe that our current understanding of the planets is more permanent – more based on “real” empirical evidence – than those of our historical antecedents. Many have found it tempting to tell the history of planetary exploration as the story of how we came to truly know our solar system. Indeed, it does seem true that the technologies of exploration, including the laboratory methods of earth and planetary science, have constrained theories about the origin and evolution of the solar system. The truth may be, however, that our science has barely scratched the surfaces of the worlds we’ve explored, and that our present understanding may one day seem rooted in our own cultural biases – defined through the epistemological lenses of capitalism, expansionism, the pursuit of resources, and a technological militarism that has defined twentieth- and twenty-first-century life. It is wise, perhaps, for historians of contemporary science to think of ourselves and our moment as just the latest chapter in a very long and (dis)continuous history of locating ourselves and our significance in our universe.