For religious institutions in Latin western Europe of the 16th century, Earth was the center of the universe, and the orderly and predictable motion of the heavenly planets about the Earth (which included the Sun) reflected divine will and an inducement to moral improvement. The discovery by Copernicus that the Earth was not at the center of the universe, but was itself a planet orbiting the Sun, was revolutionary. The invention of the telescope resulted in the discovery of more planets by Galileo and others, initially thought to be planets orbiting planets. All planets were expected to feature geological processes seen on Earth. It was even speculated that these other worlds also supported intelligent life.
The search for and discovery of a predicted “missing planet” at the beginning of the 19th century opened the door to a rapidly growing number of new small planets, which appeared as points of light in the sky, and were also referred to as “asteroids,” meaning “star-like.” All planets at this time, which then included asteroids, were thought to have formed from a disk of nebular dust and gas surrounding the early Sun. While in the 19th century it was thought by some that asteroids may have arisen from the breakup of a larger planet, it was not until the 1950s when the smaller members of this population were shown to be collisional fragments that there was a paradigm shift in the scientific literature away from their being considered a type of planet.
Near the end of the 20th century, planets around other stars were discovered. These planets now number in the many thousands, greatly expanding the diversity of planet characteristics and solar system architectures. Since then, a growing number of small planets have been discovered in the Solar System (often referred to as ice dwarfs), many of which are hypothesized to have or have had subsurface oceans. Dwarfs are also satellites of planets. Ice dwarfs are the most common type of planet in the Solar System and are hypothesized to be the most common type of planet around other stars. If life can arise in subsurface oceans of these worlds, it raises the question of whether life might be common in the universe.
The use of the term “planet” today in the scientific literature continues to reflect its heritage from the time of Galileo, that is, planets are geophysical objects, regardless of their orbits. So, by definition, planets would be those objects large enough to be gravitationally round, in generally hydrostatic equilibrium, at which point differentiation commences and geophysical processes, similar to those observed on Earth, are observed to “turn on.”
Article
Defining Planets
Mark V. Sykes, Elisabeth Adams, Kirby Runyon, S. Alan Stern, and Philip T. Metzger
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The Recognition of Meteorites and Ice Ages
Alan E. Rubin
Two important scientific questions that confronted 18th- and 19th-century naturalists were whether continental glaciation had occurred thousands of years earlier and whether extraterrestrial rocks occasionally fell to Earth. Eventual recognition of these hypotheses as real phenomena resulted from initial reports by nonprofessionals, subsequent investigation by skeptical scientists, and vigorous debate. Evidence that kilometer-thick glaciers had once covered Northern Europe and Canada included (a) the resemblance of scratched and polished rocks near mountain glaciers to those located in unglaciated U-shaped valleys; (b) the similarity of poorly sorted rocks and debris within “drift deposits” (moraines) to the sediment load of glaciers; and (c) the discovery of freezing meltwater at the base of glaciers, hypothesized to facilitate their movement. Three main difficulties naturalists had with accepting the notion that rocks fell from the sky were that (a) meteorite falls are localized events, generally unwitnessed by professional scientists; (b) mixed in with reports of falling rocks were fabulous accounts of falling masses of blood, flesh, milk, gelatin, and other substances; and (c) the phenomenon of falling rocks could neither be predicted nor verified by experiment. Five advances leading to the acceptance of meteorites were (a) Ernst Chladni’s 1794 treatise linking meteors, fireballs, and falling rocks; (b) meteor observations conducted in 1798 showing the high altitudes and enormous velocities of their meteoroid progenitors; (c) a spate of several widely witnessed meteorite falls between 1794 and 1807 in Europe, India, and America; (d) chemical analyses of several meteorites by Edward Charles Howard in 1802, showing all contained nickel (which is rare in the Earth’s crust); and (e) the discoveries of four asteroids between 1801 and 1807, providing a plausible extraterrestrial source for meteorites.
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The Planets in Alchemy and Astrology (Medieval and Renaissance)
Nicholas Campion
In the Middle Ages and Renaissance, alchemy and astrology shared a common language in the meanings and characters attributed to the celestial bodies, which provided a cosmic framework for understanding all terrestrial affairs. Astrology is the name given to a series of diverse practices based in the idea that the stars, planets, and other celestial phenomena possess significance and meaning for events on the Earth. In practical terms, astrology’s function was to predict the future, manage the present, and understand the past. In the Middle Ages and Renaissance, these three functions were not seen as separate, because time existed as a single entity—past, present, and future coexisted in the mind of God. Astrology assumed a link between Earth and sky in which all existence, spiritual, psychological, and physical, is interconnected. Time and space existed in a mutually dependent continuum, in which both were infused with the same qualities, represented in terms of astrological language by zodiac signs and planets. The practice of alchemy applied such assumptions to the material world, attempting to convert one metal into another, typically lead into gold. As all things were interconnected, including soul and body, the physical practice of alchemy was intimately connected with spiritual practice intended to purify the soul in preparation for its meeting with the divine.
The planets had roles in both astrology and alchemy. A planet, from the Greek planētai, meaning wanderer, is a star, a point, or source of light in the sky that constantly alters its position. This is in contrast to the so-called fixed stars, which appear to keep exactly the same position relative to each other, at least over historic periods of time.
Technical astrology largely developed in the Hellenistic, Greek-speaking world from the early third century BCE onward. With the collapse of the Roman Empire and classical learning in most of Western Europe from the 5th century, the complex astrology of the Classical world largely disappeared. It returned to Western Europe in two phases. The first phase, mainly in the 12th century, included the translation into Latin of major texts from Arabic, including those originally composed in Greek, and was accompanied by Aristotelian works that justified the use of astrology in naturalistic terms, thereby negating Christian disapproval. Alchemy was introduced into Western Europe at the same time. The second phase began mainly in the 15th century, during the Renaissance, as part of a wider revival of Classical thought encouraged by the translation of Platonic, neo-Platonic, and Hermetic works from Greek directly into Latin. By the end of the 17th century, the practice of both astrology and alchemy had largely disappeared. Astrology was confined to popular almanacs and alchemy was replaced by modern chemistry.
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Archaeoastronomy/Cultural Astronomy
Juan Antonio Belmonte
Archaeoastronomy and cultural astronomy are often considered synonyms, but they actually express different concepts, the former being a sub-discipline of the latter. Cultural astronomy is a fascinating but controversial discipline, which serves as an auxiliary subject to social sciences such as history, archaeology, anthropology, and ethnography, among others. The tools and methodology of astronomy play a relevant role in the discipline, but it should be inserted within social sciences epistemology.
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Space Resource Utilization
Angel Abbud-Madrid
Throughout human history, resources have been the driving force behind the exploration and settling of our planet and also the means to do so. Similarly, resources beyond Earth will make space the next destination in the quest for further exploration and economic expansion of our species. The multitude of celestial bodies surrounding Earth and the space between them hold a vast wealth of resources for a variety of applications. The unlimited solar energy, vacuum, radiation, and low gravity in space, as well as the minerals, metals, water, atmospheric gases, and volatile elements on the Moon, asteroids, comets, and the inner and outer planets of the Solar System and their moons, constitute potential valuable resources for robotic and human space missions and for future use on our own planet. In the short term, these resources could be transformed into useful materials at the site where they are found to extend mission duration and to reduce the costly dependence on materials sent from Earth. Making propellants and human consumables from local resources can significantly reduce mission mass, cost, and risk, enabling longer stays and fueling transportation systems for use within and beyond the planetary surface. Use of finely grained surficial dust and rocks can serve for habitat and infrastructure construction, radiation protection, manufacturing parts, and growing crops. In the long term, material resources and solar energy could also be brought to Earth if obtaining these resources and meeting energy demands locally prove to be no longer economically or environmentally acceptable.
However, just like on Earth, not all challenges to identify, extract, and utilize space resources are scientific and technological. As nations and private companies start working toward extracting extraterrestrial resources, an international legal framework and sound socioeconomic policies need to be put in place to ensure that these resources are used for the benefit of all humanity. Space resources promise to unleash an unprecedented wave of exploration and of economic prosperity by utilizing the full potential and value of space. As we embark on this new activity, it will be up to us, humans on planet Earth, to find the best alternatives to use resources beyond our planet effectively, responsibly, and sustainably to make this promise a reality.
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Astrobiology (Overview)
Sean McMahon
Astrobiology seeks to understand the origin, evolution, distribution, and future of life in the universe and thus to integrate biology with planetary science, astronomy, cosmology, and the other physical sciences. The discipline emerged in the late 20th century, partly in response to the development of space exploration programs in the United States, Russia, and elsewhere. Many astrobiologists are now involved in the search for life on Mars, Europa, Enceladus, and beyond. However, research in astrobiology does not presume the existence of extraterrestrial life, for which there is no compelling evidence; indeed, it includes the study of life on Earth in its astronomical and cosmic context. Moreover, the absence of observed life from all other planetary bodies requires a scientific explanation, and suggests several hypotheses amenable to further observational, theoretical, and experimental investigation under the aegis of astrobiology. Despite the apparent uniqueness of Earth’s biosphere— the “n = 1 problem”—astrobiology is increasingly driven by large quantities of data. Such data have been provided by the robotic exploration of the Solar System, the first observations of extrasolar planets, laboratory experiments into prebiotic chemistry, spectroscopic measurements of organic molecules in extraterrestrial environments, analytical advances in the biogeochemistry and paleobiology of very ancient rocks, surveys of Earth’s microbial diversity and ecology, and experiments to delimit the capacity of organisms to survive and thrive in extreme conditions.
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Condensation Calculations in Planetary Science and Cosmochemistry
Denton S. Ebel
The Sun’s chemical and isotopic composition records the composition of the solar nebula from which the planets formed. If a piece of the Sun is cooled to 1,000 K at 1 mbar total pressure, a mineral assemblage is produced that is consistent with the minerals found in the least equilibrated (most chemically heterogeneous), oldest, and compositionally Sunlike (chondritic), hence most “primitive,” meteorites. This is an equilibrium or fractional condensation experiment. The result can be simulated by calculations using equations of state for hundreds of gaseous molecules, condensed mineral solids, and silicate liquids, the products of a century of experimental measurements and recent theoretical studies. Such calculations have revolutionized our understanding of the chemistry of the cosmos.
The mid-20th century realization that meteorites are fossil records of the early solar system made chemistry central to understanding the origin of the Earth, Moon, and other bodies. Thus “condensation,” more generally the distribution of elements and isotopes between vapor and condensed solids and/or liquids at or approaching chemical equilibrium, came to deeply inform discussion of how meteoritic and cometary compositions bear on the origins of atmospheres and oceans and the differences in composition among the planets. This expansion of thinking has had profound effects upon our thinking about the origin and evolution of Earth and the other worlds of our solar system.
Condensation calculations have also been more broadly applied to protoplanetary disks around young stars, to the mineral “rain” of mineral grains expected to form in cool dwarf star atmospheres, to the expanding circumstellar envelopes of giant stars, to the vapor plumes expected to form in giant planetary impacts, and to the chemically and isotopically distinct “shells” computed and observed to exist in supernovae. The beauty of equilibrium condensation calculations is that the distribution of elements between gaseous molecules, solids, and liquids is fixed by temperature, total pressure, and the overall elemental composition of the system. As with all sophisticated calculations, there are inherent caveats, subtleties, and computational difficulties.
In particular, local equilibrium chemistry has yet to be consistently integrated into gridded, dynamical astrophysical simulations so that effects like the blocking of light and heat by grains (opacity), absorption and re-emission of light by grains (radiative transfer), and buffering of heat by grain evaporation/condensation are fed back into the physics at each node or instance of a gridded calculation over time. A deeper integration of thermochemical computations of chemistry with physical models makes the prospect of a general protoplanetary disk model as hopeful in the 2020s as a general circulation model for global climate may have been in the early 1970s.
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Composition of Earth
H. Palme
Early models of the composition of the Earth relied heavily on meteorites. In all these models Earth had different layers, each layer corresponded to a different type of meteorite or meteorite component. Later, more realistic models based on analyses of samples from Earth began with Ringwood’s pyrolite composition in the 1960s. Further improvement came with the analyses of rare MgO rich peridotites from a variety of occurrences all over the Earth, as xenoliths enclosed in melts from the upper mantle or as ultramafic massifs, tectonically emplaced on the Earth’s surface. Chemical systematics of these rocks allow the determination of the major element composition of the primitive upper mantle (PUM), the upper mantle after core formation and before extraction of basalts ultimately leading to the formation of the crust. Trace element analyses of upper mantle rocks confirmed their primitive nature. Geochemical and geophysical evidence argue for a bulk Earth mantle of uniform composition, identical to the PUM, also designated as “bulk silicate Earth” (BSE). The formation of a metal core was accompanied by the removal of siderophile and chalcophile elements into the core. Detailed modeling suggests that core formation was an ongoing process parallel to the accretion of Earth. The composition of the core is model dependent and thus uncertain and makes reliable estimates for siderophile and chalcophile element concentrations of bulk Earth difficult.
Improved stable isotope analyses show isotopic similarities with noncarbonaceous chondrites (NCC), while the chemical composition of the mantle of the Earth indicates similarities with carbonaceous chondrites (CC). In detail, however, it can be shown that no single known meteorite group, nor any mixture of meteorite groups can match the chemical and isotopic composition of Earth. This conclusion is extremely important for any formation model of the Earth.
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Records of Planetary Observations in Ancient Japan Before the 11th Century
Kiyotaka Tanikawa and Mitsuru Sôma
The records of planetary observations in Japan in the 7th century ad are treated separately from other records because they are written in the Nihongi. It is known that Japanese observational astronomy was recorded in the 7th century ad, but astronomy in Japan did not evolve straightforward in that century. There are thirty-one records that exist from that time, including four records on the Moon and planets. Correspondingly, a new interpretation of Japanese ancient history has been proposed. For the 8th, 9th, and 10th centuries, records have been compiled on the relative motion of the Moon and the planets, the motion of planets in the constellations, and stars seen in the daytime, as stated in Japanese recorded history. These records are written in Chinese, as in the case of the Nihongi, but have been translated into English. The orbits of the Moon and planets have been calculated using the NASA Jet Propulsion Laboratory (JPL) development ephemeris (DE) in order to confirm the validity of the records. The numbers of records and observations are not the same because one record may contain multiple observations. The accuracy of individual observations is discussed.
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Ethics of Planetary Science and Exploration
Jacques Arnould
Since the launch of Sputnik on October 4, 1957, the development of space activities has provided a kind of evidence for the conduct of human affairs, to the point of neglecting to question these activities from an ethical point of view: only since the beginning of the 2000s has a real ethical interrogation within the space community (French Space Agency, International Space University, COPUOS) been developed, in parallel with international law. Taking advantage of a rich cultural background and a cooperative sustained effort, space ethics contributes, for example, to better management of debris orbiting the Earth, evaluation of the social impacts of observation satellite systems, and the arrival of new private entrepreneurs apparently less aware of the impacts of managing space as a common heritage of humanity. If space law provides a possible framework and a set of principles for the current and future management of space activities, ethical principles must be considered to accurately assess their reasons for being and their consequences. The following questions are pertinent today: Has space become a trash can? Is space “Big Brother’s” ally? Is space for sale? Should space be explored at any cost? These issues require special expertise of the situation (e.g., the distribution of debris around the Earth, the capabilities of observation satellites); consideration of the global, dual (civil, military) nature of space; and reference to ethical principles (responsibility, vigilance). Human space flight, space tourism, and the search for extraterrestrial life are also subject to ethical questioning. At the beginning of the 21st century, space ethics remained a goal for the space community.
