Introduction
In modern parlance, the circumstellar habitable zone (denoted by HZ hereafter) is conventionally understood to encapsulate the area around a host star (or stars) where liquid water can theoretically exist on the surface of a terrestrial planet endowed with an appropriate atmospheric composition (e.g. Kasting Reference Kasting2010; Lingam and Loeb Reference Lingam and Loeb2019). To quote one specific example of many, Méndez et al. (Reference Méndez, Rivera-Valentín, Schulze-Makuch, Filiberto, Ramírez, Wood, Dávila, McKay, Ortiz Ceballos, Jusino-Maldonado, Nery, Heller, Byrne, Malaska, Nathan, Filipa Simões, Antunes, Martínez-Frías, Carone, Izenberg, Atri, Itic Carvajal Chitty, Nowajewski-Barra, Rivera-Hernández, Brown, Lynch, Catling, Zuluaga, Salazar, Chen, González, Kashyap Jagadeesh, Barnes, Cockell and Haqq-Misra2020) recently defined the HZ as follows:
…the circumstellar region around a star where a terrestrial planet with a suitable atmosphere could host liquid water on its surface.
The HZ paradigm has a long and labyrinthine history, further compounded by the fact that many of the early researchers in astrobiology opted to use alternative terminology instead. It is worth explicating this matter in greater detail by focusing on a select few instances. In the 1950s and 1960s, the astrophysicist Su-Shu Huang (1915–1977) authored a number of crucial papers in which the appellation ‘habitable zone’ was employed in its modern sense (Huang Reference Huang1959a, Reference Huang1959b, Reference Huang1960). During roughly the same period, however, other publications utilized terms such as the ‘ecosphere’ (Strughold Reference Strughold1953, Reference Strughold1955; Dole Reference Dole1964; Shklovskii and Sagan Reference Shklovskii and Sagan1966) and ‘liquid water belt’ (Shapley Reference Shapley1953, Reference Shapley1959; Strughold Reference Strughold1956).
In the 1960s and 1970s, among the first quantitative estimates for the inner and outer edges of the HZ for solar-type stars via numerical climate models were presented. Most of these studies arrived at inner limits close to present-day bounds (cf. Zsom et al. Reference Zsom, Seager, de Wit and Stamenković2013), but the outer edge was determined to be approximately 1 AU – a result that stands in contrast to modern formulations, which have yielded estimates of $\gtrsim \! 1.5$
AU (e.g. Zsom Reference Zsom2015; Ramirez Reference Ramirez2018). Hence, the publications of this era, when viewed collectively (Budyko Reference Budyko1969; Ingersoll Reference Ingersoll1969; Sellers Reference Sellers1969; Rasool and de Bergh Reference Rasool and de Bergh1970), implied that the HZs of stars were very narrow and that planets with temperate climates ought to be correspondingly rare (Levenson Reference Levenson2015).
The best-known examples from the above epoch are the two important papers by Michael Hart from the 1970s that also recognized that stellar luminosity grows with time and the HZ migrates outwards accordingly (Hart Reference Hart1978, Reference Hart1979), thereby necessitating the introduction of the continuously habitable zone (CHZ) – the region around a star where clement conditions can be sustained over geological timescales. The width of the 4.6 Gyr CHZ for a Sun-like star as per Hart's modelling was only about 0.06 AU (Hart Reference Hart1978). The discrepancy compared to subsequent treatments arose from a combination of inaccurate parametrization of the stabilizing carbonate-silicate cycle thereafter elucidated in Walker et al. (Reference Walker, Hays and Kasting1981), insufficient precision in the radiative transfer modelling, and not taking certain poorly constrained environmental parameters (at that time) fully into account (Schneider and Thompson Reference Schneider and Thompson1980; Kasting Reference Kasting2010; Levenson Reference Levenson2015).
It is necessary to appreciate at this juncture that the concept of the HZ is much older than the exact expression ‘habitable zone’ itself (e.g. Gonzalez Reference Gonzalez2005). In the renowned Philosophiæ Naturalis Principia Mathematica, Sir Isaac Newton (1643–1727) asserted that orbits associated with Mercury and Saturn would lead to the vaporization and freezing of Earth's liquid water, respectively (Newton Reference Newton1687). The following sentences, in particular, merit reproductionFootnote 1:
Our water, if the earth were located in the orbit of Saturn, would be frozen, if in the orbit of Mercury it would depart at once into vapours. For the light of the sun, to which the heat is proportional, is seven times denser in the orbit of Mercury than with us: and with a thermometer I have found that with a seven-fold increase in the heat of the summer sun, water boils off.
Several studies in the 19th century opted to use ‘temperate zone’ in lieu of the HZ, of which the best known is arguably Of the Plurality of Worlds (1853) by the polymath William Whewell (1794–1866). In the treatise, Whewell (Reference Whewell1855, p. 316) contended that:
The Earth's Orbit is the Temperate Zone of the Solar System. In that Zone only is the play of Hot and Cold, of Moist and Dry, possible.
Quite intriguingly, the appellation ‘temperate zone’ has witnessed a renaissance of sorts in modern times (e.g. Tasker Reference Tasker2017), partly since it apparently avoids the pitfalls of conflating the two distinct notions of the HZ and habitability (Tasker et al. Reference Tasker, Tan, Heng, Kane, Spiegel, Brasser, Casey, Desch, Dorn, Hernlund, Houser, Laneuville, Lasbleis, Libert, Noack, Unterborn and Wicks2017).
If we restrict ourselves to the terminology ‘habitable zone’ sensu stricto, the earliest mention of this expression has been attributed to Edward Walter Maunder (1851–1928) based on research by Lorenz (Reference Lorenz2019, Reference Lorenz2020), as revealed by these lines (Maunder Reference Maunder1913, p. 149):
Round our Sun there is but a narrow zone in which a habitable world may circle; in this zone there is room for but few worlds, and we actually know of three alone, the Earth, the Moon and Venus.
It is the objective of this study to illuminate a small subset of the earliest explicit allusions to the ‘habitable zone’ and analyse in what respects they are similar to, or divergent from, the modern formulation of the HZ. It is evident that this endeavour ought not be regarded as definitive because, indubitably, there are myriad writings all the way up to the 19th and 20th centuries, both in English and otherwise, that remain marginalized and inaccessible for a variety of causes.
The HZ in the 19th century
Prior to embarking on the historical voyage to comprehend the metamorphosis of the HZ paradigm, a brief detour into the statistics of Earth-sized planets is warranted for reasons that will become apparent shortly hereafter.
Number of Earth-sized planets in the HZ
In our Solar System, upon examining the number of roughly Earth-sized planets that have clement temperatures amenable to the sustenance of liquid water, we would end up with a sample size of one – to wit, the Earth. Now, if we were to take a considerable leap of faith, and loosely apply the Principle of Mediocrity (also known as the Copernican Principle), we may conjecture that the mean number of roughly Earth-sized rocky planets in the HZs of stars (often denoted by $\eta _\oplus )$
would be around unity – namely, $\eta _\oplus \sim 1$
in mathematical terms. However, the latent subtleties underpinning the Principle of Mediocrity are such that it cannot and ought not be invoked and deployed tout court, viz., without appropriate qualifications (Ćirković and Balbi Reference Ćirković and Balbi2020).
Several studies have attempted to gauge $\eta _\oplus$
for different spectral types, but the results vary by about an order of magnitude (Kaltenegger Reference Kaltenegger2017), which is primarily a consequence of variations in the statistical techniques and the definition of $\eta _\oplus$
itself. Although the magnitude and the meaning of η continue to be debated, statistical analyses indicate that $\eta _\oplus \approx 0.3$
is feasible (Dressing and Charbonneau Reference Dressing and Charbonneau2015; Zink and Hansen Reference Zink and Hansen2019), and that $\eta _\oplus \lesssim 0.9$
is realizable under optimal circumstances (Bryson et al. Reference Bryson, Kunimoto, Kopparapu, Coughlin, Borucki, Koch, Aguirre, Allen, Barentsen, Batalha, Berger, Boss, Buchhave, Burke, Caldwell, Campbell, Catanzarite, Chandrasekaran, Chaplin, Christiansen, Christensen-Dalsgaard, Ciardi, Clarke, Cochran, Dotson, Doyle, Duarte, Dunham, Dupree, Endl, Fanson, Ford, Fujieh, Gautier, Thomas, Geary, Gilliland, Girouard, Gould, Haas, Henze, Holman, Howard, Howell, Huber, Hunter, Jenkins, Kjeldsen, Kolodziejczak, Larson, Latham, Li, Mathur, Meibom, Middour, Morris, Morton, Mullally, Mullally, Pletcher, Prsa, Quinn, Quintana, Ragozzine, Ramirez, Sanderfer, Sasselov, Seader, Shabram, Shporer, Smith, Steffen, Still, Torres, Troeltzsch, Twicken, Uddin, Van Cleve, Voss, Weiss, Welsh, Wohler and Zamudio2021); in contrast, it bears mentioning that more conservative estimates have been recently expounded by the likes of Pascucci et al. (Reference Pascucci, Mulders and Lopez2019) and Kunimoto and Matthews (Reference Kunimoto and Matthews2020). Thus, on the basis of the available data, our initial ‘guesstimate’ of $\eta _\oplus \sim 1$
might not be altogether unreasonable for certain spectral types; if this relation is approximately correct, the Principle of Mediocrity would constitute a useful heuristic prima facie.
We will now proceed to delineate a select few of the writings prior to Maunder (Reference Maunder1913) that employed the appellation ‘habitable zone’ explicitly in the 19th century.
Explicit references to the HZ
In the period leading to the fin de siècle and even afterwards, the expression ‘habitable zone’ typically carried a rather divergent connotation compared to its present-day counterpart. The chief reason is that ‘habitable zone’ referred to the regions of the Earth's surface that were especially conducive and clement for humans; in other words, this phrase was almost exclusively utilized in a geographical context. This tendency was prevalent in not just English writings but also other European languages such as zona abitabile in Italian (e.g. Hugues Reference Hugues1884, p. 73), zone habitable in French (e.g. de Saint-Martin Reference de Saint-Martin1875, pp. 114–115), zona habitable in Spanish (e.g. Vilanova y Piera Reference Vilanova y Piera1874, p. 106) and bewohnbare Zone in German (e.g. Günther Reference Günther1879, p. 298). If we look further beyond into the mists of time (i.e. 18th century and earlier), the ‘habitable zone’ preserves its geographical connotations (e.g. Wyvill Reference Wyvill1672; Raleigh Reference Raleigh1677; Walsh Reference Walsh1743; Tiraboschi Reference Tiraboschi1783).
Two key examples suffice to illustrate and underscore the preceding point. Camille Flammarion (1842–1925) – who published La pluralite des mondes habites (1862), a well-known early treatise on extraterrestrial life (Crowe Reference Crowe1986) – authored a French science fiction novel entitled La Fin du monde (1894). In this novel, Flammarion (Reference Flammarion1894, p. 240) wrote:
As the habitable zone became more and more restricted to the equator, the population had still further diminished, as had also the mean length of human life, and the day came when only a few hundred millions remained, scattered in groups along the equator, and maintaining life only by the artifices of a laborious and scientific industry.
The second instance that we wish to highlight will seem quite unexpected at first sight: the famous De La Démocratie en Amérique published during 1835–1840 by the French historian and philosopher Alexis de Tocqueville (1805–1859). In this multi-volume undertaking, De Tocqueville (Reference De Tocqueville1849, p. 467) opined that:
Above its northern frontiers the icy regions of the poles extend; and a few degrees below its southern confines lies the burning climate of the equator. The Anglo-Americans are therefore placed in the most temperate and habitable zone of the continent.
Now, let us direct our attention to what may very well be the first modern, or semi-modern at the minimum, exposition of the HZ that originated in the closing decades of the 19th century. The monograph in question is World-life: Or, Comparative Geology (1883), a wide-ranging exploration of astronomy, planetary science, physics and geography by the United States geologist and biologist Alexander Winchell (1824–1891), which was reviewed favourably by contemporary scientists such as George Darwin (Darwin Reference Darwin1884). In this book, when discussing planetary habitability, Winchell (Reference Winchell1883, p. 507) contended that:
The earth, then, so far as we can reason, is in the middle of the habitable zone of the solar system, if our own natures are assumed as the criterion of habitability. On either side, the rigour of the physical conditions seems to proclaim our system a voiceless and lifeless desert.
The above quote illustrates that Winchell's conception of the HZ was markedly different from those of his contemporaries and much closer to that of our own era. Moreover, his emphasis on ‘our own natures’ to gauge the boundaries of habitability is partly compatible with the modern formulation of the HZ. In fact, Earth-centric limits for habitability have been invoked to delineate the limits of the ‘complex life habitable zone’ (Schwieterman et al. Reference Schwieterman, Reinhard, Olson, Harman and Lyons2019); see also Ramirez (Reference Ramirez2020) and Lingam (Reference Lingam2020) for cognate analyses. Winchell's tome comprises a number of other notable statements about habitability, such as the hypothesis that tenuous or non-existent atmospheres would make the likes of the Moon, Mars and Mercury inhospitable to life (Winchell Reference Winchell1883, pp. 500–508). This study additionally contained some perspicacious musings on extrasolar planetary systems that warrant reproduction (Winchell Reference Winchell1883, p. 507):
But there are other suns and other planetary systems, and other worlds which possess the conditions for habitability. When we look on the hosts of stars, and consider that if only one habitable planet wanders about each sun, we understand that the number of habitable worlds is countless.
Thus, if one sought to interpret the above lines very broadly and through a modern prism ex post facto, it might be asserted that Winchell was subscribing to the optimistic scenario with $\eta _\oplus \sim 1$
, which is perhaps not too far removed from reality, as remarked at the beginning of Section ?? ‘The HZ in the 19th century’. Among the other striking aspects of this book is the notion that one can study the geology of other worlds to comprehend Earth's geological evolution and vice versa, which was termed ‘comparative geology’ and ‘astrogeology’ (Winchell Reference Winchell1883, p. vii); the reader may consult Léveillé (Reference Léveillé2010) for a historical account of this topic.
All of the preceding facets must, however, be tempered by the sober realization that Winchell's writings, especially in the social sciences, were imbued with racist overtones and undertones (Livingstone Reference Livingstone2008, Reference Livingstone2010; Keevak Reference Keevak2011), of which the most unequivocal is probably Preadamites, or a Demonstration of the Existence of Men Before Adam (1880), a publication that continues to be adduced by white supremacistsFootnote 2.
Conclusion
It is not an exaggeration to aver that state-of-the-art delineations of the HZ constitute one of the cornerstones of the burgeoning area of exoplanets (Kasting et al. Reference Kasting, Whitmire and Reynolds1993; Kopparapu et al. Reference Kopparapu, Ramirez, Kasting, Eymet, Robinson, Mahadevan, Terrien, Domagal-Goldman, Meadows and Deshpande2013, Reference Kopparapu, Ramirez, SchottelKotte, Kasting, Domagal-Goldman and Eymet2014; Ramirez et al. Reference Ramirez, Abbot, Fujii, Hamano, Kite, Levi, Lingam, Lueftinger, Robinson, Rushby, Schaefer, Tasker, Vladilo and Wordsworth2019). Notwithstanding the critiques levelled against this concept (Moore et al. Reference Moore, Lenardic, Jellinek, Johnson, Goldblatt and Lorenz2017; Tasker et al. Reference Tasker, Tan, Heng, Kane, Spiegel, Brasser, Casey, Desch, Dorn, Hernlund, Houser, Laneuville, Lasbleis, Libert, Noack, Unterborn and Wicks2017), the HZ is arguably endowed with at least two major advantages: it has the merit of being simple (while concomitantly retaining complexity to an extent) and it functions as a pragmatic and efficient tool in guiding the detection and characterization of exoplanets that may harbour biospheres and their observable signatures. Other benefits accruing from the demarcation and usage of the HZ have been adumbrated in Ramirez (Reference Ramirez2018). It goes without saying, naturally, that habitability and the HZ are two broad frameworks with only a modest overlap and should not be conflated with each other (e.g. Cockell et al. Reference Cockell, Bush, Bryce, Direito, Fox-Powell, Harrison, Lammer, Landenmark, Martin-Torres, Nicholson, Noack, O'Malley-James, Payler, Rushby, Samuels, Schwendner, Wadsworth and Zorzano2016; Lingam and Loeb Reference Lingam and Loeb2021).
In light of these considerations, it is worth fathoming how this term came to be employed in the multidisciplinary domain of astrobiology. This short paper traces the historical development of the expression ‘habitable zone’ and shows that it belonged exclusively to the realm of geographical allusions for the majority of the 19th century and earlier epochs. However, as this century hearkened to a close, the first truly modern glimmerings of the HZ paradigm appeared in the prescient treatise by Winchell (Reference Winchell1883). Although certain claims in this monograph have proven to be incorrect, some of its prominent predictions – such as the definition of the HZ and the prevalence of Earth-sized planets in the HZ – stand up quite well to scrutiny even today; the same, however, does not hold true for some of Winchell's other contemporaneous writings.
Even though it may be tempting to shrug off the endeavour of tracing the shrouded origins of the HZ as scientifically irrelevant or merely a curio or trifle, an in-depth awareness of the convoluted historical process underlying the genesis and transmutation of the HZ can better inform us about the bygone roots of astrobiology and engender a deeper cognizance and appreciation of where the field might be headed in the turbulent 21st centuryFootnote 3.
Acknowledgements
It is a pleasure to thank Jim Kasting for the generous and insightful feedback and references shared over the course of undertaking this work. The valuable resources provided by the Harvard Library system are duly acknowledged. As an experiment, the abstract has been composed in verse – motivated by a conversation with Amedeo Balbi and Claudio Grimaldi – due to the potential benefits that might be accorded by this format (Illingworth Reference Illingworth2016; Januchowski-Hartley et al. Reference Januchowski-Hartley, Sopinka, Merkle, Lux, Zivian, Goff and Oester2018).
Conflict of interest
The author has no conflicts of interest to declare concerning this manuscript.
