Antiquity

The question of plurality of worlds dates back to classical Antiquity. However, it is necessary to clarify the meanings of this expression. With modern scientific knowledge and the current mode of thinking, there is a tendency to think that plurality of worlds means the existence of planetary systems around other stars than our Sun. During Antiquity the image of the world was very different from ours, and stars were not at all considered as being other Suns. Plurality of worlds implied the existence, outside of our visible world that contains all known celestial objects, of other worlds similar to ours, maybe an infinite number of those and unreachable.

The oldest known references concerning other worlds are those of the Greek philosophers Democritus (470–365 BC) and Epicurus (342–270 BC). According to Democritus (according to Hippolytus): ‘There are innumerable worlds of different sizes. In some there is neither Sun nor Moon, in others, they are larger than in ours, and others have more than one. These worlds are at irregular distances, more in one direction and less in another, and some are flourishing, others declining. Here they come into being, there they die, and they are destroyed by collision with one another. Some of the worlds have no animal or vegetable life nor any water.

For Epicurus, disciple of Democritus (‘Letter to Herodotus’): ‘There is an infinity of worlds both like and unlike our world. For the atoms being infinite in number, as was already proved, are found far out into space. For those atoms which are of such nature that a world could be created by them or made by them, have not been used up either on one world or a limited number of worlds… So that there nowhere exists an obstacle to an infinite number of worlds.’.

Plato (428–348) and Aristotle (384–322 BC) took an opposite view: ‘…It follows that the world would be unique. There cannot be several worlds.’ wrote Aristotle. As noted by Dick (Reference Dick1982), ‘Aristotle directly confronted the subject of other worlds only in his cosmological treatise De Caelo’.

It must be emphasized that it is sometimes difficult to understand what was at those times the representation corresponding to the word world.

Christian Era

From the Middle Ages another aspect needs to be taken into account: the agreement between the theory of plurality of worlds and religion as it was taught. Two theological interpretations exist. On the one hand, the biblical story of the creation of the world in Genesis speaks of only one world. So occurrence of other worlds cannot happen. On the other hand, it is not specified that God did not create other worlds and the almighty power of God implies that he could create other worlds. We have to note that this is not scientific reasoning, in the current meaning.

Saint Augustine (354–430 AD) was one of the first Christian authors to discuss the theory of plurality of worlds, which he opposed, as did most medieval authors, including Albert the Great (1193–1280 AD) and Thomas Aquinas (1225–1274 AD).

The question of other worlds became a subject of controversy. In 1277, the bishop of Paris condemned 219 beliefs held in the Universities, among them was ‘that the First Cause cannot make many worlds’.

In 1543, the heliocentric system was introduced by Nicolaus Copernicus (1473–1543) in the book De Revolutionibus Orbium Coelestium (Copernicus Reference Copernicus1543). This implied that our Earth was considered subsequently as a planet among others. In the late sixteenth century, Giordano Bruno (1548–1600) defended the plurality of worlds populated by a multiplicity of live, as the result of a theological approach. ‘Innumerable suns exist; innumerable earths revolve around these suns in a manner similar to the way the seven planets revolve around our sun.’ – ‘That is how the excellence of God is magnified and the greatness of His empire is demonstrated. He is not glorified in only one Sun, but in countless suns, not in only one Earth and one world, but in thousands of thousands, no, an infinity [of worlds].’

Giordano Bruno was sentenced to death and was burned by order of the Church, but nonetheless he was not a martyr for science: his religious opinions were a major factor for the sentence.

In 1609, when Galileo Galilei (1564–1642) observed the sky and some celestial objects with an optical instrument for the first time in the history of humanity, a new era of astronomy begun. He obtained extraordinary results that he very rapidly related in a book Sidereus Nuncius or Sidereal Messenger (Galilei Reference Galilei1610). Johannes Kepler (1571–1630) received this book and very quickly also answered by another book Dissertatio cum Nuncio Sidereo or Conversation with Sidereal Messenger (Kepler Reference Kepler1610). The discovery by Galileo of mountains on the Moon implies that the Moon and the Earth are similar in nature. The Moon is not a perfect sphere. The logical consequence is that there are some inhabitants in the Moon as there are inhabitants on Earth. Kepler wrote : ‘It is entirely reasonable to believe that there are some living creatures on the Moon’. He suggested that rounded reliefs on Moon could be built by Moon's inhabitants. The discovery by Galileo of Jupiter's satellites means that Jupiter is a planet similar to Earth. This led Kepler to comment (Kepler Reference Kepler1610): ‘It is not implausible to imagine that inhabitants exist not only on Moon but also on Jupiter.’ and ‘If four Jovian planets with various distances and times orbit around Jupiter, one might wonder what is the utility if there is no people to observe such admirable variety.’. So Galileo was much more prudent than Kepler who was very imaginative about his hypotheses. However, Kepler's ideas were only hypotheses…

As we shall see below, from the Galileo's discoveries, the idea of other habitable worlds became more and more acceptable.

Modern epoch

In 1686, Bernard le Bouyer de Fontenelle (1657–1757) published in France Entretiens sur la Pluralité des Mondes, i.e. A Conversation on the Plurality of Worlds.

‘Every fixed star is a sun,which diffuses light to its surrounding worlds’(translation by Gunning 1803). Fontenelle assumed that there were inhabitants on the Moon and on every planet, but these inhabitants were different from humans. He spoke also about very various life forms on Earth, even under extreme conditions. This book was written 76 years after Galileo's observations, and shows how great the progress of astronomy had been in the meantime. It was a real ‘best-seller’ in the modern sense. It was re-edited many times and translated into many languages. As an example, the first English translation was published as soon as 1687. It is important to note that Fontenelle was not a scientist, but a writer, and that the book was not intented for astronomers but for the layman. The influence of this book all over Europe was very important.

In 1753, the book Amilec ou la graine d'hommes qui sert à peupler les planètes was published in France, and the anonymous English translation Amilec or the seeds of Mankind (Reference Tiphaine de la Roche1753) is published in London on the same year. The author, Charles-François Tiphaigne de la Roche (1722–1774), was a physician and a writer who published many stories, often anonymously. In this book, a spirit sows successively all the planets. Is it a premonitory description of panspermia? In the same book, there is a description of the origin of Universe, which is reminiscent of the Big Bang : ‘…An innumerable Multitude of Vortices, or Systems, Suns, and habitable Earths, which constitute this vast Universe, all these were formely contained in a Seed of no greater Dimensions, that a small Pea. It bedded and expanded little by little, but the Development is not finished’. The meaning of books of Tiphaigne de la Roche was the subject matter of many studies. It is often considered as premonitory: in other books, the author announces inventions like photography or television. Is it related to alchemy? In any case, its evocation of panspermia, which cannot really be considered as scientific, is very interesting.

During the 18th century, the existence of life on other planets was commonly accepted.

A good example of current knowledge about planets orbiting around other stars can be found in a book of Etienne Bonnot de Condillac (1715–1780), which was the course of study for instruction of the prince of Parma (Condillac Reference Condillac1775): ‘The skies are full of luminous bodies which, like our Sun, probably make some planets moving on various orbits’. Condillac added, in the second part of this sentence: ‘…and the Universe is an immense space where there are no deserted places. Our imagination is as embarrassed to give it limits as not.’ Condillac was a French philosopher who was put in charge, with other philosophers, of teaching and educating the prince of Parma, Ferdinand. This prince was both the grandson of the king of France Louis XV and of the king of Spain. His educators and private tutors were choosen among the best scientists and philosophers and the Enlightened Europe was waiting for the results of a so brilliant education. Actually, it was a failure, as told by Elisabeth Badinter in the book: ‘L'infant de Parme’ (Badinter, Reference Badinter2008).

19th and 20th centuries

As observations of planets became more and more accurate, the planet Mars became an important object of studies. The problem of Mars canals is very famous and already extensively studied; so we shall briefly mention it.

The Italian astronomer Angelo Secchi (1818–1878) observed some dark elongated structures on the surface of Mars for which he used the Italian word canali (Secchi Reference Secchi1858). It is difficult to know if by canali, word which was later translated by canals, Secchi meant natural or artificial structures. Since 1877 the same word was used extensively by Giovanni Schiaparelli (1835–1910). However, let us note that Schiaparelli mentioned, at least in some papers, that the he did not know what precisely defined objects correspond to the so-called canals (Schiaparelli Reference Schiaparelli1882a, Reference Schiaparellib). The dark colour of some parts of the planet was first explained by the presence of seas, but when canals were seen across the seas, the dark colour of seas was attributed to some vegetation. Moreover the changes of Mars’ colours according to various seasons of Mars suggest a vegetation similar to Earth's vegetation, which colours can change with the seasons. The canals were supposed to have been built by intelligent beings for cultivation irrigation. More and more canals were supposed to be observed and a detailed nomenclature was established.

However, there were some astronomers who never believed in Mars canals. When some photographies could be obtained, curiously they confirmed the opinions of both canals’ believers and canals’ detractors.

The hypothesis of some vegetation on Mars led to many observations and studies on Mars. Not only the conditions of possible vegetation on Mars were extensively studied, as we shall see below by the very interesting example of Tikhov, but also the hypothesis of intelligent builders gave rise to a lot of imaginative and very popular stories. The idea of an extraterrestrial intelligent life became obvious or at least conceivable for everybody.

A paper in the French popular science magazine, La Nature, in Reference Sternfeld1935, Life in the Universe by Ary J. Sternfeld (1905–1980) appears surprisingly modern. This paper begins by a section entitled Beliefs, Representations, Hypotheses. The second part is entitled Origin of life and begins by: ‘The development of natural and astronomical sciences leads to the birth of a science whose object is the habitability of other worlds, astrobiology.’ This is probably the oldest use and definition of the word Astrobiology in its present meaning. The other sections of the paper concern various theories of the origin of life, transfer of life, the biological point of view and the astronomical point of view. Sternfeld also reviewed the properties of the atmospheres of various planets and stated that Titan, the largest satellite of Saturn probably has an atmosphere. The conclusion of the paper is ‘Our main conclusion is that all possibilities remain open and that nothing has been proved.’ This article took into account the very latest scientific discoveries, presented several original ideas, and did not engage in imaginative extrapolation. The Sternfeld's discussion of life in the Universe is remarkably similar to the way we view astrobiology today. Let us note that this article was written 77 years ago, the same time interval as between the publication of the Sidereus Nuncius of Galileo (Galilei Reference Galilei1610) and Conversations on the Plurality of Worlds by Fontenelle (Reference Fontenelle1686).

This paper is so interesting and is so ahead of its time that it is worth to tell more about the author. Ary Sternfeld was born in Poland in 1905, in a Jewish family. Because of a numerous clauses for Jewish students he could not attend the University that he chose. He came to France to study at the Mechanical Institute at Nancy University. After brilliant studies he worked as an engineer. He wanted to do a PhD in astronautics, but at this time, that is to say thirty years before the spatial era, French academics were not interested in astronautics. He wrote a book Introduction to Cosmonautics, cosmonautics being a word that he coined. Then, in 1935, he went to Russia (Soviet Union) to work in an institute of astronautics. As he graduated from a French University and won a French research price in astronautics, and perhaps also because he was a Jew, he was dismissed from the institute, so escaping to ferocious Stalinist purges which took place in this institute a few weeks later. Afterwards he could never work again in an institute of astronautics. He devoted his time to the writing of books and articles to popularize science. He wrote 30 books and 400 papers, which were very popular, translated into 36 languages, and published in 39 countries. He could never come again to France. He died in 1980. Nowadays Sternfeld is considered as a pioneer in astronautics but he also deserves recognition as a pioneer of astrobiology. More information can be found about Ary Sternfeld in Gruntman (Reference Gruntman2007) and Briot (Reference Briot2012a).

In 1941, 6 years later, in an article entitled Astrobiology, Laurence J. Lafleur (Reference Lafleur1941) defined astrobiology as ‘the consideration of life in the universe elsewhere than on Earth’. Lafleur (1907–1966) was a philosopher specialized in science and a translator of the French scientist and philosopher René Descartes.

It is ironic that Sternfeld means Star Field in German and in Yiddish, and Lafleur means The Flower in French !

Between 1945 and 1960, the problem of search for life in the Universe was thoroughly treated at Alma-Ata (Kazakhstan, Soviet Union), in an observatory focused on Astrobotany that is the research of a possible vegetable life on an other planet, particularly the Mars planet. This observatory was created and directed by the Russian astronomer Gavriil Adrianovich Tikhov (1875–1960).

Tikhov was born in Bielorussia. After his studies at the university in Moscow, he came to France to attend courses at the Sorbonne, the Paris University. He met some astronomers at the Paris Observatory and he worked with Jules Janssen, then director of the Meudon Observatory. He went up in a balloon to observe the Leonids and he went twice on the Mont Blanc for observing the Sun in an observatory established by Janssen on the top of this mountain. He came back in Russia and he became an astronomer at Pulkovo observatory near Saint Petersburg in 1906. In 1941, just before the beginning of the siege of Leningrad (Saint Petersburg), he went away to participate in an expedition to Alma-Ata Observatory (Kazakhstan) to observe a total solar eclipse. After the war and until his death he worked in Alma-Ata where an institute of Astronomy and Physics was created. His range of interests was very broad: besides astrobotany and astrobiology, he studied Sun, planets, comets, Earthshine, the blueness of the sky and its polarization, variable stars, interstellar absorption, development of new astronomical instruments, etc.

The most interesting part of Tikhov's research was his investigations on life on Mars then on other planets. He observed Mars during oppositions, when the Earth–Mars distance is the shortest, in 1909, 1918 and 1920. As many astronomers he believed in Mars canals, implying a Martian vegetation. However, he was unable to identify the chlorophyll in his Mars spectra. So, he researched terrestrial plants without chlorophyll and growing in Mars-like conditions. He supposed vegetation on Mars as blue or purple low plants, being some kinds of mosses or lichens and growing on a red soil.

He coined the word Astrobotany in 1945. The study of reflectance spectra of various plants became so important that in 1947 he founded a Department of Astrobotany with students, biologists and physicists. Expeditions were sent in regions where the extreme environmental conditions could be similar to those of Mars, e.g. high mountains or polar regions, to study the reflectance of plants growing in these places. Considering that microorganisms have remarkable properties of adaptation when put in different environmental conditions, astrobotanists studied extreme conditions required for life on Earth and investigated conditions on various planets for the emergence of a primitive life. So they broadened their field of research into astrobiology then cosmobiology. Tikhov used the word cosmobiology for the study of conditions for life in other planets, and particularly planets orbiting around other stars than our Sun. In 1949 he published a book entitled Astrobotany and in 1953 another named Astrobiology. Unfortunately, the laboratory was suppressed after the Tikhov's death in 1960, as the new director did not believe in life on Mars or on other planets. The work of Tikhov appears nowadays surprisingly modern. Because of the separation and lack of communication then existing between the Soviet bloc and other countries, Tikhov's work remains to this day largely unrecognized (Briot, Reference Briot and Vakoch2012b).

The first American symposium on astrobiology was held in 1957 (Wilson Reference Wilson1958, and following papers). However, the papers presented concerned not only life in other celestial bodies but also unrelated problems common to astronomy and biology, for example physiological problems of astronomical observations.

In 1965, Mamikunian (Mamikunian Reference Lederberg1965) attributed to Joshua Lederberg (Reference Lederberg1960) the coining of the word exobiology.

Importance of astrobiology in astronomical scientific publications during the 20th century

A good source of information about the importance given to the science of Astrobiology at various times can be found in the contemporary books of astronomical bibliography. The astronomy bibliographic books Astronomische Jahresbericht were published every year from 1899 until 1968 by the Astronomisches Rechen-Institut Heidelberg. These books in German made an inventory of astronomical papers published during the past year. It is difficult to be sure that the inventory is complete because many publications and journals were edited in many different languages by observatories all around the world during this period. Papers were indexed by keywords and by authors. In 1969, the volumes Astronomische Jahresbericht were replaced by Astronomy and Astrophysics Abstracts, published in English every year, which lasted until the year 2000. Since 2000, references of astronomical papers can be found on the Internet established by the Astrophysics Data System (ADS). The ADS is operated by the Smithsonian Astrophysical Observatory and NASA.

Using annual books of astronomy bibliography is a good index of how important the various sectors of astronomy were considered at different times.

It is to be noted that several words exist for the science presently studied: it can be named Astrobiology, Exobiology, Bioastronomy and sometimes Cosmobiology. So, at least three or four words exist to indicate only one science. Actually, synonyms are very rare in the language of science where one word corresponds to only one object and vice versa. More often, there is a one-to-one relationship between the word and the object. The use of these different words as keywords in the books of astronomical bibliography has to be studied.

In the volumes of Astronomische Jahresbericht (1899–1968), the word ‘astrobiology’ appeared as a keyword as early as 1953, but only in this year, whereas the word ‘astrobotany’ appeared as a keyword in 1949, and again from 1951 to 1954. Papers about astrobotany were mostly written by Tikhov or by scientists from the astrobotany section in Alma-Ata. The phrase ‘Leben im Kosmos’, i.e. ‘Life in the Universe’, was a keyword as early as 1949, but these papers were classified under the heading ‘Schriften allgemeiner Art’ which corresponds to Miscellanea, like ‘Astronomy and Philately’ for example. At the end of the forties, and during the fifties, the papers concerning this subject were still rare but slowly became more and more numerous. Many papers came from the Soviet Union or related countries. Life and vegetation on Mars was the subject of many papers and their number decreased when it appeared more and more clearly that there is no vegetation on Mars. In 1959, the item ‘Leben im Kosmos’ gained some importance and became a rubric, instead of being classified indiscriminately under ‘Schriften allgemeiner Art’. When Astronomische Jahresbericht were replaced by Astronomy and Astrophysics Abstracts in 1969, surprisingly, papers corresponding to the search of extraterrestrial life were debased: they were classified as ‘Miscellanea’. The keywords ‘Extraterrestrial intelligence’ and ‘Extraterrestrial life’ could be found in the Author and Subject Index volumes. None of the words ‘astrobiology’, ‘bioastronomy’ or ‘exobiology’ appeared among the keywords, even in the last volume of Astronomy and Astrophysics Abstracts in 2000. Astrobiology was still treated as a marginal science. However, papers about extraterrestrial life became progressively more and more numerous and were often classified under scientific headings. So, the science Astrobiology remains nearly on the fringe of the mainstream of science up till year 2000.