What does it mean for a planet to be a world? How did we come to understand that our sun is just another one of the stars? Many are familiar with the shift from an earth-centered cosmos to a sun centered one. In parallel to that story, there is a story of a plurality of worlds and the realization that each star in the sky is a sun like our own but incredibly far away.

The story touches on ideas of life on the moon, on clever advances in measuring distance, and the astonishing ability to figure out what things are made of based on the light they give off. In the later half of the 17th century, Descartes' ideas of vortices and Newtonian mechanics begged a vexing question. If the fixed stars were not part of an outer shell around our solar system but instead were suns like our own, then what was our place in the cosmos? What exactly was the scale of this new universe?

An Infinite Universe Teaming with Worlds

What is a world? In part, spurred on by Copernicus's ideas, the Dominican Friar Giordano Bruno published De l'infinito universon e mondi (On the Infinite Universe and Worlds), in 1584. As part of a suite of mystical, magical and heretical ideas, he suggested that Earth was one of many inhabited worlds in an infinite universe and that the stars were suns, which had their own worlds.

From the World to Worlds

As a result of shifting views of the universe the very idea of "world" (in Latin, Mundi) was changing. In the Aristotelian cosmos, the world was effectively synonymous with the Earth. The sphere of the world and the terrestrial realm were one in the same. Once Earth became one planet among many orbiting the sun, those planets became Earth like worlds. This new understanding of worlds is reflected in the title of The Discovery of a World in the Moon from the 1630's. As it took a long time for the Copernican model of the cosmos to win out over competing models, it took a considerable bit of time for ideas similar to Bruno's to come to fruition.

The increasing acceptance of Descartes theory of vortices in the later half of the 17th century brought with it the idea that the stars were like our sun and had their own planets orbiting around them. Bernard le Bovier de Fontenelle's popular 1686 book Entretriens sur la pluralite des mondes (Conversations on the plurality of worlds) broadly disseminated this notion, in a range of editions and translations. You can read a full-digitized copy External of an 1803 English translation of Conversations on the Plurality of Worlds online from the Library of Congress collections.

The Popularity of the Plurality of Worlds

The book Conversations on the Plurality of Worlds is organized, as the title suggests, as a series of conversations.  The book presents fictional discussions between a philosopher and his hostess, a marquise. As the two characters walk the grounds of her garden at night they discuss the stars above them. Their conversations touch on the features of the Copernican system, potential encounters with extraterrestrial life and the idea of the universe as a boundless expanse. Written in this accessible format, it found a broad audience. As the book was translated into a variety of languages and republished in new editions for hundreds of years, it presented both this cosmology and the idea of life on other worlds to a range of audiences.

The popularity of this idea, the plurality of worlds orbiting their own sun like stars, is evident in the extent to which visual representations in the frontispiece of Fontenelles' book appear in other places. Similar depictions of other solar systems shrouded in clouds beyond our own appear in a range of maps and diagrams.  For an example of one of these depictions of small cloud shrouded solar systems, see the upper right corner of the 1742 Systema Solare Et Planetarium.

Stars and Their Worlds as the Third System

Changing ideas about the structure of the universe are well illustrated in diagrams from William Derham's 1715 book Astro-Theology. Derham, an English natural philosopher, astronomer and clergymen wrote a series of works exploring connections between natural history and theology.

Derham provides a diagram of three systems of the cosmos, explaining that figure 3 shows "the Fixt Stars with their Systemes (represented by little Circles about those Stars, which Circles signify the Orbits of their respective Planets) are placed without the limits of the Solar Systeme, and the Solar Systeme is set in the Center of the Universe, and figured as a more grand and magnificent part there of." From his perspective, the shift to thinking about the plurality of worlds was significant enough that it should be set alongside the Copernican Revolution as one of the three major shifts in thinking about the nature of the universe.

Exactly how far away are the stars?

The move away from thinking about the stars as being affixed to an outer shell of the universe was tricky, in part because the stars really look to move altogether. The inability to detect any form of stellar parallax, any relative motion of the stars, had been an issue for astronomers. Simply put, it looks like the stars are fixed. This had been a long-standing argument for the idea of an Earth centered universe. The idea being that if the Earth was moving one should expect that the stars should change their position relative to us. As we orbit the sun we would expect to see their position in the sky change as we got closer and further away from different individual stars. This is where the idea of parallax becomes important.

Measuring Stellar Parallax

If you think of those two opposite sides of the earth's orbit as setting up two different lines of sight then we should see the star move relative to the other stars in that movement. Throughout the 16th century astronomers attempted to measure stellar parallax, but telescopes weren't advanced enough to be able to detect parallax. What is particularly important about measuring stellar parallax is that it gives one the ability to calculate the distance to a star. The distance between the two extremes of the Earth's orbit was known so the change in the location of the star enables one to calculate the distance to it.

In the 1830s, advances in the design of telescopes enabled scientists to detect parallax which kicked off a race to be the first to detect it. Ultimately it was Friedrich Wilhelm Bessel in 1838 who won the race and discovered that 61 Cygni had a parallax of 0.314 arcseconds.  An arcsecond is 1/3600 of a degree in a circle. That gives you a sense of just how tiny that movement is to detect. Given the diameter of the Earth's orbit suggested that the star was 10.4 lightyears away. That translates into roughly 61,000,000,000,000 miles away. He narrowly beat Fredrich Georg Wilhelm Struve and Thomas Henderson who, in the same year, measured the parallaxes of Vega and Alpha Centauri.

Prisms and Star Stuff

In 1835, the French philosopher Auguste Comte had suggested that, given that investigations of the stars would always be visual, we would never be able to know what the stars were made of.  His perspective on the limits of science makes sense; however, he would quickly be proven wrong.

Working with prisms, natural philosophers in the 18th century had learned that light was made of a series of different colors that can be broken apart with a prism. The resulting colors from the prism are the light spectrum. Studying the spectrum of light from different stellar sources would offer profound results.

In the first half of the 19th century, chemist William Wollaston noticed and wrote about the existence of black lines in the solar light spectrum. In 1814, Joseph Fraunhofer found the same kinds of lines. Through studying the spectrum of light, he found a consistent bright fixed line that appeared in the orange color of the spectrum. Given that the lines did not seem to mark the boundaries of one color to the next, he dismissed the idea that these were boundaries between different colors. When he studied the spectrum of light from the sun, he found 574 dark lines in the solar spectrum. These lines were named Fraunhofer lines after him.  Interestingly, he found that the star Sirius and several other stars differed in their spectral lines from the Sun. 

The Chemistry of the Heavens Visible in Light

Many researchers studied the spectrum of flames in their laboratories. Gustave Kirchhoff and Robert Bunsen made a series of critical advances. In 1860 they published on research that showed that different elements emitted different colors of light in the spectrum. Connecting the studies of elements spectra in the laboratory with the known spectrum of the sun and stars enabled researchers to identify the chemical composition of the stars. In short, the dark lines in the spectrum offered a kind of fingerprint for the elements that make up a star. Some of the lines in the solar spectrum change depending on the placement of the sun in the sky, indicating that some of the lines are the result of the interference of the atmosphere. Given this, Kirchhoff was able to study the spectrum of the sun and argue that iron, calcium, magnesium, sodium, nickel and chromium were present in outer layers.

Further advances were made in 1863 when Italian Astronomer, Father Pietro Angelo Secchi went on to collect spectrograms of over 4,000 stars. He found that he could distinguish a number of distinct kinds of stars that had different spectral patterns. The other stars were suns, but they weren't exactly like our sun. In his classifications he identified blue and white stars that had spectra like Sirus, stars that had spectra similar to the sun, red stars with bands, and carbon stars. Not only had astronomers figured out how to study what the sun was made of, they had also discovered that there were a range of different kinds of stars.

We now know that our sun is a star, but through increasingly sophisticated telescopes and some basic trigonometry we were able to figure out how far away the stars are. Beyond that, by understanding the properties of light we were able to deduce what the stars are made of. From our tiny home here on Earth we have been able to learn an amazing amount about the stars. Knowing just how far away they are opens up the immensity of our cosmos. However, even at this enormous scale, we still hadn't come to realize that most of those stars were just in a small corner of the universe we call the Milky Way galaxy.