Earth as a Star: Reframing the Copernican Revolution in A Universe of Earths by Dennis Danielson and Christopher M. Graney

A Universe of Earths: Our Planet and Other Worlds, from Copernicus to NASA, by Dennis Danielson and Christopher M. Graney, Oxford University Press, 2026, 202 pp, ISBN: 978-0197803516 (Hbk.).

Copernicus was correct that Earth orbits the Sun, in contrast to the prevailing model of his time in which the Sun and planets all move around Earth. But the idea that Copernicus somehow demoted Earth from its “privileged” position in the center of the universe, relegating us to mediocrity, turns out to be inverted. Dennis Danielson and Christopher M. Graney refer to this idea as the “Great Copernican Cliché,” and their book A Universe of Earths: Our Planet and Other Worlds, from Copernicus to NASA offers a new interpretation of history based on their analysis and translation of original sources. Rather than relegating Earth to a place of mediocrity, Danielson and Graney argue that Copernicus propelled Earth into “the dance of the stars” (in the words of Galileo) by elevating our status from a lowly clump of dirt and into the heavens. This realization brought the search for life in the universe into the domain of science, and this reading of history carries important lessons for those of us interested in searching for extraterrestrial life today.

Before the era of telescopes, the “wandering stars” or planets in the sky had nothing in common with Earth. The changes in the positions of the planets over time suggested that they were different than the “fixed stars” that rotated around us once per day, but both wandering and fixed stars occupied a lofty position above us in the heavens, made of different stuff than Earth. Danielson and Graney refer to this pre-Copernican model as a “two-storey Universe,” with Earth below and the stars above. The Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn all revolved around Earth in spheres at increasing distances, with the fixed stars occupying the outermost layer—beyond which lies “the dwelling of God and of all the elect.” Upward is the direction of improvement, exaltation, and heaven itself, whereas downward is the direction of deterioration, death, and hell. As an example of their numerous sources, Danielson and Graney show that this perspective was articulated as early as the 5th century in the writings of Martianus Capella, who wrote that “Earth is ‘in the middle and at the bottom’ position in the Universe.” In contrast to this medieval cosmology, the authors write, Copernicanism entailed “Earth’s exaltation, pulling it out of the filth, making it like the rest of the planets, a shining wandering star—decidedly not a sump or sentina, the term Galileo uses in his original Latin to evoke the place in a ship below the lowest decks, where bilge water collects.”

Copernicus raised our planet upward, breaking the two-storey model of the universe and propelling Earth from its miry position in the center—where filth and dirt collect from gravity—and into a celestial body comparable to the other wandering stars, the planets. This model of heliocentrism faced critics ranging from church authorities to poets, as it undermined the separation of sinful Earth from the celestial spheres and relegated the majestic Sun to the undesirable center. As an example, Danielson and Graney note that the first published critique of Copernicus’ Revolutions by Giovanni Maria Tolosani lamented that the new model “puts the indestructible Sun in a place subject to destruction.” Copernicus himself attempted to respond to such critics by suggesting that the center is not such a bad place to be: “Behold, in the midst of all resides the Sun. For who, in this beautiful temple, would set this lamp in another or a better place, whence to illuminate all things at once?” It seems that Conpernicus’ rhetoric may have worked a bit too well. His poetic plea about the Sun’s central location is precisely because he knew he was elevating Earth into one of the stars and dethroning the Sun from its privileged position. But this story, for numerous reasons discussed in the book, has now inverted itself in the form of the Great Copernican Cliché. Copernicus elevated our planet, rather than demoting it, which enabled the possibility of imagining other planets as other earths.

With the demise of the two-storey universe and the elevation of Earth into the celestial spheres, some astronomers began to speculate that the fixed stars were other suns, perhaps even with their own earths in orbit. Giordano Bruno championed these ideas, but Johannes Kepler remained unconvinced that the stars were like the Sun, writing to not let Bruno “lead us on to his belief in infinite worlds, as numerous as the fixed stars and all similar to our own.” Danielson and Graney give a detailed and convincing account of Kepler’s arguments that the stars cannot be like the Sun, using the best science of his day. A core problem revolved around the sizes of stars: Seen through a telescope, the planets and bright stars all showed measurable diameters. Mercury and Sirius showed identical diameters in a telescope, for example, but Mercury is much closer; this would require stars like Sirius to be enormous—larger than the known solar system—and located at vast distances. Anti-Copernicans criticized this conclusion, but Copernicans like Kepler embraced it. For Kepler, the “Plurality of Worlds” (as it later came to be known after the work of Bernard Le Bovier de Fontenelle) had no scientific basis, and it was more tenable to accept that the solar system is unique and the stars are vastly different. (It is a bit ironic that the planet-finding Kepler space telescope was named after a scientist who was convinced that exoplanets did not exist).

Despite the lack observable evidence, the Plurality of Worlds concept continued to inspire scientific and popular speculation that other planets might be inhabited like Earth. Danielson and Graney suggest that analogical reasoning formed the basis for much of this thinking, such as comments in the late 17th century by Christian Huygens that anyone who accepts the conclusions of Copernicus “cannot but sometimes have a fancy that it’s not improbable that the rest of the planets have their dress and furniture, nay and their inhabitants too as well as this earth of ours.” Even critics of the idea of a universe populated by intelligent beings still considered life to be a ubiquitous property of the Universe. For example, William Whewell argued in the mid-19th century that Earth resided in a narrow “temperate zone” that limited intelligent life to our planet alone; however, he also speculated on life existing on Jupiter as “aqueous, gelatinous creatures; too sluggish, almost, to be deemed alive.” Analogy may have implied a universe teeming with life, but scientific discoveries veered in the opposite direction, finding increasing evidence that life as on Earth is not commonplace among planets. Mars and Venus showed themselves to be inhospitable places to visit, and the rest of the solar system likewise appeared bereft of life. Science has also vindicated Kepler to an extent, as he was correct in surmising that that the sizes and properties of the stars are far more diverse than the Sun alone. Only a fraction of stars are like the Sun, and the population of known exoplanets shows a vast array of architectures compared with our solar system. The belief in a universe teeming with life, and intelligence, has been met with discoveries of dark skies, barren neighboring planets, and continued null results.

This historical perspective does not mean that Earth is the only inhabited planet. Danielson and Graney emphasize that the search for life is a valuable scientific pursuit with far-reaching consequences, whether this be the discovery of microorganisms on Mars or a communicative radio signal from our interstellar neighbors. But the authors implore all of us interested in looking for life to also contemplate the history of these ideas and recognize that the Plurality of Worlds has been continually undermined, rather than supported, by ongoing discoveries. We can hope to construct large enough observatories someday to find evidence of spectroscopic biosignatures—and even technosignatures—in the atmospheres of nearby exoplanets, and perhaps we will even find some. Yet this is a far cry from imagining that every star is a sun with an inhabited earth around it. We will indeed be lucky if life is commonplace enough in the universe that we have another example nearby to study. We should continue to search, as Danielson and Graney implore, but we should be realistic and honest about our motivations for searching. One cannot deny that the idea of a universe populated by a Galactic Club of sorts, that we may perhaps join, is indeed exciting. The authors also gently suggest that our desire to cling onto the concept of extraterrestrials may be a vestige of the old two-storey universe: The pre-Copernican view that Earth is mundane, dirty, and backward compared with the heavens remains prevalent in our post-Copernican discourse today. We may still find evidence of life elsewhere, and the implications would be undeniably be profound. But as we search, we should bear in mind that history has revealed Earth to be a unique and precious planet, alone in the blackness of space, and the only place in the universe that we can call home.