A Universe of Earths

Planet Earth has been a familiar concept for a mere fraction of recorded history. Until about the mid-1600s, most humans thought of Earth as immobile, likely either dim or simply invisible from the Moon or anywhere else in the heavens, and not (like the planets) participating in what Galileo called "the dance of the stars." But almost as soon as humans started to grasp that Earth is a planet, many also began wondering if perhaps the other planets might be earths. This bold conjecture ignited the whole gripping history and literature of space travel, of extraterrestrials, of other worlds.

And yet the thesis that the Universe is full of other worlds like Earth has from the start been fuelled more by imagination than by scientific evidence. For all its appeal, it has consistently been undermined by observations of the actual Universe. This talk will be based on material in the new book from Oxford University Press, A Universe of Earths: Our Planet and Other Worlds, from Copernicus to NASA by Dennis Danielson and Christopher M. Graney, and on their related cover article in the March 2026 issue of Sky & Telescope magazine.

Presenter: C.M. Graney, Vatican Observatory