---
title: "History of the extraterrestrial life debate"
chunk: 4/4
source: "https://en.wikipedia.org/wiki/History_of_the_extraterrestrial_life_debate"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:31:38.469760+00:00"
instance: "kb-cron"
---

However, some scientists had concerns over the new model. Aristotle had once stated that Earth could not move because, if it did so, birds, clouds, and falling objects would be left behind. Orbits had to be circular because the heavens had to be perfect and unchanging. And if Earth moved, the stars should leave a stellar parallax. Those concerns were addressed by Galileo Galilei. First, he explained that an object in motion stays in motion unless a force stops it; a principle nowadays included in the first of Newton's laws of motion. The idea of heavenly perfection was already being challenged by Tycho's observations. Tycho had observed a supernova, which proved that sometimes the heavens do change. The newly invented telescope also revealed "imperfections" in celestial bodies: the sun was shown with sunspots, and the Moon has many features such as craters and mountain ranges. If the heavens were not as perfect as originally considered, then the idea that orbits are not perfect circles was not so questionable. Galileo also discovered the Galilean moons of Jupiter, celestial bodies orbiting another planet, and the phases of Venus. The existence of the Galilean moons refuted the common argument that the Moon would not stay with a moving Earth. As for the stellar parallax, Galileo could not prove that the stars were more distant than estimated, but got strong evidence suggesting it: a closer look at the Milky Way revealed that it is composed of several stars.
Although those discoveries proved that Earth was not located at the center of everything, they did not completely prove that it spins around the Sun; this fact was fully confirmed when the stellar parallax was measured in detail and with stellar aberration. However, the idea generated such controversy that Galileo was summoned by the Inquisition and forced to recant his findings. Galileo, who was 70 at the time and probably fearing that his life would be at stake, did as ordered. It is said that Galileo muttered "Eppur si muove" (Italian for "And yet it moves"), but most historians doubt it, given the possible consequences Galileo would have faced if heard.
Despite the trial, by 1630 the model of Kepler and the clarifications of Galileo were unanimously accepted. However, although it was accepted that planets moved in ellipses, it was not clear why they did so. The reason was finally explained by Sir Isaac Newton in his book Philosophiæ Naturalis Principia Mathematica, which described the three laws of motion. This book also introduced the law of universal gravitation, and explains all motions in the universe. It also uses maths to explain that Kepler's laws of planetary motion are a natural consequence of the laws of gravitation and motion. With this, the geocentric model was completely discarded.
The Copernican revolution, the time between Copernicus and Newton, was almost 150 years and changed science forever. It changed the view of the universe and the place of Earth and humankind in it, shifting from a central position to just a world like many others. It also changed the way science works. Previous academics were willing to give leeway to mistakes and errors of measurement, which were strictly less tolerated by the new generations. There was also a stronger emphasis to understand not only how nature works, but also why it works that way and not another. Mere guesses like atomism or aesthetic preferences like heavenly perfection would not fly anymore. Any explanation and assumption was required to be proved before being accepted.
Although the dispute was not specifically about extraterrestrial life, the outcome kickstarted it. As there was a conflict between atomists and Aristotelians back in ancient Greece, and the Aristotelians were proved to be wrong, many assumed that this meant that atomists were right and that other worlds were just like Earth. However, the only fact about this that was found at this time was that the stars and the classical planets are not lights but celestial objects analogous to Earth, and that life in them may be plausible, if still unknown. The idea of extraterrestrial life, which was once a radical notion held by limited and specific people, became an accepted idea discussed in college classrooms. The change was also possible because of the changes in religious and philosophical thinking that took place at the time.
Besides that, there was much speculation. Galileo confused the lunar mares with seas. Kepler said that the Moon has an atmosphere and intelligent inhabitants, even writing a science fiction story about them. Dominican philosopher Giordano Bruno accepted the existence of extraterrestrial life, which became one of the charges leveled against him at the Inquisition, leading to his execution.

== Modern times ==
The study of astronomy continued after Newton, and later technological devices and math models allowed to study objects that were undreamt of at the time. Although no actual extraterrestrial life has been found, either in the Solar System or elsewhere, science currently has a far greater understanding of the context of such life or lack thereof. Biology studies the nature of life, and chemistry and biochemistry the way it works. Chemistry and biochemistry also help to understand abiogenesis, the process by which life can be generated by non-living things, which is not yet completely understood. Physics in general and planetary science in particular help to understand the conditions at places other than Earth and how they can be more beneficial or harmful for life. All those sciences are collectively studied under the umbrella science of astrobiology.
Most knowledge of astronomy is relevant in some way for the discussion of extraterrestrial life, but there are three main tenets. One, that the universe is incredibly vast and old. Second, the elements that make up life on Earth are plentiful. Third, that the laws that rule matter are the same across the universe. As a result, it can be reasoned that there is nothing special about Earth, and that life on other worlds should be plausible.

== See also ==
Plurality of worlds – Belief in numerous life-bearing "worlds"
Exotheology – Examination of theological issues as they pertain to extraterrestrial intelligence

== Notes ==

== References ==

== Bibliography ==
Bennett, Jeffrey O.; Shostak, Seth (2016). Life in the universe (4th ed.). United States: Pearson. ISBN 978-0-13-408908-9.
Crowe, Michael J., ed. (2008). The extraterrestrial life debate antiquity to 1915: A source book. Notre Dame, Indiana: University of Notre Dame Press. ISBN 978-0-268-02368-3.