---
title: "History of the extraterrestrial life debate"
chunk: 1/4
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"
---

The existence of extraterrestrial life is a scientific idea that has been debated for centuries. Initially, the question was purely speculative; in modern times a limited amount of scientific evidence provides some answers. The idea was first proposed in Ancient Greece, where it was supported by atomists and rejected by Aristotelians. The debate continued during the Middle Ages, when the discussion centered upon whether the notion of extraterrestrial life was compatible with the doctrines of Christianity. The Copernican Revolution radically altered mankind's image of the architecture of the cosmos by removing Earth from the center of the universe, which made the concept of extraterrestrial life more plausible. Today we have no conclusive evidence of extraterrestrial life, but experts in many different disciplines gather to study the idea under the scientific umbrella of astrobiology.

== Ancient Greece ==

During the early days of the history of astronomy the things seen in the night sky were explained as the actions of mythological deities. However, it soon became evident that celestial objects move and behave in regular and predictable patterns, which helped in keeping track of time, tides, and seasons, crucial for ancient agriculture. Most ancient civilizations had great knowledge of astronomy but only used it for religious and practical needs. Ancient Greek astronomy sought to go beyond that and explain the architecture of the cosmos.
Thales of Miletus sought to explain the nature of the universe without relying on supernatural explanations, and reasoned that Earth was a flat disk floating on an ocean of water. The idea was not widely accepted even then, but it established the underlying idea that the universe is intrinsically understandable. Greek philosophers did not follow the scientific method but based their ideas on pure thought instead. However, their discussions laid some principles that would eventually lead to it, such as the rejection of supernatural explanations and that ideas would not be valid if they were contradicted by observable facts. They also developed geometry, which helped with architecture and other practical tasks, but also with astronomic observations.
The initial idea of a flat Earth covered by a celestial dome was soon discarded. Thales' student Anaximander proposed a full celestial sphere instead. He also noticed evidences of the curved surface of the world and proposed that the Earth was shaped like a cylinder. Most other Greeks, however, preferred the proposal of Pythagoras that Earth was a perfect sphere, as they associated circles and spheres with mathematical perfection. The model of the celestial sphere works for distant stars, which seem to be at fixed locations in the sky to the naked eye, but the Sun and the Moon move at different speeds and the other classical planets follow complex paths and vary in their brightness. This was explained by adding other layers to the celestial sphere. This was detailed in the Ptolemaic model. Aristarchus of Samos proposed instead that it is Earth that spins around the sun, which makes it easier to explain the retrograde motion of the classical planets, but this was rejected by other Greeks. They pointed out that if Earth moves a stellar parallax would change the location of stars in the sky during the year. Although stellar parallax does exist, stars are too far away from Earth, more than Greeks considered, to be noticeable by the naked eye.