Where to Go: The Search for Habitable Planets

Philosophers and scientists speculated for centuries about the existence of extra solar systems. This speculation was not very common among ancient Greek scientists. Given the scale of the universe generally accepted at their time, the existence of extra solar systems gathered very little support. According to Aristotle and Plato, the universe was a finite sphere. Its ultimate limit was the “primum mobile”, whose diurnal rotation was conferred upon it by a transcendental God, not part of the universe, a motionless prime mover and first cause.