The whole universe may once have been warm enough for liquid water
For roughly seven million years, starting about 10 million years after the Big Bang, the cosmic background glowed between the boiling and freezing points of water. Astrophysicist Avi Loeb has speculated that primitive life could in principle have arisen then, in a cosmos-wide habitable epoch, long before the Sun existed.
No life beyond Earth has ever been detected, yet many scientists expect it. The case, argued by figures such as Carl Sagan and Stephen Hawking, rests on three pillars: the universe is vast and old enough for many Earth-like histories to unfold; carbon, water and the other ingredients of life are forged in stars everywhere; and physical laws are the same throughout. The opposing Rare Earth hypothesis holds that our planet's lucky combination of galactic position, solar system layout and local features is unlikely to recur, and that belief in aliens is more hope than inference.
Speculation is ancient. Augustine discussed the countless worlds imagined by Democritus and Epicurus; in 1440 Nicholas of Cusa suggested every celestial body, even the Sun, might be inhabited; and in the 15th century William Vorilong wondered whether Jesus might redeem beings on other worlds. Descartes admitted no one could prove the stars were empty of intelligent creatures.
Where to look is contested. A star's habitable zone, where surface water can stay liquid, is neither sufficient nor necessary: Venus lies inside ours yet is dry, while Europa, far outside, probably hides an ocean kept liquid by tidal heating. Earth's own extremophiles, thriving around hydrothermal vents, acidic springs and deep underground, suggest life adapts widely, though its origin may demand stricter conditions. Researchers focus on Venus, Mars, Europa and Titan.
Distance is the great obstacle. New Horizons needed nine years to reach Pluto, and Voyager 2, at about 50,000 km per hour, would take some 100,000 years to cover the 4.4 light-years to Alpha Centauri. In 1961 Frank Drake wrote his famous equation to spark discussion at a SETI meeting, multiplying factors from star formation rates to how long civilisations broadcast. His own guesses gave 10,000 communicating civilisations, but most terms are unknown, and some may be unknowable. Whether humanity should actively signal others remains hotly debated.
Source: Extraterrestrial life
