20th century

=== Astronomy ===
* A much better understanding of the evolution of the [[universe]] was achieved, its [[Age of the universe|age]] (about 13.8&nbsp;billion years) was determined, and the [[Big Bang]] theory on its origin was proposed and generally accepted.
* The age of the [[Solar System]], including Earth, was determined, and it turned out to be much older than believed earlier: more than 4&nbsp;billion years, rather than the 20 million years suggested by [[Lord Kelvin]] in 1862.<ref>{{cite journal |last=Thomson |first=Sir William |title=On the Age of the Sun's Heat |journal=Macmillan's Magazine |year=1862 |volume=5 |pages=288–293 |url=http://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html}}</ref>
* The planets of the Solar System and their moons were closely observed via numerous [[space probe]]s. [[Pluto]] was discovered in 1930 on the edge of the Solar System, although in the early 21st century, it was reclassified as a [[dwarf planet]] instead of a planet proper, leaving eight planets.
* No trace of life was discovered on any of the other planets orbiting [[Sun|the Sun]] (or elsewhere in the universe), although it remained undetermined whether some forms of primitive life might exist, or might have existed, somewhere in the Solar System. [[Extrasolar planets]] were observed for the first time.