Short answers, pulled from the story.
Extraterrestrial life refers to any life that originates from a world other than Earth. No extraterrestrial life has yet been detected, despite ongoing scientific searches spanning the Solar System and beyond.
The Drake equation was devised in 1961 by astronomer Frank Drake to estimate the number of active, communicating civilizations in the Milky Way. It multiplies factors such as the rate of star formation, the fraction of stars with planets, and the fraction of planets where life and intelligence evolve, though most of its values remain unknown or speculative.
Mars, Europa, Enceladus, and Titan are the strongest candidates. Europa likely has a subsurface ocean kept liquid by tidal flexing. Enceladus releases plumes containing organic molecules, hydrogen, and, as of December 2023, hydrogen cyanide. Mars may have been habitable in the past and could shelter microbes deep underground.
The Fermi paradox describes the contradiction between high estimates of the probability that extraterrestrial civilizations exist and the complete lack of evidence for any. Given that the observable universe contains nearly 2 trillion galaxies, the silence remains unexplained.
The Rare Earth hypothesis, detailed by Peter Ward and Donald Brownlee in their 2000 book Rare Earth: Why Complex Life is Uncommon in the Universe, argues that Earth's habitability depends on an unlikely combination of galactic, solar, and planetary factors. Under this view, complex life is rare in the universe even if microbial life is common.
The Wow! signal was a radio signal detected in 1977 by a SETI project that has never been explained or repeated. It remains a subject of speculative debate among researchers studying the possibility of extraterrestrial intelligence.