A fringe theory called "panspermia" suggests that lifeforms can spread to new planets by hitching rides on meteors. New research lays out a roadmap for finding where these hypothetical, planet-hopping aliens may reside.

This summer, director Fede Alvarez ­returns the notorious Alien franchise to its monster-movie roots, and feeds yet another batch of hapless young space colonists to a nest of “xenomorphs”. It remains to be seen if Alien: Romulus will do more than pay loving tribute to Ridley Scott’s original 1979 Alien, but it hardly matters – Alien is a franchise...