The twentieth century has seen a resurgence in interest in a question which was once the province of religion rather than science: are we alone in the universe, or is there a plurality of worlds? The SETI (Search for Extraterrestrial Intelligence) project is one attempt to answer the question, listening out for radio messages that might indicate the presence of other beings.
A more recent one has been the search for direct evidence of planets circling other stars. They are much too small to be observed directly, even if the light from their parent stars didn’t obscure them, so they have to be sought by subtle astronomical observations that detect changes in the light coming from their stars brought about by wobbles caused by the invisible companion.
In the past eighteen months the discovery of several such planets has been announced. Their formal name is extrasolar planets, but this has been abbreviated to exoplanet, employing the Greek prefix exo-, “outside; external”, which turns up in a number of English words such as exotic, exoskeleton, and exogenous. The adjective is exoplanetary.