The word exoplanet first appeared in the mid-nineties, when initial discoveries of such planets were announced. It quickly became adopted as an abbreviation of the more formal expression extrasolar planet (extra- = ‘outside’ or ‘beyond’, compare e.g. extraterrestrial, extracurricular), but has still largely remained outside of public radar until the breakthrough discovery in February 2014 brought it more firmly into the mainstream spotlight. There’s also some evidence for a derived adjective exoplanetary and related terms exocomet and exomoon.