The IAU working definition of an exoplanet

In antiquity, all of the enduring celestial bodies that were seen to move relative background sky stars considered planets. During Copernican revolution, this definition was altered objects orbiting around Sun, removing Sun and Moon but adding Earth list known The concept planet is thus not simply a question nature, origin, composition, mass or size, historically related motion one body other, in hierarchical configuration. After discussion within IAU Commission F2 "Exoplanets Solar System", criterion star-planet ratio has been introduced term "exoplanet", thereby requiring structure our System for an object be referred as exoplanet. Additionally, planetary brown dwarfs, provided they follow criterion, are now exoplanets. Therefore, current working exoplanet, amended August 2018 by reads follows: - Objects with true masses below limiting thermonuclear fusion deuterium (currently calculated 13 Jupiter solar metallicity) orbit stars, dwarfs stellar remnants have central $L_4$/$L_5$ instability ($M/M_{\rm central}$$<$$2/(25+\sqrt{621}$)$\approx$1/25) "planets", no matter how formed. minimum mass/size required extrasolar should same used System, which sufficient both self-gravity overcome rigid forces clearing neighborhood object's orbit. Here we discuss history rationale behind definition.