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