The phrase refers to an orbiting body (a planet or
protoplanet) "sweeping out" its
orbital region over time, by
gravitationally interacting with smaller
bodies nearby. Over many orbital cycles, a large body will tend to cause small bodies either to
accrete with it, or to be disturbed to another orbit, or to be captured either as a
satellite or into a
resonant orbit. As a consequence it does not then share its orbital region with other bodies of significant size, except for its own satellites, or other bodies governed by its own gravitational influence. This latter restriction excludes objects whose orbits may cross but that will never collide with each other due to
orbital resonance, such as
Jupiter and
its trojans,
Earth and
3753 Cruithne, or
Neptune and the
plutinos. If > 1, then the body will likely clear out the small bodies in its orbital zone. Stern and Levison used this discriminant to separate the
gravitationally rounded, Sun-orbiting bodies into
überplanets, which are "dynamically important enough to have cleared [their] neighboring planetesimals", and
unterplanets. The überplanets are the eight most massive solar orbiters (i.e. the IAU planets), and the unterplanets are the rest (i.e. the IAU dwarf planets).
Soter's Steven Soter proposed an observationally based measure (
mu), which he called the "
planetary discriminant", to separate bodies orbiting stars into planets and non-planets. Like Stern–Levison's , is a measure of the ability of the body to clear its orbit, but unlike , it is solely based on theory and does not use empirical data from the Solar System. is based on properties that are feasibly determinable even for exoplanetary bodies, unlike Soter's , which requires an accurate census of the orbital zone. \Pi = \frac{m}{M^{5/2}a^{9/8}}\,k where '
is the mass of the candidate body in Earth masses, ' is its semi-major axis in
AU, '
is the mass of the parent star in solar masses, and ' is a constant chosen so that > 1 for a body that can clear its orbital zone. '''' depends on the extent of clearing desired and the time required to do so. Margot selected an extent of 2\sqrt{3} times the
Hill radius and a time limit of the parent star's lifetime on the
main sequence (which is a function of the mass of the star). Then, in the mentioned units and a main-sequence lifetime of 10 billion years, '''' = 807. The body is a planet if > 1. The minimum mass necessary to clear the given orbit is given when = 1. is based on a calculation of the number of orbits required for the candidate body to impart enough energy to a small body in a nearby orbit such that the smaller body is cleared out of the desired orbital extent. This is unlike , which uses an average of the clearing times required for a sample of asteroids in the
asteroid belt, and is thus biased to that region of the Solar System. 's use of the main-sequence lifetime means that the body will eventually clear an orbit around the star; 's use of a
Hubble time means that the star might disrupt its planetary system (e.g. by going nova) before the object is actually able to clear its orbit. The formula for assumes a circular orbit. Its adaptation to elliptical orbits is left for future work, but Margot expects it to be the same as that of a circular orbit to within an order of magnitude. To accommodate planets in orbit around brown dwarfs, an updated version of the criterion with a uniform clearing time scale of 10 billion years was published in 2024. The values of for Solar System bodies remain unchanged. In 2025, Hwang cited Margot's idea to define a planet as celestial body that has largest Margot's . ==Numerical values==