Clearing the neighbourhood
"Clearing the neighbourhood" is one of three necessary criteria for a celestial body to be considered a planet in the Solar System, according to the definition adopted in 2006 by the International Astronomical Union (IAU).
[2] The phrase stems from a paper presented to the 2000 IAU general assembly by the planetary scientists Alan Stern and Harold F. Levison.
[3] Steven Soter prefers to use the term dynamical dominance,[4] and Jean-Luc Margot notes that such language "seems less prone to misinterpretation".
[2] Prior to 2006, the IAU had no specific rules for naming planets, as no new planets had been discovered for decades, whereas there were well-established rules for naming an abundance of newly discovered small bodies such as asteroids or comets.
The naming process for Eris stalled after the announcement of its discovery in 2005, because its size was comparable to that of Pluto.
[2] In their paper, Stern and Levison sought an algorithm to determine which "planetary bodies control the region surrounding them".
In the domain of the solar planetary disc, there is little variation in the average values of k for small bodies at a particular distance from the Sun.
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.
Steven Soter proposed an observationally based measure μ (mu), which he called the "planetary discriminant", to separate bodies orbiting stars into planets and non-planets.
where μ is a dimensionless parameter, M is the mass of the candidate planet, and m is the mass of all other bodies that share an orbital zone, that is all bodies whose orbits cross a common radial distance from the primary, and whose non-resonant periods differ by less than an order of magnitude.
[4] The order-of-magnitude similarity in period requirement excludes comets from the calculation, but the combined mass of the comets turns out to be negligible compared with the other small Solar System bodies, so their inclusion would have little impact on the results.
Π is based on properties that are feasibly determinable even for exoplanetary bodies, unlike Soter's μ, which requires an accurate census of the orbital zone.
Π's use of the main-sequence lifetime means that the body will eventually clear an orbit around the star; Λ'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.
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.
He argued that the IAU's wording is vague, and that — like Pluto — Earth, Mars, Jupiter and Neptune have not cleared their orbital neighbourhoods either.
Earth co-orbits with 10,000 near-Earth asteroids (NEAs), and Jupiter has 100,000 trojans in its orbital path.
In the paper proposing Stern and Levison's Λ discriminant, they stated, "we define an überplanet as a planetary body in orbit about a star that is dynamically important enough to have cleared its neighboring planetesimals ..." and a few paragraphs later, "From a dynamical standpoint, our solar system clearly contains 8 überplanets" — including Earth, Mars, Jupiter, and Neptune.
