Clearing the neighbourhood

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"text": "In the study of celestial mechanics, the term \"clearing the neighbourhood\" refers to a specific state of gravitational dominance. An object has cleared its neighbourhood when it is large enough to control its orbital zone. This means no other bodies of a comparable size exist in its path, except for its own natural satellites. This concept is one of three essential criteria used by the International Astronomical Union (IAU) to define a planet in our Solar System. If a large object meets the other planetary requirements but fails to clear its orbital zone, it is classified as a dwarf planet.

\n\nThe process of clearing an orbit occurs through continuous gravitational interaction. As a large body moves along its path, it acts on smaller nearby objects over many orbital cycles. This interaction typically results in three different outcomes for the smaller bodies. First, the objects may accrete, meaning they collide with and merge into the larger body. Second, the gravity of the larger body can disturb the small objects, pushing them into entirely different orbits. Third, the smaller bodies might be captured as satellites or pulled into a resonant orbit. Through these actions, the large body effectively sweeps its orbital region clear of significant mass.\n\nIt is important to note that no planet can achieve a state of perfect, absolute emptiness. Astronomer Jean-Luc Margot explains that gravitational and radiative forces constantly perturb the orbits of comets and asteroids. These forces push small objects into planet-crossing orbits, meaning the orbital zone is never truly finished being cleared. Furthermore, some objects are allowed to remain in the neighbourhood if they are in orbital resonance. For example, Jupiter shares space with \"Trojans,\" and Neptune shares its region with \"plutinos.\" These objects avoid collisions because their orbital periods are mathematically synchronized with the larger planet.\n\nThe need for a formal definition arose from a naming crisis in the early 2000s. Before 2006, the IAU lacked specific rules for naming planets because no new ones had been found for decades. However, the discovery of Eris in 2005 changed everything. Eris was found to be a comparable size to Pluto, which forced scientists to decide how to classify such objects. To resolve this, the IAU sought a taxonomical definition to distinguish true planets from minor planets. This led to the formal adoption of the three-criteria definition in 2006.\n\nSeveral scientists have proposed mathematical ways to measure this gravitational dominance. In 2000, Alan Stern and Harold F. Levison developed an algorithm using a value called lambda ($\lambda$). This measure calculates a body's ability to scatter smaller masses over a period equal to the Hubble time, or the age of the Universe. They used this to divide bodies into \"überplanets,\" which are the eight massive planets, and \"unterplanets,\" which are the dwarf planets. Steven Soter later proposed the \"planetary discriminant\" ($\mu$). This method compares the mass of a candidate planet to the total mass of all other objects sharing its orbital zone. Soter suggested that a body should be a planet if its $\mu$ value is greater than 100.\n\nJean-Luc Margot proposed a different method called the planetary discriminant ($\Pi$). Unlike Soter's method, which requires a census of all nearby objects, Margot's $\Pi$ can be calculated using only the mass of the body, the mass of its star, and its distance from that star. This makes it a very useful tool for studying exoplanets in other star systems. For the eight planets in our Solar System, the value of $\Pi$ is many orders of magnitude greater than 1. In contrast, for all known dwarf planets, the value is many orders of magnitude less than 1. This mathematical gap provides a clear way to separate the two groups.\n\nDespite these mathematical models, there is still scientific disagreement regarding these rules. Alan Stern, who led the New Horizons mission to Pluto, has argued against the IAU's definition. He points out that even major planets like Earth and Jupiter share their orbits with other objects, such as near-Earth asteroids or Trojan asteroids. Stern believes that the definition should rely on the intrinsic attributes of the object rather than its dynamical environment. He has advocated for a category of \"überplanets\" that more closely aligns with the eight most massive orbiters in our system. This debate highlights how difficult it is to draw precise lines in the vastness of space.",

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"File:TheKuiperBelt 75AU All.svg"

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}