Clearing_the_neighbourhood

# Clearing the neighbourhood  **Microsim — three.js (Wikitube framework):** *What the Solar System holds (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=census&embed=1" data-title="What the Solar System holds (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=census`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: under show, choose small bodies and look at where the swarms sit, packed between Mars and Jupiter, strung along Jupiter's orbit and spread across the Kuiper belt beyond Neptune; switch show back to everything and press l to label the planets and dwarf planets; then scroll out to Pluto and see that its orbit lies inside the Kuiper belt's crowd while Neptune's path is kept clear.* **Clearing the neighbourhood**, also called dynamical dominance, is the condition in which a body orbiting a star has become gravitationally dominant along its orbit, so that no other body of comparable size shares its orbital zone except its own satellites or bodies held under its control, such as those locked in resonance with it.[^soter2006] It is the third of the three criteria in the [[IAU_definition_of_planet|IAU definition of planet]] adopted in 2006: a round body orbiting the [[Sun]] that has cleared its neighbourhood is a planet, and one that has not is a [[Dwarf_planet|dwarf planet]].[^iau-res] The IAU attached no number to the phrase, but the quantitative criteria proposed since then all find the eight planets and the dwarf planets separated by several orders of magnitude.[^margot2015] The wording goes back to a paper that Alan Stern and Harold Levison presented to the IAU General Assembly of 2000, which asked how likely a body is to scatter the small bodies near its orbit, given its mass and orbital period.[^stern2002] Steven Soter prefers the term "dynamical dominance", and Jean-Luc Margot finds that phrase less prone to misreading.[^soter2006][^margot2015] Margot extended the idea to [[Planetary_system|planets of other stars]] in 2015.[^margot2015] The explorer at the top of this page opens on its census view, which colours bodies by class; the eight planets and the swarms of small bodies in the belts between and beyond them show at a glance which orbits are crowded and which are not. ## Criteria A massive body sweeps its orbital zone through repeated gravitational encounters with small bodies nearby. Over many orbits each small body is accreted by the large one, thrown onto a different orbit, captured as a satellite, or trapped in an orbital [[Resonance|resonance]] that keeps it from ever colliding with the large body. Once that process has run its course, whatever is left in the zone is under the large body's control.[^stern2002] That is why resonant companions do not count against a planet. [[Jupiter]] shares its orbit with thousands of [[Jupiter_trojan|trojans]] that are held near the points 60° ahead of and behind it; [[Earth]] has 3753 Cruithne, an asteroid whose orbit is linked to Earth's by a 1:1 resonance; and [[Neptune]] crosses paths with [[Pluto]] and the other [[Plutino|plutinos]], which make two orbits for every three of Neptune's and so never meet it.[^wiegert1997][^stern2002] Nor is an empty orbit required. Margot notes that gravitational and radiative forces keep nudging asteroids and comets onto planet-crossing orbits, so no planet can ever finish the job, and that the IAU did not mean perfect clearing.[^margot2015] ### Stern–Levison's Stern and Levison looked for a way to decide which bodies "control the region surrounding them". Their parameter Λ (lambda) measures how far a body can scatter small bodies from its zone within the Hubble time, about the age of the universe. It scales as the square of the body's mass divided by its orbital period, Λ = k m² / a^{3/2}, where *a* is the semi-major axis and *k* depends only weakly on the orbits of the small bodies.[^stern2002][^soter2006] A body with Λ greater than 1 is likely to clear its zone. On this basis Stern and Levison divided round bodies orbiting the Sun into "überplanets", dynamically important enough to have cleared their neighbourhoods, and "unterplanets"; the überplanets were the eight bodies the IAU now calls planets.[^stern2002] ### Soter's Soter's planetary discriminant µ (mu) is observational rather than theoretical: the mass of the candidate divided by the total mass of all other bodies in its orbital zone. He defines the zone as the set of bodies whose orbits cross a common distance from the Sun and whose periods, outside resonances, differ by less than a factor of ten. That rule leaves comets out, but their combined mass is too small to matter. Soter proposed µ > 100 as the mark of a planet.[^soter2006] The weakness of µ is that it needs a census of everything in the zone, which is rarely available. ### Margot's Margot's Π (pi) uses only the body's mass *m* in Earth masses, its semi-major axis *a* in [[Astronomical_unit|AU]] and the star's mass *M* in solar masses: Π = k m / (M^{5/2} a^{9/8}).[^margot2015] It asks how many orbits a body needs to transfer enough energy to a neighbour to move it out of a band 2√3 Hill radii wide, and requires that this happen within the star's main-sequence lifetime; for a lifetime of 10 billion years the constant *k* is 807, so that Earth's own Π is 807.[^margot2015] A body with Π > 1 is a planet. Because Π depends only on quantities that can be measured for an [[Planetary_system|exoplanet]], it works for other stars, although it assumes a circular orbit. A 2024 revision by Margot, Gladman and Yang adopts a uniform clearing time of 10 billion years, so that planets around brown dwarfs can be classified too; the values for the Solar System do not change.[^margot2024] ## Numerical values The table gives the three measures for the eight planets and the best-known dwarf planets. Π and Λ are computed here from each body's mass and semi-major axis, using Margot's constant for Π and scaling Λ from Earth's published value; µ is Soter's.[^margot2015][^soter2006][^nasa-fs][^jpl-sbdb] | Body | Mass (Earth = 1) | a (AU) | Π (derived) | Λ (derived) | µ | |---|---|---|---|---|---| | [[Jupiter]] | 317.8 | 5.20 | 40,100 | 1.3 × 10⁹ | 6.25 × 10⁵ | | [[Saturn]] | 95.2 | 9.54 | 6,070 | 4.7 × 10⁷ | 1.9 × 10⁵ | | [[Venus]] | 0.815 | 0.723 | 948 | 1.66 × 10⁵ | 1.35 × 10⁶ | | [[Earth]] | 1 | 1.00 | 807 | 1.53 × 10⁵ | 1.7 × 10⁶ | | [[Uranus]] | 14.5 | 19.2 | 422 | 3.8 × 10⁵ | 2.9 × 10⁴ | | [[Neptune]] | 17.1 | 30.1 | 300 | 2.7 × 10⁵ | 2.4 × 10⁴ | | [[Mercury_(planet)|Mercury]] | 0.055 | 0.387 | 130 | 1,940 | 9.1 × 10⁴ | | [[Mars]] | 0.107 | 1.52 | 54 | 940 | 1.8 × 10⁵ | | [[Ceres_(dwarf_planet)|Ceres]] | 0.00016 | 2.77 | 0.040 | 8.2 × 10⁻⁴ | 0.33 | | [[Pluto]] | 0.0022 | 39.5 | 0.028 | 2.9 × 10⁻³ | 0.077 | | [[Eris_(dwarf_planet)|Eris]] | 0.0028 | 67.9 | 0.020 | 2.1 × 10⁻³ | 0.10 | | [[Haumea]] | 0.00066 | 43.1 | 0.0077 | 2.4 × 10⁻⁴ | — | The gap is the point. The smallest planetary Π, Mars's 54, is more than a thousand times the largest dwarf-planet value, Ceres's 0.04 (derived), and Mars's µ of 180,000 is more than half a million times Ceres's. Nothing lies in between. The masses come from the NASA fact sheet for the planets and from spacecraft or satellite-orbit measurements for Ceres, Pluto, Eris and Haumea.[^nasa-fs][^park2019][^brozovic2024][^brown2007][^proudfoot2024] Setting Π = 1 gives the distance at which a body of a given mass would stop qualifying. For Earth that distance is about 380 AU; for Mars about 53 AU; for Pluto about 1.7 AU, inside the orbit of Mars (all derived).[^margot2015] An Earth-mass body in the far [[Kuiper_belt|Kuiper belt]] would therefore still be a planet, while one in the inner [[Oort_cloud|Oort cloud]] would not. The explorer draws the planets on orbits from JPL elements and the dwarf planets on orbits from the JPL Small-Body Database, but its population clouds are ILLUSTRATIVE samples, not a census of the zones these numbers describe.[^jpl-t1][^jpl-sbdb] ## Disagreement Stern, principal investigator of NASA's New Horizons mission to Pluto, opposed demoting Pluto on this criterion. He called the IAU's wording vague and argued that Earth, Mars, Jupiter and Neptune have not cleared their zones either, pointing to about 10,000 [[Near-Earth_object|near-Earth asteroids]] near Earth's orbit and about 100,000 trojans in Jupiter's. "If Neptune had cleared its zone," he said, "Pluto wouldn't be there."[^rincon2006] The objection sits awkwardly with Stern's own earlier work. The paper that introduced Λ described an überplanet as a body "dynamically important enough to have cleared its neighboring planetesimals", and concluded that the Solar System contains eight of them, Earth, Mars, Jupiter and Neptune among them.[^stern2002] The IAU's planets and Stern's überplanets are the same eight bodies. The disagreement is not about the dynamics but about their role: Stern intended dynamical dominance to divide planets into subclasses, and he rejects it as the test of what a planet is, preferring intrinsic properties such as roundness.[^space2011] Supporters of the IAU wording answer that the trojans and plutinos are exactly the bodies a dominant planet controls, which is what the criterion means by clearing.[^stern2002][^margot2015] ## See also - [[IAU_definition_of_planet]] - [[Dwarf_planet]] - [[Hill_sphere]] - [[Small_Solar_System_body]] - List of gravitationally rounded objects of the Solar System (plain text: not yet on Wikitube) ## Notes Π and Λ in the table are computed here, not copied from the papers. Π uses Π = 807 m / a^{9/8} for a star of one solar mass, with *m* in Earth masses and *a* in AU; Λ uses Λ = 1.53 × 10⁵ m² / a^{3/2}, the constant being Earth's published Λ. Both reproduce the published values to within rounding. Semi-major axes for the planets come from JPL Table 1 and for the dwarf planets from the JPL Small-Body Database. Soter gives no µ for Haumea. ## References [^iau-res]: International Astronomical Union (24 August 2006). "IAU 2006 General Assembly: Resolutions 5 and 6". https://iauarchive.eso.org/static/resolutions/Resolution_GA26-5-6.pdf [^stern2002]: Stern, S. A.; Levison, H. F. (2002). "Regarding the criteria for planethood and proposed planetary classification schemes". *Highlights of Astronomy* 12: 205–213 (presented at the XXIVth General Assembly of the IAU, Manchester, August 2000). https://doi.org/10.1017/S1539299600013289 [^soter2006]: Soter, S. (2006). "What is a planet?". *The Astronomical Journal* 132: 2513–2519. https://doi.org/10.1086/508861 [^margot2015]: Margot, J.-L. (2015). "A quantitative criterion for defining planets". *The Astronomical Journal* 150: 185. https://doi.org/10.1088/0004-6256/150/6/185 [^margot2024]: Margot, J.-L.; Gladman, B.; Yang, T. (2024). "Quantitative criteria for defining planets". *The Planetary Science Journal* 5: 159. https://doi.org/10.3847/PSJ/ad55f3 [^wiegert1997]: Wiegert, P. A.; Innanen, K. A.; Mikkola, S. (1997). "An asteroidal companion to the Earth". *Nature* 387: 685–686. https://doi.org/10.1038/42662 [^nasa-fs]: NASA NSSDCA. "Planetary Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/ (fetched 2026-09-18). [^jpl-t1]: JPL Solar System Dynamics. "Approximate Positions of the Planets", Table 1. https://ssd.jpl.nasa.gov/planets/approx_pos.html [^jpl-sbdb]: JPL Solar System Dynamics. Small-Body Database Lookup. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html (elements fetched 2026-09-18). [^park2019]: Park, R. S.; Vaughan, A. T.; Konopliv, A. S.; Ermakov, A. I.; Mastrodemos, N.; Castillo-Rogez, J. C.; et al. (2019). "High-resolution shape model of Ceres from stereophotoclinometry using Dawn imaging data". *Icarus* 319: 812–827. https://doi.org/10.1016/j.icarus.2018.10.024 [^brozovic2024]: Brozović, M.; Jacobson, R. A. (2024). "Post-New-Horizons orbits and masses for the satellites of Pluto". *The Astronomical Journal* 167: 256. https://doi.org/10.3847/1538-3881/ad39f0 [^brown2007]: Brown, M. E.; Schaller, E. L. (2007). "The mass of the dwarf planet Eris". *Science* 316: 1585. https://doi.org/10.1126/science.1139415 [^proudfoot2024]: Proudfoot, B. C. N.; Ragozzine, D. A.; Giforos, W.; Grundy, W. M.; MacDonald, M.; Oldroyd, W. J. (2024). "Beyond point masses. III. Detecting Haumea's nonspherical gravitational field". *The Planetary Science Journal* 5: 69. https://doi.org/10.3847/PSJ/ad26e9 [^rincon2006]: Rincon, P. (25 August 2006). "Pluto vote 'hijacked' in revolt". *BBC News*. https://news.bbc.co.uk/2/hi/science/nature/5283956.stm [^space2011]: Wall, M. (24 August 2011). "Pluto's planet title defender: Q & A with planetary scientist Alan Stern". *Space.com*. http://www.space.com/12710-pluto-defender-alan-stern-dwarf-planet-interview.html ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Clearing_the_neighbourhood) : [Wikitube](https://en.wikitube.io/wiki/Clearing_the_neighbourhood) · pinned revision [1372723834](https://en.wikipedia.org/w/index.php?oldid=1372723834) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-033 · explorer state `?view=census`.*