NationMaster - Encyclopedia: Clearing the neighbourhood

"Clearing the neighbourhood" is an informal description of part of the process of planet formation. The phrase may be derived from a paper presented to the general assembly of the International Astronomical Union in 2000 by Alan Stern and Harold Levison. The authors used several similar phrases as they developed a theoretical basis for determining if an object orbiting a star is likely to "clear its neighboring region" of planetesimals, based on the object's mass and its orbital period.^[1] The concept was used by the IAU in its August 24, 2006, redefinition of the term planet as the criterion differentiating a planet from a dwarf planet: A planet is a body with sufficient mass to have "cleared the neighbourhood around its orbit."^[2] The IAU also explicitly stated that Pluto will now be considered a dwarf planet. Pluto has not cleared the neighborhood of its orbit (vis-à-vis Kuiper Belt Objects such as the Plutinos). The IAU's definition did not specify how this factor was to be measured in the general case, beyond that future decisions over borderline objects were to be made by "an IAU process," presumably in committee. The accretion theory, in astrophysics, is a scientific theory of the formation of our Solar system. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... This page may meet Wikipedias criteria for speedy deletion. ... Harold F. Hal Levison is a planetary scientist specializing in planetary dynamics. ... ÃˆÃ ÃŒÃ Ã¬Ã²Ã²In cosmogony, planetesimals are objects thought to exist within solar nebulae. ... Unsolved problems in physics: What causes anything to have mass? Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... August 24 is the 236th day of the year in the Gregorian Calendar (237th in leap years), with 129 days remaining. ... 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... The final definition left the solar system with eight planets, pictured above (not to scale) Displays the remaining eight planets with the celestial bodies that have now been designated as dwarf planets. ... The eight planets and three dwarf planets of our Solar System, alongside the Sun. ... Artists impression of Pluto (background) and its satellite Charon (foreground). ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... The Kuiper belt (KYE per) is an area of the solar system extending from within the orbit of Neptune (at 30 AU) to 50 AU from the sun, at inclinations consistent with the ecliptic. ... In astronomy, a plutino is a trans-Neptunian object that has a 3:2 orbital resonance with Neptune. ...

Clearly distinguishing "planets" from "dwarf planets" and other minor planets had become necessary because the IAU had adopted different rules for naming newly discovered major planets and newly discovered minor planets, without establishing a basis for telling them apart. The naming process for Eris stalled after the announcement of its discovery in 2005, pending clarification of this first step. Minor planets, or planetoids are minor bodies of the Solar system orbiting the Sun (or of other planetary systems orbiting other stars) that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a... Eris (IPA or ), officially designated 136199 Eris, is the largest known dwarf planet in the solar system. ...

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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. 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 which will never collide with each other due to orbital resonance, such as Jupiter and the Trojan asteroids, Earth and 3753 Cruithne or Neptune and the Plutinos.^[1] In cosmogony, a protoplanet is a quasi-planetoid which is slightly larger than a planetesimal and orbits within a solar nebulas protoplanetary discs. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ... Gravity or Gravitation is a property by which all objects attract each other. ... ... See also: Accretion (finance) Accretion is increase in size by gradual addition of smaller parts. ... MILSTAR:A Communciation Satellite A satellite is any object that orbits another object (which is known as its primary). ... In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Image of the Trojan asteroids in front of and behind Jupiter along its orbital path. ... Earth (IPA: , often referred to as the Earth, Terra, or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ... 3753 Cruithne (English crew-een-ya; Modern Irish crih-na) is an asteroid in orbit around the Sun. ... Atmospheric characteristics Surface pressure â‰«100 MPa Hydrogen - H2 80% Â±3. ...

Steven Soter of the Department of Astrophysics, American Museum of Natural History, has written that "A heliocentric body with Λ > 1 [viz., a planet] has cleared a substantial fraction of small bodies out of its orbital neighborhood."^[3] Λ, pronounced "lambda," is a parameter proposed by Stern and Levison^[1] that measures the extent to which a body scatters smaller masses out of its orbital zone over a long period of time. Mathematically Λ is defined as Steven Soter is an astronomer who argued for a distinction between dwarf planets and the other eight planets based on their inability to clear the neighborhood around their orbits. This article belongs in one or more categories. ... The American Museum of Natural History is a landmark of Manhattans Upper West Side in New York, USA, at 79th Street and Central Park West. ...

where k is approximately constant and M and P are the scattering body's mass and orbital period, respectively. Two bodies are defined to share an orbital zone if their orbits cross a common radial distance from the primary, and their non-resonant periods differ by less than an order of magnitude. The order-of-magnitude similarity in period requirement excludes comets from the calculation, but the combined mass of the comets turn out to be negligible compared to the other small solar system bodies anyway so their inclusion would have little impact on the results. Stern and Levison found a gap of five orders of magnitude in Λ between the smallest terrestrial planets and the largest asteroids and Kuiper Belt Objects (KBOs). Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ...

Soter went on to propose a parameter he called the "planetary discriminant" (designated with the symbol µ, pronounced "mu") that represented an experimental measure of the actual degree of cleanliness of the orbital zone. µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone.

*Λ/ΛE = M²/P, in Earth masses squared per year.
**µ = M/m, where M is the mass of the body, and m is the aggregate mass of all the other bodies that share its orbital zone. Note: This article contains special characters. ... (*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ... Earth, also known as the Earth or Terra, is the third planet outward from the Sun. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ... 1 Ceres (IPA , Latin: ) is a dwarf planet in the asteroid belt. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Atmospheric characteristics Atmospheric pressure 120 kPa Hydrogen 83% Helium 15% Methane 1. ... Atmospheric characteristics Surface pressure â‰«100 MPa Hydrogen - H2 80% Â±3. ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... Eris (IPA or ), officially designated 136199 Eris, is the largest known dwarf planet in the solar system. ...

Controversy

Stern, currently leading the NASA New Horizons mission to Pluto, objects to the reclassification of Pluto on the basis that—like Pluto—Earth, Mars, Jupiter and Neptune have not cleared their orbital neighbourhoods either. Earth co-orbits with 10,000 near-Earth asteroids, and Jupiter has 100,000 Trojan asteroids in its orbital path. "If Neptune had cleared its zone, Pluto wouldn't be there," he now says.^[4] Image File history File links TheKuiperBelt_75AU_All. ... Image File history File links TheKuiperBelt_75AU_All. ... NASA logo Listen to this article Â· (info) This audio file was created from an article revision dated 2005-09-01, and does not reflect subsequent edits to the article. ... New Horizons is a NASA unmanned mission to fly by Pluto and its moons. ... Earth (IPA: , often referred to as the Earth, Terra, or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ... Note: This article contains special characters. ... Near-Earth asteroids (NEAs) are asteroids whose orbits are close to Earths orbit. ...

In 2000 Stern himself wrote, "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.^[1] Most planetary scientists understand "clearing the neighborhood" to refer to an object being the dominant mass in its vicinity, for instance Earth being many times more massive than all of the NEA's combined, and Neptune "dwarfing" Pluto and the rest of the KBO's.^[3]

Stern and Levison's paper shows that it's possible to estimate whether an object is likely to dominate its neighborhood given only the object's mass and orbital period, known values even for extrasolar planets. In any case, the recent IAU definition specifically limits itself only to objects orbiting the Sun.^[2]

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