Information about Clearing The Neighbourhood
In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. The current definition of a planet adopted by the International Astronomical Union (IAU) only includes those bodies which have "cleared the neighbourhood of its orbit"[1]. A large body which meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. This includes Pluto, which shares its orbital neighbourhood with Kuiper Belt Objects such as the plutinos. The IAU's definition does not attach specific numbers or equations to this term, however the extent to which all the planets have cleared their neighbourhoods is much greater, by any measure, than that of any dwarf planet or any candidate for dwarf planet known so far.
The phrase may be derived from a paper presented to the general assembly of the IAU 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.[2]
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.
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 (Lambda) is a parameter proposed by Stern and Levison[2] 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
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).
Soter went on to propose a parameter he called the "planetary discriminant", designated with the symbol µ (mu) , that represents 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.
Here is a list of planets by planetary discriminant, as defined by Steven Soter, in decreasing order, where the planetary discriminant μ is the ratio between the mass of the body and the total mass of the other non-resonating and non-satellite bodies in the same orbital zone, as defined by Soter. Also listed is the Stern-Levinson parameter Λ, the square of the mass over the orbital period, normalized to the Earth values(Λ/ΛE). (Note that ΛE ~ 1.5105, so that the unnormalized values of Λ for the eight planets defined by the IAU are orders of magnitude greater than 1, and the values of Λ for the dwarf planets are orders of magnitude less than 1.)<ref name="Soter 2006" />
The calculated parameters for major solar system bodies are:[3]
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.[5]
However, 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.[2] 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.[1]
The phrase may be derived from a paper presented to the general assembly of the IAU 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.[2]
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.
Details
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.[2]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 (Lambda) is a parameter proposed by Stern and Levison[2] 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
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).
Soter went on to propose a parameter he called the "planetary discriminant", designated with the symbol µ (mu) , that represents 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.
Here is a list of planets by planetary discriminant, as defined by Steven Soter, in decreasing order, where the planetary discriminant μ is the ratio between the mass of the body and the total mass of the other non-resonating and non-satellite bodies in the same orbital zone, as defined by Soter. Also listed is the Stern-Levinson parameter Λ, the square of the mass over the orbital period, normalized to the Earth values(Λ/ΛE). (Note that ΛE ~ 1.5105, so that the unnormalized values of Λ for the eight planets defined by the IAU are orders of magnitude greater than 1, and the values of Λ for the dwarf planets are orders of magnitude less than 1.)<ref name="Soter 2006" />
The calculated parameters for major solar system bodies are:[3]
| Rank | Name | Image | Stern-Levinson parameter Λ/ΛE; |
Planetary discriminant µ |
Mass (kg) | Type of object |
|---|---|---|---|---|---|---|
| 1 | Earth | ) | 1.00 | 1.7106 | (5.97361024 kg) | 3rd planet |
| 2 | Venus | ) | 1.08 | 1.35106 | (4.86851024 kg) | 2nd planet |
| 3 | Jupiter | ) | 8510 | 6.25105 | (1.8991027 kg) | 5th planet |
| 4 | Saturn | ) | 308 | 1.9105 | (5.68461026 kg) | 6th planet |
| 5 | Mars | ) | 0.0061 | 1.8105 | (6.41851023 kg) | 4th planet |
| 6 | Mercury | ){kind=small} | 0.0126 | 9.1104 | (3.3021023 kg) | 1st planet |
| 7 | Uranus | ) | 2.51 | 2.9104 | (8.68321025 kg) | 7th planet |
| 8 | Neptune | ){kind=small} | 1.79 | 2.4104 | (1.02431026 kg) | 8th planet |
| 9 | Ceres | ){kind=small} | 8.710−9 | 0.33 | 9.51020 kg | 1st dwarf planet |
| 10 | Eris | ) | 3.510−8 | 0.10 | (1.51022 kg)[4] | 3rd dwarf planet |
| 11 | Pluto | ) | 1.9510−8 | 0.077 | (1.291022 ± 10% kg) | 2nd dwarf planet |
Controversy
)
Orbits of celestial bodies in the Kuiper Belt with approximate distances and inclination. Objects marked with red are in orbital resonances with Neptune, with Pluto (the largest red circle) located in the "spike" of plutinos at the 2:3 resonance
However, 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.[2] 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.[1]
See also
- Planet
- List of planetary bodies
- List of solar system objects by mass
- List of solar system objects by radius
- List of solar system objects
- List of noteworthy asteroids
- Definition of planet
- 2006 redefinition of planet
- Mesoplanet
References
- Soter, Steven (2006-08-16). What is a Planet? (PDF). Retrieved on 2006-08-24. submitted to The Astronomical Journal, 16 August 2006
1. ^ "The Final IAU Resolution on the definition of "planet" ready for voting", IAU, 24 August 2006. Retrieved on 2006-08-26.2006">
2. ^ Stern, S. Alan; and Levison, Harold F. (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (PDF). Highlights of Astronomy 12: 205-213, as presented at the XXIVth General Assembly of the IAU - 2000 [Manchester, UK, 7 August-18 August 2000].
3. ^ Soter, Steven (2006-08-16). What is a Planet? (PDF). Retrieved on 2006-08-24. submitted to The Astronomical Journal, 16 August 2006
4. ^ Very rough estimate based on a diameter of 2400 km and composition similar to that of Pluto.
5. ^ Rincon, Paul (25 August 2006). Pluto vote 'hijacked' in revolt. BBC News. Retrieved on 2006-09-03.
2. ^ Stern, S. Alan; and Levison, Harold F. (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (PDF). Highlights of Astronomy 12: 205-213, as presented at the XXIVth General Assembly of the IAU - 2000 [Manchester, UK, 7 August-18 August 2000].
3. ^ Soter, Steven (2006-08-16). What is a Planet? (PDF). Retrieved on 2006-08-24. submitted to The Astronomical Journal, 16 August 2006
4. ^ Very rough estimate based on a diameter of 2400 km and composition similar to that of Pluto.
5. ^ Rincon, Paul (25 August 2006). Pluto vote 'hijacked' in revolt. BBC News. Retrieved on 2006-09-03.
| Footer SolarSystem |
|---|
){kind=small} |
| The Sun Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Eris |
| Planets Dwarf planets Moons: Terrestrial Martian Jovian Saturnian Uranian Neptunian Plutonian Eridian |
| Small bodies: Meteoroids Asteroids/Asteroid moons (Asteroid belt) Centaurs TNOs (Kuiper belt/Scattered disc) Comets (Oort cloud) |
| See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the |
External links
- al-Jazeera: Pluto central in battle of the planets (Associated Press) Thursday 24 August 2006, 10:48 Makka Time, 7:48 GMT
- Ottawa Citizen: The case against Pluto (P. Surdas Mohit) Thursday, August 24 2006
nebular hypothesis is the currently accepted argument about how Earth's Solar System formed. It was first proposed in 1734 by Emanuel Swedenborg[1]. In 1755 Immanuel Kant, who was familiar with Swedenborg's work, developed the theory further.
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planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of
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satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.
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definition of "planet" became the subject of intense debate. Although the word itself dates back millennia, it had never had a formal definition until, beginning in 2005, a confluence of circumstances forced the astronomical community to take action.
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International Astronomical Union (IAU) unites national astronomical societies from around the world. It also acts as the internationally recognized authority for assigning designations to celestial bodies (stars, planets, asteroids, etc.
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dwarf planet is a celestial body within the Solar System that satisfies the following four conditions:[1]
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- is in orbit around the Sun
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium
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Pluto
Map of Pluto based on Charon eclipses, approximately true colour and giving the highest resolution currently possible
Discovery
Discovered by: Clyde W.
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Map of Pluto based on Charon eclipses, approximately true colour and giving the highest resolution currently possible
Discovery
Discovered by: Clyde W.
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Kuiper belt (pronounced IPA: /ˈkaɪpɚ/, to rhyme with "viper"),[1] sometimes called the Edgeworth-Kuiper belt
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plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino.
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S. Alan Stern is the Associate Administrator of NASA's Science Mission Directorate. Formerly a scientist at the Southwest Research Institute, he remains the Principal Investigator of the New Horizons mission to Pluto.
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Harold F. "Hal" Levison is a planetary scientist specializing in planetary dynamics. He argued for a distinction between what are now called dwarf planets and the other eight planets based on their inability to "clear the neighborhood around their orbits," although his proposal
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Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.
A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of dust grains that collide and stick to form
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A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of dust grains that collide and stick to form
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Mass is a fundamental concept in physics, roughly corresponding to the intuitive idea of "how much matter there is in an object". Mass is a central concept of classical mechanics and related subjects, and there are several definitions of mass within the framework of relativistic
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The orbital period is the time taken for a planet (or another object) to make one complete orbit.
When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.
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When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.
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Minor planets, or asteroids or planetoids, are minor celestial bodies of the Solar system orbiting the Sun (mostly Small solar system bodies) that are smaller than major planets, but larger than meteoroids (commonly defined as being 10 meters across or less
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Eris
Eris (centre) and Dysnomia (left of centre).
Hubble Space Telescope.
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Discovered by: M. E. Brown,
C. A. Trujillo,
D. L.
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Eris (centre) and Dysnomia (left of centre).
Hubble Space Telescope.
Discovery
Discovered by: M. E. Brown,
C. A. Trujillo,
D. L.
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Protoplanets are moon-sized planet embryos within protoplanetary discs. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide.
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Gravitation is a natural phenomenon by which all objects with mass attract each other. In everyday life, gravitation is most familiar as the agency that endows objects with weight.
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Astronomical objects are significant physical entities, associations or structures which current science has confirmed to exist in space. This does not necessarily mean that more current science will not disprove their existence.
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In astrophysics, the term accretion is used for at least two distinct processes.
The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc.
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The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc.
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satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.
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orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers.
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This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. The colors have been enhanced to bring out detail.
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This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. The colors have been enhanced to bring out detail.
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Trojan asteroids are a large group of objects that share the orbit of the planet Jupiter around the Sun. Viewed from a coordinate system that is fixed on Jupiter, they appear to orbit one of the two Lagrangian points of stability, L4 and L5
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Herod_Archelaus
