"Clearing the neighbourhood" (or dynamical dominance) around a
celestial body
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its
natural satellite
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
s or those otherwise under its gravitational influence.
"Clearing the neighbourhood" is one of three necessary criteria for a celestial body to be considered a
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
in the
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
, according to
the definition adopted in 2006 by the
International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU).
In 2015, a proposal was made to extend the definition to
exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s.
In the end stages of
planet formation
The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting t ...
, a
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
, as so defined, will have "cleared the neighbourhood" of its own orbital zone, i.e. removed other bodies of comparable size. A large body that meets the other criteria for a planet but has not cleared its neighbourhood is classified as a
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
. This includes
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
, whose orbit intersects with
Neptune's orbit and shares its orbital neighbourhood with many
Kuiper belt
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 tim ...
objects. The IAU's definition does not attach specific numbers or equations to this term, but all IAU-recognised planets have cleared their neighbourhoods to a much greater extent (by
orders of magnitude
An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...
) than any dwarf planet or candidate for dwarf planet.
The phrase stems from a paper presented to the 2000 IAU general assembly by the
planetary scientist
Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their fo ...
s
Alan Stern
Sol Alan Stern (born November 22, 1957) is an American engineer and planetary scientist. He is the principal investigator of the ''New Horizons'' mission to Pluto and the Chief Scientist at Moon Express.
Stern has been involved in 24 suborbita ...
and
Harold F. Levison. The authors used several similar phrases as they developed a theoretical basis for determining if an object orbiting a
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
is likely to "clear its neighboring region" of
planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ...
s based on the object's
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...
and its
orbital period
The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting pla ...
.
Steven Soter
Steven Soter is an astrophysicist currently holding the positions of scientist-in-residence for New York University's Environmental Studies Program and of Research Associate for the Department of Astrophysics at the American Museum of Natural Hi ...
prefers to use the term "''dynamical dominance''",
and
Jean-Luc Margot
Jean-Luc Margot (born 1969) is a Belgian-born astronomer and a UCLA professor who specializes in planetary sciences.
Career
Margot has discovered and studied several binary asteroids with radar and optical telescopes. His discoveries include ( ...
notes that such language "seems less prone to misinterpretation".
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. The IAU sought to resolve the naming of Eris by seeking a taxonomical definition to distinguish planets from
minor planet
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s.
Criteria
The phrase refers to an orbiting body (a planet or
protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitation ...
) "sweeping out" its
orbital region over time, by
gravitation
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
ally interacting with smaller
bodies
Bodies may refer to:
* The plural of body
* ''Bodies'' (2004 TV series), BBC television programme
* Bodies (upcoming TV series), an upcoming British crime thriller limited series
* "Bodies" (''Law & Order''), 2003 episode of ''Law & Order''
* B ...
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, or to be captured either as a
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...
or into a
resonant 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 that will never collide with each other due to
orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...
, such as
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
and
its trojans,
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
and
3753 Cruithne, or
Neptune and the
plutino
In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the l ...
s.
As to the extent of orbit clearing required,
Jean-Luc Margot
Jean-Luc Margot (born 1969) is a Belgian-born astronomer and a UCLA professor who specializes in planetary sciences.
Career
Margot has discovered and studied several binary asteroids with radar and optical telescopes. His discoveries include ( ...
emphasises "a planet can never completely clear its orbital zone, because gravitational and radiative forces continually perturb the orbits of asteroids and comets into planet-crossing orbits" and states that the IAU did not intend the impossible standard of impeccable orbit clearing.
Stern–Levison's ''Λ''
In their paper,
Stern and
Levison sought an algorithm to determine which "planetary bodies control the region surrounding them".
They defined ''Λ'' (
lambda
Lambda (}, ''lám(b)da'') is the 11th letter of the Greek alphabet, representing the voiced alveolar lateral approximant . In the system of Greek numerals, lambda has a value of 30. Lambda is derived from the Phoenician Lamed . Lambda gave ri ...
), a measure of a body's ability to scatter smaller masses out of its orbital region over a period of time equal to the age of the Universe (
Hubble time
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...
). Λ is a dimensionless number defined as
:
where ''m'' is the mass of the body, ''a'' is the body's semi-major axis, and ''k'' is a function of the orbital elements of the small body being scattered and the degree to which it must be scattered. 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.
If Λ > 1, then the body will likely clear out the small bodies in its orbital zone. Stern and Levison used this discriminant to separate the
gravitionally rounded, Sun-orbiting bodies into ''überplanets'', which are "dynamically important enough to have cleared its neighboring planetesimals", and ''unterplanets''. The überplanets are the eight most massive solar orbiters (i.e. the IAU planets), and the unterplanets are the rest (i.e. the IAU dwarf planets).
Soter's ''µ''
Steven Soter
Steven Soter is an astrophysicist currently holding the positions of scientist-in-residence for New York University's Environmental Studies Program and of Research Associate for the Department of Astrophysics at the American Museum of Natural Hi ...
proposed an observationally based measure ''µ'' (
mu), which he called the "''planetary discriminant''", to separate bodies orbiting stars into planets and non-planets.
He defines mu as
:
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.
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 to the other small Solar System bodies, so their inclusion would have little impact on the results. µ is then calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone. It is a measure of the actual degree of cleanliness of the orbital zone. Soter proposed that if µ > 100, then the candidate body be regarded as a planet.
Margot's Π
Astronomer
Jean-Luc Margot
Jean-Luc Margot (born 1969) is a Belgian-born astronomer and a UCLA professor who specializes in planetary sciences.
Career
Margot has discovered and studied several binary asteroids with radar and optical telescopes. His discoveries include ( ...
has proposed a discriminant, Π (
pi), that can categorise a body based only on its own mass, its semi-major axis, and its star's mass.
Like Stern–Levison's Λ, Π is a measure of the ability of the body to clear its orbit, but unlike Λ, it is solely based on theory and does not use empirical data from the Solar System. 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.
:
where ''m'' is the mass of the candidate body in
Earth masses, ''a'' is its semi-major axis in
AU, ''M'' is the mass of the parent star in
solar masses, and ''k'' is a constant chosen so that Π > 1 for a body that can clear its orbital zone. ''k'' depends on the extent of clearing desired and the time required to do so. Margot selected an extent of
times the
Hill radius and a time limit of the parent star's lifetime on the
main sequence (which is a function of the mass of the star). Then, in the mentioned units and a main-sequence lifetime of 10 billion years, k = 807.
[This expression for ''k'' can be derived by following Margot's paper as follows:
The time required for a body of mass ''m'' in orbit around a body of mass ''M'' with an orbital period ''P'' is:
With and ''C'' the number of Hill radii to be cleared.
This gives
requiring that the clearing time ''tclear'' to be less than a characteristic timescale ''t*'' gives:
this means that a body with a mass ''m'' can clear its orbit within the designated timescale if it satisfies
This can be rewritten as follows
so that the variables can be changed to use solar masses, Earth masses, and distances in AU by
and
Then, equating ''t*'' to be the main-sequence lifetime of the star ''tMS'', the above expression can be rewritten using
with ''tSun'' the main-sequence lifetime of the Sun, and making a similar change in variables to time in years
This then gives
Then, the orbital-clearing parameter is the mass of the body divided by the minimum mass required to clear its orbit (which is the right-hand side of the above expression) and leaving out the bars for simplicity gives the expression for Π as given in this article:
which means that
Earth's orbital period can then be used to remove ''aEarth'' and ''PEarth'' from the expression:
which gives
so that this becomes
Plugging in the numbers gives ''k'' = 807.] The body is a planet if Π > 1. The minimum mass necessary to clear the given orbit is given when Π = 1.
Π is based on a calculation of the number of orbits required for the candidate body to impart enough energy to a small body in a nearby orbit such that the smaller body is cleared out of the desired orbital extent. This is unlike Λ, which uses an average of the clearing times required for a sample of asteroids in the
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, c ...
, and is thus biased to that region of the Solar System. 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
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...
means that the star might disrupt its planetary system (e.g. by going nova) before the object is actually able to clear its orbit.
The formula for Π assumes a circular orbit. Its adaptation to elliptical orbits is left for future work, but Margot expects it to be the same as that of a circular orbit to within an order of magnitude.
Numerical values
Below is a list of planets and dwarf planets ranked by Margot's planetary discriminant Π, in decreasing order.
For all eight planets defined by the IAU, Π is orders of magnitude greater than 1, whereas for all dwarf planets, Π is orders of magnitude less than 1. Also listed are Stern–Levison's Λ and Soter's µ; again, the planets are orders of magnitude greater than 1 for Λ and 100 for µ, and the dwarf planets are orders of magnitude less than 1 for Λ and 100 for µ. Also shown are the distances where Π = 1 and Λ = 1 (where the body would change from being a planet to being a dwarf planet).
The mass of Sedna is not known; it is very roughly estimated here as , on the assumption of a density of about 2 g/cm
3.
Disagreement
Stern, the
principal investigator
In many countries, the term principal investigator (PI) refers to the holder of an independent grant and the lead researcher for the grant project, usually in the sciences, such as a laboratory study or a clinical trial. The phrase is also often us ...
of the ''
New Horizons'' mission to Pluto, disagreed with the reclassification of Pluto on the basis of its inability to clear a neighbourhood. He argued that the IAU's wording is vague, and that — like Pluto —
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
,
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
, 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. "If Neptune had cleared its zone, Pluto wouldn't be there", he said.
The IAU category of 'planets' is nearly identical to Stern's own proposed category of 'überplanets'. 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.
Although Stern proposed this to define dynamical subcategories of planets, he rejected it for defining what a planet is, advocating the use of intrinsic attributes over dynamical relationships.
See also
*
List of Solar System objects
The following is a list of Solar System objects by orbit, ordered by increasing distance from the Sun. Most named objects in this list have a diameter of 500 km or more.
*The Sun, a spectral class G2V main-sequence star
*The inner Solar Sy ...
*
List of gravitationally rounded objects of the Solar System
This is a list of most likely gravitationally rounded objects of the Solar System, which are objects that have a rounded, ellipsoidal shape due to their own gravity (but are not necessarily in hydrostatic equilibrium). Apart from the Sun itself, ...
*
List of Solar System objects by size
This article includes a list of the most massive known objects of the Solar System and partial lists of smaller objects by observed mean radius. These lists can be sorted according to an object's radius and mass and, for the most massive objects, ...
*
List of notable asteroids
Notes
References
{{Solar System
Astronomical controversies
Celestial mechanics
Definition of planet
Dynamics of the Solar System
Planetary science
Pluto's planethood