Clearing the neighbourhood
"Clearing the neighbourhood around its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union (IAU) in its 2006 definition of planet. In 2015, a proposal was made to utilise the criterion in extending the definition to exoplanets.
In the end stages of planet formation, a planet (as so defined) will have "cleared the neighbourhood" of its own orbital zone (see below), 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. A large body that meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. This includes Pluto, which is constrained in its orbit by the gravity of Neptune and 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, but all the planets have cleared their neighbourhoods to a much greater extent (by orders of magnitude) than any dwarf planet, or any candidate for dwarf planet.
The phrase may be derived from a paper presented to the general assembly of the IAU in 2000 by Alan Stern and Harold F. 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. Steven Soter prefers to use the term "dynamical dominance" and Jean-Luc Margot notes that such language "seems less prone to misinterpretation".
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 and 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.
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, or to be captured either as a satellite 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, such as Jupiter and its trojans, Earth and 3753 Cruithne, or Neptune and the plutinos. As to the extent of orbit clearing required, Jean-Luc Margot 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.
In their paper, Stern and Levison sought an algorithm to determine which "planetary bodies control the region surrounding them". They defined Λ (lambda), 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). 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).
Steven Soter proposed an observationally based measure µ (mu), which he called the "planetary discriminant", to separate bodies orbiting stars into planets and non-planets. Per Soter, 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 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.
Astronomer Jean-Luc Margot has proposed a discriminant, Π, 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. Π 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 = 812.[a] 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, 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 of the star; Λ's use of a Hubble time 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 within an order of magnitude of the circular assumption.
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).
|Mass (kg)||Type of object||Π = 1
|Λ = 1
|1||Jupiter||4.0 × 104||6.25 × 105||1.30 × 109||1.8986 × 1027||5th planet||65,000||6,220,000|
|2||Saturn||6.1 × 103||1.9 × 105||4.68 × 107||5.6846 × 1026||6th planet||22,000||1,250,000|
|3||Venus||9.5 × 102||1.35 × 106||1.66 × 105||4.8685 × 1024||2nd planet||320||2,180|
|4||Earth||8.1 × 102||1.7 × 106||1.53 × 105||5.9736 × 1024||3rd planet||390||2,870|
|5||Uranus||4.2 × 102||2.9 × 104||3.84 × 105||8.6832 × 1025||7th planet||4,200||102,000|
|6||Neptune||3.0 × 102||2.4 × 104||2.73 × 105||1.0243 × 1026||8th planet||4,800||127,000|
|7||Mercury||1.3 × 102||9.1 × 104||1.95 × 103||3.3022 × 1023||1st planet||30||60|
|8||Mars||5.4 × 101||1.8 × 105||9.42 × 102||6.4185 × 1023||4th planet||53||146|
|9||Ceres||4.0 × 10−2||0.33||8.32 × 10−4||9.43 × 1020||dwarf planet||0.16||0.0245|
|10||Pluto||2.8 × 10−2||0.077||2.95 × 10−3||1.29 × 1022||dwarf planet||1.7||0.812|
|11||Eris||2.0 × 10−2||0.10||2.15 × 10−3||1.67 × 1022||dwarf planet||2.1||1.13|
|12||Haumea||7.9 × 10−3||0.02||2.41 × 10−4||4.0 × 1021||dwarf planet||0.59||0.168|
|13||Makemake||7.4 × 10−3||0.02||2.22 × 10−4||~4 × 1021||dwarf planet||0.58||0.168|
Stern, currently leading NASA's New Horizons mission to Pluto, disagrees with 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 (NEAs), and Jupiter has 100,000 trojans in its orbital path. "If Neptune had cleared its zone, Pluto wouldn't be there", he now says.
This is a shift from Stern's statement in 2000: "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.
- List of Solar System objects
- List of gravitationally rounded objects of the Solar System
- List of Solar System objects by size
- List of notable asteroids
- , which is rounded to 1.9·10−4 in the paper. Using the unrounded figure gives 812, whereas using the rounded figure would give 816.
- These values are based on a value of k estimated for Ceres and the asteroids belt: k equals 1.53 × 105 AU1.5/M⊕2, where AU is the astronomical unit and M⊕ is the mass of Earth. Accordingly, Λ is dimensionless.
- "IAU 2006 General Assembly: Result of the IAU Resolution votes". IAU. 24 August 2006. Retrieved 2009-10-23.
- Margot, Jean-Luc (2015-10-15). "A Quantitative Criterion for Defining Planets" (PDF). The Astronomical Journal 150 (6): 185. arXiv:1507.06300v4. doi:10.1088/0004-6256/150/6/185.
- Stern, S. Alan; 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, 7–18 August 2000]. Bibcode:2002HiA....12..205S.
- Soter, Steven (2006-08-16). "What is a Planet?". The Astronomical Journal 132 (6): 2513–2519. arXiv:astro-ph/0608359. Bibcode:2006AJ....132.2513S. doi:10.1086/508861.
- Calculated using the estimate for the mass of the Kuiper belt found in Iorio, 2007 of 0.033 Earth masses
- Rincon, Paul (25 August 2006). "Pluto vote 'hijacked' in revolt". BBC News. Retrieved 2006-09-03.