Double planet
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Iridescent
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Reverted edits by [[Special:Contributions/86.29.244.81|86.29.244.81]] (using [[WP:HG|Huggle]])
:''This article is about planets that orbit very close to each other. For 'double planet systems' (stars orbited by two planets), see [[planetary system]].''
[[Image:Plutoncharon1.jpg|thumb|250px|[[Pluto]] (left) and [[Charon (moon)|Charon]] are sometimes informally considered to be a double (dwarf) planet system since the [[barycenter]] lies in space between the two bodies.]]
[[Image:Moon Earth Comparison.png|thumb|250px|right|Since the barycenter of the Earth-Moon system lies beneath the surface of the Earth, these two bodies are informally referred to as a planet-satellite system.]]
"'''Double planet'''" is an informal term used to describe two [[planet]]s very close to each other: The formal term is "[[Binary system (astronomy)|binary system]]". Similarly, there are also '''double [[asteroid]]''' (or double [[Asteroid group|minor planet]]) systems such as [[90 Antiope]].
== Definition of a double planet ==
There has been some debate in the past on precisely where to draw the line between a double-planet and a planet-moon system. In most cases, this is not an issue because the satellite has a small mass relative to its host planet. In particular, with the exception of the Earth-Moon and Pluto-Charon systems, all satellites in our Solar System have masses less than 0.00025 the mass of the host planet or dwarf planet. On the other hand, the [[Earth]] and [[Moon]] have a mass ratio of 0.01230, and [[Pluto]] and its moon [[Charon (moon)|Charon]] have a mass ratio of 0.147.
A commonly accepted cutoff point for deciding between a planet-satellite or double-planet system is based on the location of the center of mass of the two objects (that is, the [[barycenter]]). If the barycenter is not located under the surface of either body, then one usually refers to the system as a double-planet system. In this case, both bodies in their entirety orbit about a point in the free space between the two. By this definition, Pluto and Charon would be seen as a "double" (dwarf) planet system, whereas the Earth and Moon would not. In [[2006 in science|2006]] the [[International Astronomical Union]] briefly considered a formal definition of the term double planet which could have formally included Pluto and Charon, but this definition was not ratified.
== Tug-of-war definition ==
{{Original research|date=September 2007}}
The late [[Isaac Asimov]] suggested a distinction between planet-moon and double-planet systems based on what he called a "tug-of-war" value.<ref>Asimov, Isaac (1975). ''Just Mooning Around'', In: Of time and space, and other things. Avon.</ref> This quantity is simply the ratio of the gravitational force between the planet and satellite, and the satellite and Sun. In the case of the [[Moon]], the Sun "wins" the "tug of war," and the Earth and Moon form a double-planet system. The opposite is true for most (but not all) of the other satellite systems in our Solar System, including the Pluto-Charon system, which would be classified as planet-satellite systems.
This definition was only intended to discuss the unique nature of the Earth-Moon system, and was not proposed as a final double planet definition. It has not received wide attention in the scientific community, and does not adequately describe the physics of three gravitationally interacting bodies (see [[N-body problem#Three-body problem|three-body problem]] and [[Hill sphere]]).
A major criticism of this definition is that two identical planet-moon systems possessing different distances from the central star could mean that one would be classified as a double planet, whereas the other would not.
Another criticism is that this definition does not account for the proportional difference in size between the planet and satellite. As an example, if we could replace Earth's moon with a satellite the size of [[Phobos (moon)|Phobos]], the system would still be considered a double-planet by this definition.
==See also==
*[[2006 definition of planet]]
*[[Definition of planet|Definition of a planet]]
*[[Hill sphere]]
*[[Natural satellite]]
*[[Orbit of the Moon]]
*[[3753 Cruithne]]
==References==
<references/>
* "[[Clyde Tombaugh]] (1906-97) Astronomer who discovered the Solar System's ninth planet", ''[[Nature (journal)|Nature]]'' 385 (1997) 778 (Pluto and Charon are "the only known example of a true double planet".)
* "It's not easy to make the Moon", ''Nature'' 389 (1997) 327 (comparing double planet theory of Moon formation and Pluto-Charon as double planet)
[[Category:Types of planet]]
[[Category:Binary planets|*]]
[[Category:Moons]]
[[Category:Moon]]
[[ar:كوكب مضاعف]]
[[ca:Planeta doble]]
[[de:Doppelplanetensystem]]
[[et:Kaksikplaneet]]
[[es:Planeta doble]]
[[fr:Planète double]]
[[it:Pianeta doppio]]
[[nl:Dubbelplaneet]]
[[ja:二重惑星]]
[[pl:Planeta podwójna]]
[[pt:Planeta duplo]]
[[sk:Dvojplanéta]]
[[fi:Kaksoisplaneetta]]
[[zh:雙行星]]