A double planet is an informal term used to describe two planets that orbit each other about a common center of mass that is not located within the interior of either planet: The formal term is "binary system". Similarly, there are also double asteroid (or double 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 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 arycenter). 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 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.
A second, and unrelated, usage of the term "double planet system" has arisen since 1995 when extrasolar planets began to be discoved in other solar systems. In this context, the term "double planet system" is used to refer to another solar system in which two planets have been discovered orbiting the central star. As of 2003, there were ten known star systems outside our own with at least two detected planets, qualifying at least as double planet systems. Multiple planet systems with more than two planets have been discovered as well, including the Upsilon Andromedae, Rho-1 Cancri (or 55 Cancri), and Mu Arae systems.
Asimov's proposed definition
The late Isaac Asimov suggested a distinction between planet-moon and double-planet systems based on what he called a "tug-of-war" value. 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 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, the system would still be considered a double-planet by this definition.
- ↑ Asimov, Isaac (1975). Just Mooning Around, In: Of time and space, and other things. Avon.
- "Clyde Tombaugh (1906-97) Astronomer who discovered the Solar System's ninth planet", 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)