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Encyclopedia > Double planet
Pluto and Charon are sometimes informally considered to be a double (dwarf) planet.
Pluto and Charon are sometimes informally considered to be a double (dwarf) planet.

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". The term "double planet" has also been used to refer more generally to two interacting planets or dwarf planets of comparable mass. There are also double asteroids (or double minor planets), such as 90 Antiope. Image File history File links Plutoncharon1. ... Image File history File links Plutoncharon1. ... Atmospheric characteristics Atmospheric pressure 0. ... Media:Example. ... The eight planets and three dwarf planets of the Solar System. ... In physics, the center of mass (or centre of mass) of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it were concentrated. ... A binary system is an astronomy term referring to two objects in space, usually stars, which are so close that their gravitational forces attract one another into a mutual orbit. ... Artists impression of Pluto (background) and Charon (foreground). ... Asteroid, minor planet, and planetoid are synonyms, and are used to indicate a diverse group of small celestial bodies that drift in the solar system in orbit around the Sun. ... Minor planets, or planetoids are minor bodies of the Solar system orbiting the Sun (or of other planetary systems orbiting other stars) that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a... 90 Antiope (an-tye-a-pee) is an asteroid discovered on October 1, 1866 by Robert Luther. ...

Contents

Debate

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, it is not an issue because the moon is of very small mass relative to its host planet. In particular, the Earth-Moon and Pluto-Charon systems are the only examples in our present Solar System where the mass of a moon is larger than one fortieth of one percent of the mass of the host planet or dwarf planet(i.e. mass ratio of 0.00025 or less). On the other hand, the Earth and the 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 is when the center of mass that the two objects orbit around (the barycenter) is not located under the surface of either body, in which case the barycenter is in space between the two bodies. This literally makes the difference between whether one body orbits around the other body, or whether both bodies orbit about a point in space between them. By this definition, Pluto and Charon could be seen as a "double" (dwarf) planet and Earth and Moon would not.[1] Earth (IPA: , often referred to as the Earth, Terra, or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ... Bulk composition of the Moons mantle and crust estimated, weight percent Oxygen 42. ... Atmospheric characteristics Atmospheric pressure 0. ... Media:Example. ... It has been suggested that Center of gravity be merged into this article or section. ... It has been suggested that Center of gravity be merged into this article or section. ...


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. 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ...


Two planets colliding with each other

The second usage for the term double planet, in the context of the theory of the origin of the Moon (i.e. Earth's Moon), is a set of two planets of comparable mass that collide with each other - i.e. with at least transiently overlapping orbits. A double planet in this sense occurred in the very early solar system, consisting of the proto-Earth and a second, Mars-sized planet that collided with it at an oblique angle, in the consensus hypothesis of the formation of the Earth-Moon system. The second body was not a proto-Moon because most of its mass was incorporated into the Earth, while the Moon formed from a small fraction of debris kicked up from the Earth by the collision. These double planet precursor bodies to the Earth-Moon system had roughly comparable mass - i.e. a mass ratio in the neighborhood of 10:1. This happens to be similar to the mass ratio of Pluto-Charon. For other moons in the solar system see natural satellite. ... Major features of the Solar System (not to scale): The Sun, the eight planets, the asteroid belt containing the dwarf planet Ceres, outermost there is the dwarf planet Pluto (the dwarf planet Eris not shown), and a comet. ... Note: This article contains special characters. ...


Two planets orbiting a star

A third usage has arisen since 1995 when we began to discover extrasolar planets 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 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. Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ... 2003 is a common year starting on Wednesday of the Gregorian calendar, and also: The International Year of Freshwater The European Disability Year Events January events January 1 Luíz Inácio Lula Da Silva becomes the 37th President of Brazil. ... Upsilon Andromedae (Ï… And ) is a star, approximately 44 light-years from Earth, and approximately 3 billion years old, two thirds the age of our Sun. ... 55 Cancri (abbreviated 55 Cnc; Bayer designation ρ1 Cancri, Rho-1 Cancri) is a nearby 6th magnitude star in the constellation Cancer. ... Mu Arae (NASA Nstars database) Mu Arae (μ Ara / μ Arae) is a 5th magnitude yellow-orange G-type star similar to our Sun. ...


Asimov's proposed definition

The late Isaac Asimov suggested a distinction between planet-moon systems and double-planet systems based on what he called a tug-of-war value that describes whether the presumed satellite is more firmly under the gravitational influence of the presumed planetary primary or the Sun. In the case of the Moon, the Sun "wins" the tug of war, i.e., its gravitational hold on the Moon is greater than that of Earth. The opposite is true for other presumed satellites in the Solar System (with a few exceptions), including the Pluto-Charon system. By this definition, the Earth and Moon form a double-planet system, but Pluto and Charon represent a true primary with a satellite. Isaac Asimov, Ph. ...


This definition has not received wide attention in the professional literature. A major criticism of this is that two pairs of bodies of identical size but at different distances from their star could mean one pair would be classed as a double planet and the other pair not. Another criticism is that the definition does not account for the proportional difference in size between the two bodies, so that a circumstance may arise where either: a, two similarly sized objects would be considered a planet-moon system, if close enough to each other, or b. a grossly disproportionate pair could be considered a double-planet, if given sufficient distance between them.


This definition was only intended to discuss the unique nature of the earth moon system. Asimov only concluded that the moon had the sun as its primary; that the earth had lost the tug of war and that the moon was not a satellite of the earth. It was not proposed as a final double planet definition but as only one test that would distinguish between a double planet and a satellite/primary system. This part of the required final definition uses a mathematical test. The relative size requirement was not discussed. Relative size involves an arbitrary limit rather than a clean mathematical one.


References

  1. ^ http://tidesandcurrents.noaa.gov/restles3.html

Sources

  • "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)
  • [1] - "Geochemical implications of the formation of the Moon by a single giant impact", Nature 338 (1989) 29
  • "Occurrence and Stability of Apsidal Resonance in Multiple Planetary Systems", Astrophysical Journal 598 (2003) 1290

  Results from FactBites:
 
Double planet - Wikipedia, the free encyclopedia (983 words)
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 second usage for the term double planet, in the context of the theory of the origin of the Moon (i.e.
A double planet in this sense occurred in the very early solar system, consisting of the proto-Earth and a second, Mars-sized planet that collided with it at an oblique angle, in the consensus hypothesis of the formation of the Earth-Moon system.
Pluto & Charon - Double Planet Meets Triple Star (1515 words)
Seen from the Earth, this planet moved in front of two different stars on July 20 and August 21, respectively, providing observers at various observatories in South America and in the Pacific area with a long awaited and most welcome opportunity to learn more about tenuous atmosphere of that cold planet.
When the star moves behind the planet, the stellar rays suffer minute deviations as they are refracted (i.e., bent and defocussed) by the planet's atmospheric layers.
This effect, together with the large distance to the planet, manifests itself as a gradual decline of observed intensity of the stellar light, rather than an abrupt drop as this would be the case if the planet had no atmosphere.
  More results at FactBites »

 
 

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