Double planet

Double planet system (also short double planet ) is in astronomy a hitherto informal name for the system of a planet with a large moon, in comparison with the regular size ratios in the solar system has a relatively similar degree.

In the literal sense, a double planet would be a system of two planets that are gravitationally bound together because of their proximity and move around a common center of gravity according to Kepler 's laws of orbits. In the theoretical ideal case of two celestial bodies of exactly the same mass of the common center of gravity lies exactly at the geometrical center of the system and the distance between the two centers in the body is double that of an average orbital radius.

Term

The term is originally arisen for the system of the earth and its unusually large moon. Their size ratio is 1:3,7 and the mass ratio is 1:81; which has the classic question of the origin of the moon raised. According to the most conservative view according to the cosmogony Immanuel Kant's 1755 Earth and Moon have emerged from a common local compression within the presolar primordial cloud and have formed a priori a double aggregation. This hypothesis is therefore sometimes referred to in the literature with respect to the other views, of a double planet hypothesis. The modern computer-based so-called collision theory describes the formation of the Moon by the collision of a Mars-sized planet about the earth. The use of the term double planet takes place in the course of time more and more independent of the original cause.

With regard to the very erdmondähnlichen hemispheres of Mercury and its very eccentric elliptical orbit, there is a hypothesis that there is an escaped moon of Venus with him. That would explain why the two innermost planets are entirely without companions.

For planets of other stars, called exoplanets, no satellites are due to the difficult observation methods still detectable.

Pluto - Charon

Since the discovery of relatively large especially Pluto moon is also the system Pluto - Charon sometimes referred to as a double planet. The minimum diameter ratio is approximately 1:2 and the masses about 1:6,6. This means that the center of gravity ( barycenter ) of the Pluto -Charon system is 1200 km on Pluto's surface. Strictly speaking, thus Pluto and Charon orbit each other, so these two celestial bodies in accordance with the idea of ​​a double planet much stronger than Earth and Moon. For the formation of Charon increasingly a major collision is also favored.

In August 2006 the International Astronomical Union discussed in the context of the debate on the first scientific definition of a planet if Pluto and Charon should get together as a double planet the status of the planet Pluto class. In the discussion draft of a planet concept was proposed as a criterion for the existence of a binary system that the common center of gravity of the two planets must be outside the major.

Ultimately, the IAU decided against this design. With the adopted definition of planet Pluto was the planet status is withdrawn and placed in the same time newly created class of dwarf planets; Charon was not affected by this redefinition as a companion, so that it continues to officially regarded as the moon and, by definition, is not a double dwarf planet with Pluto system.