In astronomy, double planet and binary planet are informal terms used to describe a binary system of two astronomical objects that each satisfy the definition of planet and that are near enough to each other to have a significant gravitational effect on each other compared with the effect of the star(s) they orbit. As of 2010, there are no officially classified double planets in our Solar system. One unofficial definition is that the objects orbit a common center of gravity, the barycenter, that is above both their surfaces.
Similarly, there are also binary asteroids (also known as double minor planets) such as 90 Antiope, and binary Kuiper belt objects (KBOs) such as (79360) 1997 CS29 and 1998 WW31. The European Space Agency has referred to the Earth–Moon system as a type of double planet. The IAU General Assembly in August, 2006, considered a proposal that Pluto and Charon be reclassified as a type of double planet, however the proposal was abandoned.
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 (1⁄4000) the mass of the host planet or dwarf planet. On the other hand, the Moon to Earth mass ratio is 0.01230 (≈ 1⁄81), while the Charon to Pluto mass ratio is 0.117 (≈ 1⁄9).
A commonly accepted cutoff point by the vast majority of scientists for deciding between a planet-satellite or double-planet system is based on the location of the barycenter of the two objects (that is, the center of gravity). If the barycenter is not located under the surface of either body, then one may refer to the system as a double-planet system. In this case, both bodies orbit about a point in the free space between them. By this definition, Pluto and Charon would be seen as a double dwarf-planet system, whereas the Earth and Moon would remain defined as a planet-satellite system. Due to the increasing distance from Earth of the Moon's orbital path (because of tidal forces, the Moon currently drifts away from Earth about 3.74 cm or 1.5 in per year), the barycenter that now lies under the surface of the Earth will someday move outside the surface. So by this definition, the Earth-Moon system might be seen as a double planet billions of years from now. In 2006 the International Astronomical Union briefly considered a formal definition of double planet which would have included Pluto and Charon, but this definition was not ratified.
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