Double planet
In astronomy, a double planet (also binary planet) is a binary system where both objects are of planetary mass. The term is not recognized by the International Astronomical Union and is therefore not an official classification. At its 2006 General Assembly, the International Astronomical Union considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned in favor of the current definition of planet. In promotional materials advertising the SMART-1 mission and pre-dating the IAU planet definition, the European Space Agency once referred to the Earth–Moon system as a double planet.
Some binary asteroids with components of roughly equal mass are sometimes informally referred to as double minor planets. These include binary asteroids 69230 Hermes and 90 Antiope and binary Kuiper belt objects (KBOs) 79360 Sila–Nunam and 1998 WW31.
There is debate as to what criteria should be used to distinguish "double planet" from a "planet–moon system". The following are considerations.
A definition proposed in the Astronomical Journal calls for both bodies to individually satisfy an orbit-clearing criterion in order to be called a double planet.
One important consideration in defining "double planet" is the ratio of the masses of the two bodies. A mass ratio of 1 would indicate bodies of equal mass, and bodies with mass ratios closer to 1 are more attractive to label as "doubles." Using this definition, the satellites of Mars, Jupiter, Saturn, Uranus, and Neptune can all easily be excluded; they all have masses less than 0.00025 ( 1⁄4000) of the planets around which they revolve. Some dwarf planets, too, have satellites substantially less massive than the dwarf planets themselves.
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