Double planet

In astronomy, a double planet (also binary planet) is a binary satellite system where both objects are planets, or planetary-mass objects, and whose joint barycenter is external to both planetary bodies.

Although up to a third of the star systems in the Milky Way are binary,^[1] double planets are expected to be much rarer given the typical planet to satellite mass ratio is around 1:10000, they are influenced heavily by the gravitational pull of the parent star^[2] and according to the giant-impact hypothesis are gravitationally stable only under particular circumstances.

The Solar System does not have an official double planet, however the Earth–Moon system is sometimes considered to be one. In promotional materials advertising the SMART-1 mission, the European Space Agency referred to the Earth–Moon system as a double planet.^[3]

Several dwarf planet candidates can be described as binary planets. At its 2006 General Assembly, the International Astronomical Union considered a proposal that Pluto and Charon be reclassified as a double planet,^[4] but the proposal was abandoned in favor of the current IAU definition of planet. Other trans-Neptunian systems with proportionally large planetary-mass satellites include Eris–Dysnomia, Orcus–Vanth and Varda–Ilmarë.

Binary asteroids with components of roughly equal mass are sometimes 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.