Double planet

In astronomy, a double planet (also binary planet) is a binary satellite system where both objects are planets, or planetary-mass objects, that share an orbital axis external to both planetary bodies.

Visual comparison of the sizes of Earth and the Moon (above right) and PlutoCharon (below right)

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 and are gravitationally stable only under particular circumstances.

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.

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