A double planet is a planetary binary system, in which both objects are of planetary mass. Binary asteroids with equal mass are sometimes imformally refered to as being "double minor planets". These include binary asteroids 69230 Hermes and 90 Antiope and binary Kuiper belt objects (KBOs) 79360 Sila-Nunam and 1998 WW31.
Double planet meaning[]
To distinguish "double planety" from a "planet-moon system:
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". 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.