双行星和联行星是非正式的天文学术语，用来描述一颗有着够大卫星的行星，因而必须考虑那颗卫星是否也算是行星。一个非官方的定义需要考虑轨道的重力中心，质心，是否落在两者的表面之外。正式的名称是"联星系"，相似的，也称为双小行星（或双迷你行星）系统，像是安地欧普，和双开普带天体（KBO）系统，例如79360 1997CS29和1998 WW31。迄2009年，在太阳系中还没有被官方认可的双行星。欧洲太空总署曾经提议将地月系统视为双行星。在2006年8月召开的国际天文联合会会员大会也曾经选出冥王星和冥卫一（卡伦）系统是双行星的一种类型。

In astronomy, 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.