The terms “binary planet” and “double planet” are informal terms used to describe a binary system of two astronomical objects that fit the definition of a planet and are close enough to each other that their gravitational fields affect each other.
There is some debate over what classifies a double planet system versus a planet-moon system, and there have been calls for drawing a precise line between the two types of systems. There is usually no trouble distinguishing between the two, as moons (or “satellites,” as they are formally known) typically have masses incredibly smaller than the planet they orbit. However, in the case of the Earth-Moon and Pluto-Cheron systems, the planets and satellites are much closer in mass. There are several different ways to distinguish planet-moon systems and double planet systems.
In the center-of-mass definition of double planet systems, the center of mass, or barycenter, must not be located under the surface of either planet. Under this definition, Pluto-Charon would be considered a double dwarf-planet system, as neither is a full-sized planet. The Earth-Moon system would remain a planet-satellite system because, currently, the barycenter lies under the surface of the Earth. However, the Moon and Earth are moving away from each other, and according to Princeton, they may eventually be considered a double-planet system under this definition, as they will both be orbiting a common center of gravity.
Another definition is the tug-of-war definition, which does not consider the sizes of the planets. Instead, it uses the mass of the primary planet and the Sun combined with the squared distances between the smaller object and its planet and the sun. The resulting formula reflects the effects of gravity on the smaller planet from the larger planet and the sun. If the Sun has a greater “tug” on the smaller planet than the larger planet does, it is considered a double planet system.
One definition, which has been abandoned, is the co-accretion definition, which says that the two planetary bodies should be considered a double planet system if the formed, or accreted, from the same protoplanetary disk, or the rotating disk of dust and gas that surrounds the core of a developing planet.
Though the local solar system does not have any official double planet systems, the Kepler Space Telescope has discovered one confirmed system as of August 2010. The Kepler, launched in 2009, is NASA’s first spacecraft designed to hunt for extra-solar planets around other stars. It does so by watching for a transit, or for something moving in front of a star and blocking some of the starlight. The double planet system that the Kepler found has planets close to the size of Saturn, and if they were placed in the local solar system they would fall within the orbit of Mercury, the planet closest to the Sun. The planets have a gravitational influence on each other. For every two orbits that the inner planet makes, the outer makes one orbit, and when they swing past each other their transits may slow up or speed down.