Editing Double planet (section)

=== Formation of the system ===

A final consideration is the way in which the two bodies came to form a system. Both the Earth–Moon and Pluto–Charon systems are thought to have been formed as a result of [[giant impact hypothesis|giant impacts]]: one body was impacted by a second body, resulting in a debris disk, and through accretion, either two new bodies formed or one new body formed, with the larger body remaining (but changed). However, a giant impact is not a sufficient condition for two bodies being "double planets" because such impacts can also produce tiny satellites, such as the four small outer satellites of Pluto.

A now-abandoned hypothesis for the [[origin of the Moon]] was actually called the "double-planet hypothesis"; the idea was that the Earth and the Moon formed in the same region of the [[Solar System]]'s proto-planetary disk, forming a system under gravitational interaction. This idea, too, is a problematic condition for defining two bodies as "double planets" because planets can "capture" moons through gravitational interaction. For example, the [[moons of Mars]] ([[Phobos (moon)|Phobos]] and [[Deimos (moon)|Deimos]]) are thought to be asteroids captured long ago by [[Mars]]. Such a definition would also deem Neptune–Triton a double planet, since [[Triton (moon)|Triton]] was a [[Kuiper belt]] body the same size and of similar composition to Pluto, later [[Capture of Triton|captured]] by [[Neptune]].