Double planet 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 and Deimos) are thought to be asteroids captured long ago by Mars. Such a weak definition would also deem Neptune-Triton a double planet,
Ice planet Ice planet An ice planet is a theoretical type of exoplanet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere. They are bigger versions of the small icy worlds of the Solar System, which includes the moons Europa, Enceladus, and Triton, the dwarf planets Pluto and Eris, and many other small Solar System bodies such as comets. Ice planets usually appear nearly white with geometric albedos of more than 0.9. An ice planet's surface can be composed of water, methane, ammonia, carbon dioxide (known as "dry ice"), carbon monoxide, and