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,
Solar System Exploration Research Virtual Institute on one or more aspects of lunar science investigations of the Moon (including lunar samples), from the Moon, and on the Moon. In 2013 the NLSI became the Solar System Exploration Research Virtual Institute and expanded its scope to include Near-Earth asteroids and Phobos and Deimos. The Solar System Exploration Research Virtual Institute (SSERVI) is composed of thirteen U.S. teams, and ten international partners. The teams are listed below: US TEAMS: Focus: Formation of terrestrial planets and asteroid belt, modeling of the Moon's origin and Phobos/Deimos, history of NEAs and lunar bombardment, NEA origins, identification and characterization boulder.swri.edu Focus: Studies