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,
Moons of Mars Sun's angular diameter as seen from Mars, by contrast, is about 21'. Thus there are no total solar eclipses on Mars, as the moons are far too small to completely cover the Sun. On the other hand, total lunar eclipses of Phobos are very common, happening almost every night. The motions of Phobos and Deimos would appear very different from that of our own Moon. Speedy Phobos rises in the west, sets in the east, and rises again in just eleven hours, while Deimos, being only just outside synchronous orbit, rises as expected in the east but very slowly. Despite