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,
The Angry Red Planet find a recording of an alien voice, which announces that the ship's crew were allowed to leave Mars so they could deliver a message to their home planet. The voice then states that "we of Mars" have been observing human development on Earth for many thousands of years and have determined that humanity's technology has far outpaced progress in cultural advancement. The alien then accuses humankind of invading Mars, warning that if future expeditions ever return to the Red Planet, the Earth would be destroyed in retaliation. "The Angry Red Planet" began as "The Planet Mars", a treatment written by