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,
Wonders of the Solar System he examines how the Cassini–Huygens space-probe's imagery provides an insight on the highly complex structure of the ice rings of Saturn, and its diverse array of moons, and how they can reveal insights into the formation and evolution of the solar system. He also explains how Enceladus, as the most reflective object in the known universe, has been of interest due to its continental divide-like canyons and geysers (as also observed in parts of Iceland). In the Sahara Desert the formative effect of winds on sand-dune morphology is paralleled to that of the gravitational effect of Saturn's 61 known moons