Double planet

Although up to a third of the star systems in the Milky Way are binary,[1] double planets are expected to be much rarer given the typical planet to satellite mass ratio is around 1:10000, they are influenced heavily by the gravitational pull of the parent star[2] and according to the giant-impact hypothesis are gravitationally stable only under particular circumstances.

In promotional materials advertising the SMART-1 mission, the European Space Agency referred to the Earth–Moon system as a double planet.

Binary asteroids with components of roughly equal mass are sometimes referred to as double minor planets.

[5] One important consideration for defining "double planets" is the ratio of the masses of the two bodies.

Using this definition, the satellites of Mars, Jupiter, Saturn, Uranus, and Neptune can all easily be excluded; they all have masses less than 0.00025 (1⁄4000) of the planets around which they revolve.

The International Astronomical Union (IAU) earlier classified Charon as a satellite of Pluto, but had also explicitly expressed the willingness to reconsider the bodies as double dwarf planets in the future.

However, the Moon currently migrates outward from Earth at a rate of approximately 3.8 cm (1.5 in) per year; in a few billion years, the Earth–Moon system's center of mass will lie outside Earth, which would make it a double-planet system.

Jupiter is too light to be a fusor; were it thirteen times heavier, it would achieve deuterium fusion and become a brown dwarf.

[8] Isaac Asimov suggested a distinction between planet–moon and double-planet structures based in part on what he called a "tug-of-war" value, which does not consider their relative sizes.

He showed that even the largest gas giant, Jupiter, had only a slightly better hold than the Sun on its outer captured satellites, some with tug-of-war values not much higher than one.

All the other satellites, without exception, "fall away" from the Sun through part of their orbits, caught as they are by the superior pull of their primary planets – but not the Moon.

If the Earth–Moon system happened to orbit farther away from the Sun than it does now, then Earth would win the tug of war.

Also, several tiny moons discovered since Asimov's proposal would qualify as double planets by this argument.

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.

Such a definition would also deem Neptune–Triton a double planet, since Triton was a Kuiper belt body the same size and of similar composition to Pluto, later captured by Neptune.