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In astronomy, double planet and binary planet are informal terms used to describe a binary system where both objects are of planetary mass. Though not an official classification, the European Space Agency has referred to the Earth–Moon system as a double planet. The IAU General Assembly in August 2006 considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned.
There has been some debate in the past on precisely where to draw the line between a double-planet and a planet–moon system. In most cases, this is not an issue because the satellite has a small mass relative to its host planet. In particular, with the exception of the Earth–Moon and Pluto–Charon systems, all satellites in the Solar System have masses less than 0.00025 (1⁄4000) the mass of the host planet or dwarf planet. On the other hand, the Moon to Earth mass ratio is 0.01230 (≈ 1⁄81), while the Charon to Pluto mass ratio is 0.117 (≈ 1⁄9).
Isaac Asimov suggested a distinction between planet–moon and double-planet systems based in part on what he called a "tug-of-war" value. This quantity is simply the relationships between the masses of the primary planet and the Sun combined with the squared distances between the satellite and its planet and the Sun:
where m1 is the mass of the primary planet, m2 is the mass of the Sun, d1 is the distance between the satellite and the Sun, and d2 is the distance between the satellite and its primary planet. Note that the tug-of-war value does not rely on the mass of the satellite or smaller body.
This formula actually reflects the relation of the gravitational forces the satellite "feels" from the planet and from the Sun. The tug-of-war figure for Saturn's moon Titan is 380, which means that Saturn's hold on Titan is 380 times as strong as the Sun's hold on Titan. This figure does not reflect the orbital stability, which is better estimated by the ratio of the satellite's semi-major axis to the Hill radius of the planet with respect to the Sun. Titan's value may be compared with that of Saturn's moon Phoebe, which has a tug-of-war value of just 3.5. So Saturn's hold on Phoebe is only 3.5 times as strong as the Sun's hold on Phoebe.
Asimov figured tug-of-war values for several satellites of all the planets that have them except for dwarf planet Pluto, because very little was known about Pluto and its satellite Charon at the time. He showed that even the great 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. Yet in every case the tug-of-war value was found to be greater than one, so in every case the Sun "loses" the tug of war with the planets.
In the case of the Earth's Moon, the Sun actually "wins" the tug of war with a value of only 0.46, which means that Earth's hold on the Moon is less than half the Sun's hold. Since the Sun's gravitational effect on the Moon is more than twice that of Earth's, Asimov reasoned that the Earth and Moon must form a double-planet system. This was one of several arguments in Asimov's writings for considering the Moon to be a planet rather than a satellite.
We might look upon the Moon, then, as neither a true satellite of the Earth nor a captured one, but as a planet in its own right, moving about the Sun in careful step with the Earth. To be sure, from within the Earth-Moon system, the simplest way of picturing the situation is to have the Moon revolve about the Earth; but if you were to draw a picture of the orbits of the Earth and Moon about the Sun exactly to scale, you would see that the Moon's orbit is everywhere concave toward the Sun. It is always "falling toward" the Sun. 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.— Isaac Asimov
See the Path of Earth and Moon around Sun section in the "Orbit of the Moon" article for a more detailed explanation.
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