Double planet
 |
Pluto and Charon are often considered as a double planet. |
The term
double planet is an informal term with several accepted usages which share the meaning of two interacting planets of comparable mass.
There are also
double asteroids (or double
minor planets), such as
90 Antiope.
In the most common usage, a double planet is a set of two
planets of comparable mass orbiting one another. That is, a
binary planet.
Pluto and
Charon have consistently been recognized in the literature as representing the only real double planet system in our
Solar System today.
Debate
In this usage, there has been some debate on precisely where to draw the line between a double planet and a planet-moon system. In most cases, it is not an issue because the moon is of very small mass relative to its host planet. In particular, the Earth-Moon and Pluto-Charon systems are the only examples in our present Solar System where the mass of a moon is larger than one fortieth of one percent of the mass of the host planet (i.e. mass ratio of 0.00025 or less). On the other hand, the
Earth and the
Moon have a mass ratio of 0.01230, and
Pluto and its moon
Charon have a mass ratio of 0.147. A commonly accepted cutoff point is when the center of mass that the two objects orbit around (the
barycenter) is not located under the surface of either body, in which case the barycenter is in space between the two bodies. This literally makes the difference between whether one body orbits around the other body, or whether both bodies orbit about a point in space between them. By this definition, Pluto and Charon qualify as a double planet and Earth and Moon do not.
The second usage for the term double planet, in the context of the theory of the origin of
the Moon (i.e. Earth's Moon), is a set of two planets of comparable mass that collide with each other - i.e. with at least transiently overlapping orbits. A double planet in this sense occurred in the very early
Solar System, consisting of the
proto-Earth and a second,
Mars-sized planet that collided with it at an oblique angle, in the consensus theory of the formation of the
Earth-
Moon system. The second body was not a proto-Moon because most of its mass was incorporated into the Earth, while the Moon formed from a small fraction of debris kicked up from the Earth by the collision. These double planet precursor bodies to the Earth-Moon system had roughly comparable mass - i.e. a mass ratio in the neighborhood of 10:1. This happens to be similar to the mass ratio of Pluto-Charon.
A third usage has arisen since 1995 when we began to discover
extrasolar planets in other Solar Systems. In this context, the term double planet system is used to refer to another Solar System in which two planets have been discovered orbiting the star.
As of 2003, there were ten known star systems outside our own with at least two detected planets, qualifying at least as double planet systems. Multiple planet systems with more than two planets have been discovered as well, including the
Upsilon Andromedae, Rho-1 Cancri (or
55 Cancri), and
Mu Arae systems.
The late
Isaac Asimov suggested a distinction between planet-moon systems and double-planet systems based on what he called a
tug-of-war (TOW) value that describes whether the presumed satellite is more firmly under the gravitational influence of the presumed planetary primary or the Sun. In the case of the Moon, the Sun "wins" the tug of war,
i.e., its gravitational hold on the Moon is greater than that of Earth. The opposite is true for other presumed satellites in the Solar System (with a few exceptions), including the Pluto-Charon system. By this definition, the Earth and Moon form a double-planet system, but Pluto and Charon represent a true primary with a satellite.
This definition has not received wide attention in the professional literature.
*[
1] - "
Clyde Tombaugh (1906-97) Astronomer who discovered the Solar System's ninth planet",
Nature 385 (1997) 778 (Pluto and Charon are "the only known example of a true double planet".)
*[
2] - "It's not easy to make the Moon",
Nature 389 (1997) 327 (comparing double planet theory of Moon formation and Pluto-Charon as double planet)
*[
3] - "Geochemical implications of the formation of the Moon by a single giant impact",
Nature 338 (1989) 29
*[
4] - "Occurrence and Stability of Apsidal Resonance in Multiple Planetary Systems",
Astrophysical Journal 598 (2003) 1290
