Double planet

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Not to be confused with a star orbited by two planets, called a double-planet system.
The EarthMoon system—sometimes informally referred to as a double planet[1]

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 EarthMoon system as a double planet.[1] 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.[2]

There are also double minor planets, such as binary asteroids 90 Antiope and binary Kuiper belt objects (KBOs) 79360 Sila–Nunam and 1998 WW31.

Definition of a double planet[edit]

There has been debate on where to draw the line between a double planet and a planet–moon system. In most cases, this is not an issue because most satellites have small masses relative to their planets. In particular, with the exception of the Earth–Moon system, all satellites of planets in the Solar System have masses less than 0.00025 (14000) the mass of the host planet. The Moon-to-Earth mass ratio is 0.01230 (≈ 181). In comparison, the Charon-to-Pluto mass ratio is 0.117 (≈ 19).

Co-accretion definition[edit]

The now-abandoned co-accretion hypothesis of the origin of the Moon is also called the double-planet hypothesis. The idea is that two bodies should be considered a double planet if they accreted together directly from the proto-planetary disk, much as a double star typically forms together.

Once it was realized that both the Moon and Pluto's Charon likely formed from giant impacts, this parallel was noted when calling them double planets. However, an impact may also produce tiny satellites, such as the small outer satellites of Pluto, so this does not determine where the line should be drawn.

Center-of-mass definition[edit]

Pluto and Charon orbit their common center of gravity, which lies outside of Pluto. Images captured by the New Horizons spacecraft in July 2014.[3]

A common proposal for a double planet is a system where the center of mass lies outside the primary. This was the basis for the argument that the Pluto–Charon system be considered a double planet when the IAU was debating whether dwarf planets should be considered a class of planet. Under this definition, the Earth–Moon system is not a double planet. However, the center of mass varies with the distance between the bodies. As the Moon migrates outward from Earth, the center of mass of the system will migrate outward as well, until in a few hundred million years Earth will fit the definition. It has been suggested that such a definition would call into question Jupiter's status as a planet, as the center of mass of the Jupiter–Sun system lies outside the surface of the Sun.[4]

Tug-of-war definition[edit]

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.[5] This quantity is simply the ratio of the force exerted on the smaller body by the larger (primary) body to the force exerted on the smaller body by the Sun. This can be shown to equal

tug-of-war value = mpms × (dsdp)2

where mp is the mass of the primary (the larger body), ms is the mass of the Sun, ds is the distance between the smaller body and the Sun, and dp is the distance between the smaller body and the primary.[5] Note that the tug-of-war value does not rely on the mass of the satellite (the smaller body).

This formula actually reflects the relation of the gravitational effects on the smaller body from the larger body 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. Titan's tug-of-war 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 calculated tug-of-war values for several satellites of the planets. 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. In nearly every one of Asimov's calculations the tug-of-war value was found to be greater than one, so in those cases the Sun loses the tug-of-war with the planets. The one exception was Earth's Moon, where the Sun wins the tug-of-war with a value of 0.46, which means that Earth's hold on the Moon is less than half that of the Sun's. Asimov included this with his other arguments that Earth and the Moon should be considered a binary planet.[5]

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. 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.[5][6][Footnote 1]

— Isaac Asimov

See the Path of Earth and Moon around Sun section in the "Orbit of the Moon" article for a more detailed explanation.

Note that this definition of double planet depends on the pair's distance from the Sun. 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. For example, at the orbit of Mars, the Moon's tug-of-war value would be 1.05. Also, several tiny moons discovered since Asimov's proposal would qualify as double planets by this argument. Neptune's small outer moons Neso and Psamathe, for example, have tug-of-war values of 0.42 and 0.44, less than that of Earth's Moon. Yet their masses are tiny compared to Neptune's, with an estimated ratio of 1.5×10−9 (1700,000,000) and 0.4×10−9 (12,500,000,000).

See also[edit]


  1. ^ Asimov uses the term "concave" to describe the Earth–Moon orbital pattern around the Sun, whereas Aslaksen uses "convex" to describe the exact same pattern. Which term one uses relies solely upon the perspective of the observer. From the point-of-view of the Sun, the Moon's orbit is concave; from outside the Moon's orbit, say, from planet Mars, it is convex.


  1. ^ a b "Welcome to the double planet". ESA. 2003-10-05. Retrieved 2009-11-12. 
  2. ^ "The IAU draft definition of "planet" and "plutons"". International Astronomical Union. 2006-08-16. Retrieved 2008-05-17. 
  3. ^ "A Moon over Pluto (Close up)". August 7, 2014. 
  4. ^ Herbst, T. M.; Rix, H.-W. (1999). Guenther, Eike; Stecklum, Bringfried; Klose, Sylvio, ed. Star Formation and Extrasolar Planet Studies with Near-Infrared Interferometry on the LBT. San Francisco, Calif.: Astronomical Society of the Pacific. pp. 341–350. Bibcode:1999ASPC..188..341H. ISBN 1-58381-014-5. 
  5. ^ a b c d Asimov, Isaac (1975). "Just Mooning Around", collected in Of Time and Space, and Other Things. Avon. Formula derived on p. 89 of book. p. 55 of .pdf file. Retrieved 2012-01-20.
  6. ^ Aslaksen, Helmer (2010). "The Orbit of the Moon around the Sun is Convex!". National University of Singapore: Department of Mathematics. Retrieved 2012-01-23. 


  • Stern, S. Alan (27 February 1997). "Clyde Tombaugh (1906–97) Astronomer who discovered the Solar System's ninth planet". Nature 385 (6619): 778. Bibcode:1997Natur.385..778S. doi:10.1038/385778a0 Pluto–Charon is "the only known example of a true double planet". 
  • Lissauer, Jack J. (25 September 1997). "It's not easy to make the Moon". Nature 389 (6649): 327–328. Bibcode:1997Natur.389..327L. doi:10.1038/38596 Compares the double-planet theories of Earth–Moon and Pluto–Charon formations. 

Further reading[edit]