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Definition of "double planet"Edit



Both bodies satisfy planet criterionEdit

A definition proposed in the Astronomical Journal calls for both bodies to individually satisfy an orbit-clearing criterion in order to be called a double planet.[3]

Mass ratios closer to 1Edit

One important consideration in defining "double planet" is the ratio of the masses of the two bodies. A mass ratio of 1 would indicate bodies of equal mass, and bodies with mass ratios closer to 1 are more attractive to label as "doubles". 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 (​14000) of the planets around which they revolve. Some dwarf planets, too, have satellites substantially less massive than the dwarf planets themselves.


The Moon-to-Earth mass ratio of 0.01230 (≈ ​181) is also notably close to 1 when compared to all other satellite-to-planet ratios. Consequently, some scientists view the Earth-Moon system as a double planet as well, though this is a minority view. Eris's lone satellite, Dysnomia, has a radius somewhere around ​14 that of Eris; assuming similar densities (Dysnomia's compositional make-up may or may not differ substantially from Eris's), the mass ratio would be near ​140, a value intermediate to the Moon–Earth and Charon–Pluto ratios.


PlutoCharon system: the barycenter lies outside of Pluto.

Center-of-mass positionEdit

Currently, the most commonly proposed definition for a double-planet system is one in which the barycenter, around which both bodies orbit, lies outside both bodies. Under this definition, Pluto and Charon are double dwarf planets, since they orbit a point clearly outside of Pluto, as visible in animations created from images of the New Horizons space probe in June 2015.


The center of mass of the Jupiter–Sun system lies outside the surface of the Sun, though arguing that Jupiter and the Sun are a double star is not analogous to arguing Pluto-Charon is a double dwarf planet. The problem is that Jupiter is not a star, or even a brown dwarf, and due to its low mass it is unable to achieve any form of fusion.[4]

Tug-of-war valueEdit



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] The tug-of-war value does not rely on the mass of the satellite (the smaller body).




— Isaac Asimov



Formation of the systemEdit


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 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.

See alsoEdit


Informational notes



  3. ^ Margot, J.L. "A Quantitative Criterion for Defining Planets". Astronomical Journal. 150: 185. arXiv:1507.06300. Bibcode:2015AJ....150..185M. doi:10.1088/0004-6256/150/6/185.
  4. ^ Herbst, T. M.; Rix, H.-W. (1999). Guenther, Eike; Stecklum, Bringfried; Klose, Sylvio (eds.). 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.
  6. ^ Aslaksen, Helmer (2010). "The Orbit of the Moon around the Sun is Convex!". National University of Singapore: Department of Mathematics. Archived from the original on 2013-01-16. 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:10.1038/385778a0Pluto–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:10.1038/38596Compares the double-planet theories of Earth–Moon and Pluto–Charon formations.

Further reading

External linksEdit