Double planet: Difference between revisions
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imported>OAbot m Open access bot: url-access updated in citation with #oabot. |
imported>Shem.iz m Added citation from Stern (1992) to verify the barycenter based definition of a double planet system |
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{{Short description|Binary system where two planetary-mass objects share an orbital axis external to both}} | {{Short description|Binary system where two planetary-mass objects share an orbital axis external to both}} | ||
{{More references needed|date=September 2025}} | |||
{{For|a star orbited by two planets|Double-planet system}} | {{For|a star orbited by two planets|Double-planet system}} | ||
{{See also|Satellite system (astronomy)}} | {{See also|Satellite system (astronomy)}} | ||
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In astronomy, a '''double planet''' (also '''binary planet''') is a [[Binary system (astronomy)|binary]] [[Satellite system (astronomy)|satellite system]] where both [[astronomical object|objects]] are [[planets]], or [[planemo|planetary-mass objects]], and whose [[barycenter]] is external to both planetary bodies. | In astronomy, a '''double planet''' (also '''binary planet''') is a [[Binary system (astronomy)|binary]] [[Satellite system (astronomy)|satellite system]] where both [[astronomical object|objects]] are [[planets]], or [[planemo|planetary-mass objects]], and whose [[barycenter]] is external to both planetary bodies. | ||
Although up to a third of the [[star system]]s in the [[Milky Way]] are binary,<ref>[http://www.cfa.harvard.edu/news/2006-11 Most Milky Way Stars Are Single], Harvard-Smithsonian Center for Astrophysics</ref> double planets are expected to be much rarer | Although up to a third of the [[star system]]s in the [[Milky Way]] are binary,<ref>[http://www.cfa.harvard.edu/news/2006-11 Most Milky Way Stars Are Single], Harvard-Smithsonian Center for Astrophysics</ref> double planets are expected to be much rarer. Given the typical planet to satellite mass ratio is around 1:10,000, they are influenced heavily by the gravitational pull of the parent star<ref>{{Cite journal|last1=Canup|first1=Robin M.|author1-link=Robin Canup |last2=Ward|first2=William R.|date=June 2006|title=A common mass scaling for satellite systems of gaseous planets|url=https://www.nature.com/articles/nature04860|journal=Nature|language=en|volume=441|issue=7095|pages=834–839|doi=10.1038/nature04860|pmid=16778883|bibcode=2006Natur.441..834C|s2cid=4327454|issn=1476-4687|url-access=subscription}}</ref> and according to the [[giant-impact hypothesis]] are gravitationally stable only under particular circumstances. | ||
The [[Solar System]] does not have an official double planet, however the [[Earth]]–[[Moon]] system is sometimes considered to be one. In promotional materials advertising the [[SMART-1]] mission, the [[European Space Agency]] referred to the Earth–Moon system as a double planet.<ref name="esa-double">{{cite web |date=2003-10-05 |title=Welcome to the double planet |url=http://www.esa.int/esaMI/SMART-1/SEMO1VMKPZD_0.html |access-date=2009-11-12 |publisher=[[European Space Agency|ESA]]}}</ref> | The [[Solar System]] does not have an official double planet, however the [[Earth]]–[[Moon]] system is sometimes considered to be one. In promotional materials advertising the [[SMART-1]] mission, the [[European Space Agency]] referred to the Earth–Moon system as a double planet.<ref name="esa-double">{{cite web |date=2003-10-05 |title=Welcome to the double planet |url=http://www.esa.int/esaMI/SMART-1/SEMO1VMKPZD_0.html |access-date=2009-11-12 |publisher=[[European Space Agency|ESA]]}}</ref> | ||
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=== Center-of-mass position === | === Center-of-mass position === | ||
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.{{ | 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.<ref>{{cite journal |last=Stern |first=S. Alan |title=The Pluto–Charon System |journal=Annual Review of Astronomy and Astrophysics |volume=30 |issue=1 |pages=185–223 |date=1992 |doi=10.1146/annurev.aa.30.090192.001153 |quote=The most common dynamical definition of a binary or double planet is one in which the center of mass (barycenter) of the system lies in the space between the two objects rather than within the interior of either.}}</ref> 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. | ||
Under this definition, the Earth–Moon system is not currently a double planet; although the Moon is massive enough to cause the Earth to make a noticeable revolution around this center of mass, this point nevertheless lies well within Earth. However, the Moon currently migrates outward from Earth at a rate of approximately {{convert|1.5|in|cm|order=flip|abbr=on}} 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. | Under this definition, the Earth–Moon system is not currently a double planet; although the Moon is massive enough to cause the Earth to make a noticeable revolution around this center of mass, this point nevertheless lies well within Earth. However, the Moon currently migrates outward from Earth at a rate of approximately {{convert|1.5|in|cm|order=flip|abbr=on}} 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. | ||
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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 as strong as the Sun's. Asimov included this with his other arguments that Earth and the Moon should be considered a binary planet.<ref name="Asimov"/> | 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 as strong as the Sun's. Asimov included this with his other arguments that Earth and the Moon should be considered a binary planet.<ref name="Asimov"/> | ||
{{Blockquote|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{{spaced ndash}}but not the Moon.<ref name="Asimov"/><ref name="Aslaksen">{{Cite web |title=The Orbit of the Moon around the Sun is Convex! |last=Aslaksen |first=Helmer |url=http://www.math.nus.edu.sg/aslaksen/teaching/convex.html |year=2010 |location=National University of Singapore |publisher=Department of Mathematics |access-date=2012-01-23 |archive-url=https://web.archive.org/web/20130116204505/http://www.math.nus.edu.sg/aslaksen/teaching/convex.html |archive-date=2013-01-16 |url-status=dead }}</ref><ref name="PoV" group="Footnote">Asimov uses the term "[[Wikt:concave|concave]]" to describe the Earth–Moon orbital pattern around the Sun, whereas Aslaksen uses "[[Wikt:convex|convex]]" to describe the | {{Blockquote|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{{spaced ndash}}but not the Moon.<ref name="Asimov"/><ref name="Aslaksen">{{Cite web |title=The Orbit of the Moon around the Sun is Convex! |last=Aslaksen |first=Helmer |url=http://www.math.nus.edu.sg/aslaksen/teaching/convex.html |year=2010 |location=National University of Singapore |publisher=Department of Mathematics |access-date=2012-01-23 |archive-url=https://web.archive.org/web/20130116204505/http://www.math.nus.edu.sg/aslaksen/teaching/convex.html |archive-date=2013-01-16 |url-status=dead }}</ref><ref name="PoV" group="Footnote">Asimov uses the term "[[Wikt:concave|concave]]" to describe the Earth–Moon orbital pattern around the Sun, whereas Aslaksen uses "[[Wikt:convex|convex]]" to describe the 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.</ref>| Isaac Asimov}} | ||
See the [[Orbit of the Moon#Path of Earth and Moon around Sun|Path of Earth and Moon around Sun]] section in the "Orbit of the Moon" article for a more detailed explanation. | See the [[Orbit of the Moon#Path of Earth and Moon around Sun|Path of Earth and Moon around Sun]] section in the "Orbit of the Moon" article for a more detailed explanation. | ||