<p>Is it likely to ever discover three planets of similar mass revolving around a common barycenter, or would such a system be too unstable? Such triple planet system would look like a triangle once in a time.</p>

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2021-05-21T17:34:09.847Z
We haven't found any yet, so probably very unlikely.
WarpPrime
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2021-05-22T00:21:41.373Z
mathematically, a 3-body system is unstable unless the ratios of mass between the 3 bodies is very high. Read up about L4 and L5 Lagrangian point requirements. Any other configuration is unstable.
PcMan
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2021-05-22T01:52:24.637Z
@PcMan and yet there are *plenty* of 3-body star systems, so...?
uhoh
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2021-05-22T02:07:58.143Z
@uhoh, three-body star systems are always a pair of stars orbiting each other, with a third star orbiting the pair.  Such a system is always unstable in the presence of a nearby massive body (ie. a star).
Mark
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2021-05-22T02:23:32.567Z
@Mark so that's a *four-body system*. Just trying to clarify the "a 3-body system is unstable unless" comment. *Thanks!*
uhoh
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2021-05-22T05:12:24.330Z
@uhoh I think what Mark and PcMan mean is that a system of three planets (or stars) with a _similar_ mass would be too unstable to be likely. In case of triple star systems however, mass doesn't matter, that's why Alpha Centauri is one, with Proxima Centauri being much less massive than the other two stars. But in triple _planet_ systems, the planets should have similar masses, otherwise the third one would be a moon instead of a planet.
Giovanni
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2021-05-22T07:55:59.533Z
@Giovanni I have a hunch that these thee comments say *different* thing, though related. Comments are too short for this, we can just wait for someone to spell it all out in an answer.
uhoh
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2021-05-22T16:03:35.597Z
A double planetary system, as opposed to a planet with a moon, is a system where the center of rotation is outside of the surface of either of them. A stable triple planet system would probably look like two planets relatively close together, and a third one far enough away that the pair pretty much looks like a point mass to it, and far enough away from the sun that it can be stable. Search term is "Hill sphere", or "subsatellite".
Greg
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2021-05-22T17:14:50.597Z
@Greg So you consider Pluto-Charon a double planet? And Sun-Jupiter a binary half-star/half-planet system? (yes, the Sun-Jupiter barycenter is outside the Sun) And the Moon would become a planet once the Earth-Moon barycenter would go outside the Earth in a billion years or so. As you see, having the barycenter outside either body is not enough. I consider it a double planet if the less massive body has more than 50% of the parent body's mass. By comparison, Charon has 12.2% of Pluto's mass, therefore it's a moon.
Giovanni
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2021-05-22T18:30:50.827Z
It's not my definition. I'm not even totally on board with IAU's definition which demoted Pluto to a dwarf planet, and I'm not alone in that. If you want to  say that Pluto and Charon are a double planet, I'd buy that. But if the center of mass is within one of the bodies you have a planet with a moon, for as much as a new label might be enlightening.
https://en.wikipedia.org/wiki/Double_planet
Greg
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2021-05-23T05:59:25.643Z
@Greg You're right, the IAU's planet definition is nonsense. Pluto, Ceres, Eris and other celestial bodies _are_ planets.
Giovanni
<planet><mass><binary-star><orbital-elements><multiple-star-systems>
How likely are triple planet systems?
Giovanni
# How likely are triple planet systems?

(Asked by: username_0 on 2021-05-21)

Is it likely to ever discover three planets of similar mass revolving around a common barycenter, or would such a system be too unstable? Such triple planet system would look like a triangle once in a time.

**Tags:** planet, mass, binary-star, orbital-elements, multiple-star-systems

> **Comments:**
> **username_1**: We haven't found any yet, so probably very unlikely.  
> **username_2**: mathematically, a 3-body system is unstable unless the ratios of mass between the 3 bodies is very high. Read up about L4 and L5 Lagrangian point requirements. Any other configuration is unstable.  
> **username_3**: @username_2 and yet there are *plenty* of 3-body star systems, so...?  
> **username_4**: @username_3, three-body star systems are always a pair of stars orbiting each other, with a third star orbiting the pair.  Such a system is always unstable in the presence of a nearby massive body (ie. a star).  
> **username_3**: @username_4 so that's a *four-body system*. Just trying to clarify the "a 3-body system is unstable unless" comment. *Thanks!*  
> **username_0**: @username_3 I think what Mark and username_2 mean is that a system of three planets (or stars) with a _similar_ mass would be too unstable to be likely. In case of triple star systems however, mass doesn't matter, that's why Alpha Centauri is one, with Proxima Centauri being much less massive than the other two stars. But in triple _planet_ systems, the planets should have similar masses, otherwise the third one would be a moon instead of a planet.  
> **username_3**: @username_0 I have a hunch that these thee comments say *different* thing, though related. Comments are too short for this, we can just wait for someone to spell it all out in an answer.  
> **username_5**: A double planetary system, as opposed to a planet with a moon, is a system where the center of rotation is outside of the surface of either of them. A stable triple planet system would probably look like two planets relatively close together, and a third one far enough away that the pair pretty much looks like a point mass to it, and far enough away from the sun that it can be stable. Search term is "Hill sphere", or "subsatellite".  
> **username_0**: @username_5 So you consider Pluto-Charon a double planet? And Sun-Jupiter a binary half-star/half-planet system? (yes, the Sun-Jupiter barycenter is outside the Sun) And the Moon would become a planet once the Earth-Moon barycenter would go outside the Earth in a billion years or so. As you see, having the barycenter outside either body is not enough. I consider it a double planet if the less massive body has more than 50% of the parent body's mass. By comparison, Charon has 12.2% of Pluto's mass, therefore it's a moon.  
> **username_5**: It's not my definition. I'm not even totally on board with IAU's definition which demoted Pluto to a dwarf planet, and I'm not alone in that. If you want to  say that Pluto and Charon are a double planet, I'd buy that. But if the center of mass is within one of the bodies you have a planet with a moon, for as much as a new label might be enlightening. https://en.wikipedia.org/wiki/Double_planet  
> **username_0**: @username_5 You're right, the IAU's planet definition is nonsense. Pluto, Ceres, Eris and other celestial bodies _are_ planets.  

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