An Exoplanet with Three Suns?

Astronomers have confirmed the existence of a triple-star exoplanet located 1,800 light-years from Earth. Planets parked in multi-star systems are rare, but this object is particularly unusual owing to its inexplicably weird orbital alignment.

. Around 10% of all star systems involve three stars, according to NASA. Planets have been spotted in triple-star systems before, and also within binary star systems, but such discoveries remain rare. Multiple star systems, it would seem, don’t tend to host a lot of planets. This could mean that the conditions for the formation of planets are not ideal in these settings, but it could be the result of an observational selection effect, in that it might be tougher for astronomers to spot planets in multiple-star systems compared to single-star systems.

Can the orbital dynamics of systems like this be long term stable? Or are they always ephemeral? (@physicists )

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Hi Josh. This blog post by Sean Raymond may be of some use: Building the ultimate Solar System part 6: a system with multiple stars – PLANETPLANET

About mult-star systems in general. Doesn’t address the stability of this particular system.

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Reminds me of the three-body problem (note: all I know about this is from the eponymous, and excellent, novel).


This article is pretty good too.

The whole exchange reminds me of Picard and the octuplet system.


Can the orbital dynamics of systems like this be long term stable?

Not according to this. (The trilogy is worth the read. Really gets mind-bendy by the end.)


Yes, the orbital dynamics of such systems can be long term stable. The secret is in hierarchical orbits. Look at my terrible MS Paint drawing:

Because gravity goes down with distance, as long as L is much smaller than L’ and L’ is much smaller than L’', the planet will orbit the red star as if the other stars do not exist and the red star-yellow star system will orbit each other as if the blue star does not exist, i.e., there is a “hierarchy” of orbits.

(Note that in reality, the gravity of faraway objects actually do matter a little bit)


So the idea is that as long as you can recursively bifurcate a system, such that all bifurcations are well approximated as two single points in orbit, the system is stable?

Are their any cases of stable systems where this isn’t true?

Are hierarchical orbital systems attractors? Or are they just metaestable (like a pencil balancing upright)?

Yes, this is the easiest way to get a stable orbital system. Again, in real life, the “bifurcations” are never really perfect, and there are small effects on the orbital stability of a system due to the influence of a faraway system.

Yes, there are theoretical orbital systems which are stable and does not use this strategy (though I don’t think we have found one yet). If you would allow me to “cheat”, a star cluster such as a globular cluster is technically a system of many stars that are in a sense stable.

They are not generally attractors. Indeed, they are not even “stable” in the strict mathematical sense! For example, in the 2-body case: if you perturb an orbiting planet by increasing the velocity a little bit, the planet’s orbit will move outwards until it equilibrate at a different orbital radius. Another example would be: if you move the planet perpendicularly up or down from its orbital plane, there are no forces that will restore it back to where it was.

The sense of stability that are typically used in astronomy is instead:

After a long period of time, no members of the system is flung to infinity

Sometimes people also add: “After a long period of time, no members of the system collides with each other”.

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