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Two Planets in Same Orbit?


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Here's a question for you who are savvy in orbital mechanics and such. Is it possible for two planets to form at the same distance from their parent star and, on the same plane,

so that they end up in roughly the same orbit but, at different points in said orbit?, eventually colliding i presume.

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17 hours ago, Sunshine said:

Here's a question for you who are savvy in orbital mechanics and such. Is it possible for two planets to form at the same distance from their parent star and, on the same plane,

so that they end up in roughly the same orbit but, at different points in said orbit?, eventually colliding i presume.

From a terminology perspective the answer to this is no.  One aspect of the definition of a planet is that it is the primary gravitational body within its orbital path (summarised).  As such you can't have two 'planets' on the same orbital path.  Though potentially two dwarf planets.

What you are describing really is one of the theories of planetary system formation in that many bodies are formed and they eventually merge to former a few larger bodies/planets (or get ejected from the system).  

If the question is that whether such a system could form and be stable for hundreds of millions of years then in principle if you have two perfectly circular orbits, a perfect star system, and they are exactly opposite each other in ther orbit then in principle the answer is yes, but the chance of such a system forming is so infinitesimally small that the answer really is no.

13 hours ago, KevS said:

Yes, the co-orbital configuration; if my memory serves me correctly its something to do with "Lagrangian points" at which a body in a co-planer orbit will maintain its distance from the others within said orbit. 

Lagrangian points need two larger bodies though (e.g. Jupiter / Sun) so needs another body as well.  So strictly speaking you can't have planets at lagrangian points as at best they would be dwarf planets (the other larger body would be the planet).

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The Saturn moons Janus and Epimethius have weird orbits they circle the planet every 13 hours in separate orbits so closely matched that every four years or so the meet up swing round each other and swap orbits.😲

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What I've never understood about the planetary definition, is that Jupiter has not cleared its orbit because of the Trajon and Greek asteroids which orbit around two Lagrange points on Jupiter's orbit. Does that make it a minor planet? 😉🤪🤣

I guess there is no reason why these asteroids could not be another planet sharing a stable orbit.

InnerSolarSystem-en.png

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On 15/01/2020 at 07:45, Stu said:

What I've never understood about the planetary definition, is that Jupiter has not cleared its orbit because of the Trajon and Greek asteroids which orbit around two Lagrange points on Jupiter's orbit. Does that make it a minor planet? 😉🤪🤣

I guess there is no reason why these asteroids could not be another planet sharing a stable orbit.

Might be that definition is related to planet formation period not later captures. It also probably refers to independent bodies in same orbit rather than satellites (although not sure if L-point objects can indeed be called satellites?).

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On 15/01/2020 at 06:45, Stu said:

What I've never understood about the planetary definition, is that Jupiter has not cleared its orbit because of the Trajon and Greek asteroids which orbit around two Lagrange points on Jupiter's orbit. Does that make it a minor planet? 😉🤪🤣

I guess there is no reason why these asteroids could not be another planet sharing a stable orbit.

What defines Jupiter as a planet is that it is the primary gravitational body within that region (it doesn't have to be the only object that crosses that path).  The Trojan / Greek asteroids are there because Jupiter is the dominant body (without it they would be scattered all over the place).  The same goes for Neptune.  Strictly speaking Pluto 'crosses' Neptune's orbit but when they come to be close together Neptune will hardly notice Pluto, whereas Pluto will definitely notice Neptune!

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  • 4 weeks later...

I recall reading up on secondary, no make the tertiary, bodies at Lagrangian orbital positions are only semi-stable. They oscillate or orbit about a gravitational "well". In fact it was a diagram of the topography of curved space in a 3 body system that eventually helped me understand how Lagrangian points work. The "3rd party" object needs to be so much smaller than the primary pair, it allows it to hide away from the complexities of the 3 body problem - seen but not felt!

lagrange-point-1_rsvb.jpg

Try putting an Earth size body at L3 and the whole system become chaotic and bodies go flying all over :)

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