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Could there be a stable orbit shared by two planets with the same mass and density? Regardless of whether the formation of such system is possible or not.

For example, if Earth and Venus could have the same orbit around the sun, but each planet in the opposite side of the orbit, would that orbit be stable?

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  • $\begingroup$ Are you talking about a binary/double planet or some other configuration? $\endgroup$
    – called2voyage
    Oct 2, 2015 at 19:26
  • $\begingroup$ @called2voyage I added an example using Earth and Venus, thanks for the comment. $\endgroup$
    – rraallvv
    Oct 2, 2015 at 19:31
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    $\begingroup$ "For example, if Earth and Venus could have the same orbit around the sun, but each planet in the opposite side of the orbit.." See also the Lagrangian point(s).. $\endgroup$ Oct 2, 2015 at 19:33
  • $\begingroup$ Strictly technically, by current International Astronomical Union's (IAU) definition of a planet (Resolution 5A), no. Co-orbital planet-sized bodies within the Solar system would fail the cleared the neighbourhood around its orbit requirement. And outside of the Solar system, according to IAU's rationale, planets don't even exist, since they're not in orbit around the Sun. $\endgroup$
    – TildalWave
    Oct 3, 2015 at 8:02
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    $\begingroup$ Similar question on physics.SE: physics.stackexchange.com/questions/25978/… $\endgroup$ Oct 3, 2015 at 13:14

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The term for bodies sharing an orbit in this manner is co-orbital configuration. It is technically possible for two objects of planetary size to share an orbit like this, and we may have even found an example in an extrasolar system.

It is speculated that early in Earth's history that it shared an orbit with a Mars-sized planet named Theia which later collided with Earth to produce the Moon.

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    $\begingroup$ Note that the the update at the top of that linked article suggests the discovery to be in error. Though you did say "may". $\endgroup$ Oct 4, 2015 at 11:09
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Saturn has co-orbital moons, Epimetheus and Janus. They orbit in the same direction and swap orbits approximately every four years as the inner body catches up with the outer body.

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I believe the two bodies could be orbiting a central bodies at the stable Lagrange points. Planet A would be at Planet B's L4 Lagrange point, and Planet B would be at Planet A's L5 point.

Lagrange Points

from Wikipedia

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    $\begingroup$ Would an orbit containing two bodies of equal mass even have the same lagrange points? The math only works because there's nothing of non-negligible mass in the neighborhood. $\endgroup$
    – Random832
    Oct 3, 2015 at 7:13
  • $\begingroup$ @Random832 That question has been on my mind for the last couple of days. I was thinking about asking it as a question on its own. But I am unsure whether here or physics is the best place to ask it. $\endgroup$
    – kasperd
    Oct 3, 2015 at 8:27
  • $\begingroup$ @Random832 That's a good point, it probably wouldn't have the same lagrange points. I think you're right that the third body is assumed to have negligible mass. $\endgroup$
    – Adam Wuerl
    Oct 3, 2015 at 23:47

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