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I read this and wonder now, what if we would introduce a new planet to our solar system; would the force of gravitation demand a similar plane as that of the other planets?

Mercuries 6.3° indicate that a tilt is possible to some degree, but there are obviously limits. Would the plane of a new planet eventually be "sandwiched down", even if it were tilted 90° at the beginning?

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  • $\begingroup$ This being hard-science, you can generally get a lot of info over at astronomy.SE $\endgroup$ – Carl Witthoft Sep 27 '19 at 14:50
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Yes it could be out of plane

Astronomers are reporting today the discovery of a planetary system way out of tilt, where the orbits of two planets are at a steep angle to each other. This surprising finding will impact theories of how multi-planet systems evolve, and it shows that some violent events can happen to disrupt planets' orbits after a planetary system forms, say researchers.
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Upsilon Andromedae c and d. Much more startling, though, is their finding that not all planets orbit this star in the same plane. The orbits of planets c and d are inclined by 30 degrees with respect to each other.
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The new, exact masses, found by astrometry are 14 Jupiters for planet c and 10 Jupiters for planet d.

But only for a little while, a few billion years perhaps. Over time the interactions with the other planets would eventually pull it into plane with the others. Depending on the mass of your new planet it would create a new plane to a greater or lesser extent, if it has sufficient mass to perturb the orbit of the gas giants, then a significantly new equilibrium would be reached, otherwise it would be pulled into something approximating the current plane.

Consider a planet orbiting at 90 degrees to the plane.

At two points on its orbit it has a chance to have a "close" encounter with another planet. Every time it does so the two planets pull each other slightly closer to their own orbit. Each time the planet in the plane has a close encounter with another planet in the system it has a similar interaction, but these will happen more often.

The eccentric planet won't have this interaction on every orbit, and it could be years between each encounter. Each encounter will only pull the planet slightly closer to the main orbital plane (assuming it's not a tiny planet passing close to a gas giant). Hence millions, possibly billions of years to pull it into line.

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  • $\begingroup$ You're off by some 4-5 orders of magnitude in regards with the required time, given that the age of the Universe is about 14 billion years. $\endgroup$ – Adrian Colomitchi Sep 27 '19 at 11:17
  • $\begingroup$ @AdrianColomitchi, I'm terrible for exaggerating, made it something more sensible :) $\endgroup$ – Separatrix Sep 27 '19 at 11:25
  • $\begingroup$ @AdrianColomitchi, I think the timespan limiting factor has to be the age of the primary star rather than the age of the universe. $\endgroup$ – Henry Taylor Sep 27 '19 at 12:09
  • $\begingroup$ @HenryTaylor Separatrix'es "terrible exaggeration" - since adjusted - was initially set to "hundreds of billions of years". I gave the closest age available to that. $\endgroup$ – Adrian Colomitchi Sep 27 '19 at 12:12
  • $\begingroup$ @AdrianColomitchi, the thing is this isn't dependent on past events, it's open ended into the future. A planet on a large highly eccentric orbit, the linked article suggests one on a 10,000 year orbit, could take an indefinite amount of time to come into plane. $\endgroup$ – Separatrix Sep 27 '19 at 12:42
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If there are some gas giants, the things will evolve pretty quickly towards an ecliptic plane - tidal forces will dissipate the energy fast enough to get the system evolving to a maximum moment of inertia with the minimum kinetic energy. Rocky planets orbiting far from the gas giants may persist longer on inclined orbits, but eventually will fall "in line".

See why a long cylinder filled with liquid will not keep spinning along the "length" axis but one orthogonal to it: https://youtu.be/1VPfZ_XzisU?t=664 Similar considerations apply on the planet system.

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No need to introduce a new one.

Eris is the goddess of discord. She discords of other gods so much that her planet has an orbital inclination of 44.1444 degrees. She doesn't want to orbit in the same plane as other planets.

Makemake and Haumea hace inclinations of 28 degrees, for consideration.

The trick here is that those three planets are way far from the sun and never come close to the more massive planets such as the gas and ice giants - so they will take a cosmologically long time to come to a closer inclination to the rest, if ever.

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There was a report some time ago about a planetary system where there was no ecliptic plane: some planets were as off as 60 degrees from the orbital plane of the others (I don't remember the exact value by heart)

That can happen if you have no medium to dissipate momentum.

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    $\begingroup$ A gas giant planet will be enough to dissipate the energy and align the planets close to an ecliptic. $\endgroup$ – Adrian Colomitchi Sep 27 '19 at 11:21

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