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.
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.
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.