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I have heard some talk about a tenth planet, supposedly called "X". What I have heard is that this planet orbits (When thinking of our Solar System on an XY Axis) On the Y axis as opposed to the other planets which rotate on the X axis. I have also heard that this planet could have caused the annihilation of the dinosaurs. Is this something true or just a myth?
Answer 1:

The controversy regarding planet X is an interesting one that goes way back.
Basically, one studies the motions in detail of the known planets to see if the predictions match the realities.
In fact, this is exactly how Neptune was found...
the motion of URANUS could not be explained by accounting for the gravitational effects of the Sun and the other known planets. In 1845-1846, two astronomers independently estimated by some laborious calculations that ,to explain the motion of Uranus, another planet should exist at about 30 AU from the sun and have a mass of such and such.
On the evening of Sept 23, 1846 a German astronomer in Berlin was able to first see NEPTUNE.
Now there are some who have looked at the orbit of Neptune and Pluto who have argued that there must be still another massive planet that explains what some believe to be irregularities in the orbit.
This is very controversial, because in order to compute a reliable orbit one need to have watched the planet for at least one orbital period. The problem with the outer planets is that their periods exceed a few hundred years---we simply do not have enough data to reliably claim that their orbit really are different that what we predict...because the predictions have inherent uncertainty!!!
Nevertheless some have taken the older data that may not be very good and have determined that if a planet X was in a high inclination orbit then it would be hard to find and could be the source of a gravitational perturbation that could affect the orbit of Neptune, that some say has RESIDUAL anomalies" in its orbit.
So the JURY is still out on the presence of a planet X large enough to perturb the orbit of Neptune ...

As far as the dinosaur thing... the idea is that IF such a planet does exist, then when it passes through the plane of the ecliptic it could shake rattle and roll some big asteroids and send them onto Earth crossing orbits.
But this is speculative to say the least!!!!
The search goes on for planet X although most astronomers think that it does not exist...
Your question indicates that you understand this issue... what books have you been reading ? There are many around on basic astronomy; if interested I can give you some references.

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