Nothing No One Can Hit A Planet
Since "Planet X" is entirely imaginary, probably nothing at all.
It would be better to ask what would happen if Earth hit a star, as stars are much larger than Earth is. The planet would be vaporized by the intense heat.
It would spin out of earths orbit. And most likely hit a planet, star, comet, or keep on going.
The ball would travel in a straight line until it hit some planet, star, black hole, asteroid or comet, which could take billions of years.
If a dwarf star crashed into a planet,the planet would likely explode.
They will hit Earth if, in their orbit around the Sun, they happen to cross Earth's orbit.
At that distance, there would be no planet - just vaporised gasses.
we would all die.
A large gas planet like Saturn would probably not last as an inner planet - the sun would probably pull Saturn into it and destroy it.
The object would crash into the planet.
the planet will shake
It's days would be shorter