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.
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.
we would all die.
At that distance, there would be no planet - just vaporised gasses.
The object would crash into the planet.
the planet will shake
It's days would be shorter
We all die
Yes. But the odds are pretty small that it will happen.