Can two planets collide? There are around 100 billion stars in the Milky Way and an equal number of planets. Lots of those planets probably have moons. So there must be billions of worlds in our galaxy alone. Of course, the distances between them are huge. But imagine for a moment that you could observe their movements over billions of years. Some of those planets would bump into each other a few times – it's just inevitable. 

Planetary collisions occur in several ways. The gravity of one planet can alter the orbit of another over time. Stable planetary orbits around a star can cross over. Sometimes, several things combine to throw planets out of their star system entirely. There’s a small chance that in 3 billion years Mercury’s orbit will become unstable and disrupt the entire inner Solar System!

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