Astronomers of the Paris Observatory have studied how orbits of planets in our solar system might evolve billions of years into the future. The calculations show there's a tiny chance that Mars or Venus could collide with Earth around billions of years from now, and there's also a possibility that Mercury could strike Venus and merge into a larger planet.
The researchers carried out more than 2,500 simulations. They found that in some, Mars and Venus collided with the Earth.
"It will be complete devastation," said Professor Laskar.
"The planet is coming in at 10km per second - 10 times the speed of a bullet - and of course Mars is much more massive than a bullet."