BBC science correspondent Pallab Ghosh explains how Earth could collide with Mars or Venus
Astronomers calculate there is a tiny chance that Mars or Venus could collide with Earth - though it would not happen for at least a billion years.
The finding comes from simulations to show how orbits of planets might evolve billions of years into the future.
But the calculated chances of such events occurring are tiny.
Writing in the journal Nature, a team led by Jacques Laskar shows there is also a chance Mercury could strike Venus and merge into a larger planet.
Professor Laskar of the Paris Observatory and his colleagues also report that Mars might experience a close encounter with Jupiter - whose massive gravity could hurl the Red Planet out of our Solar System.
Astronomers had thought that the orbits of the planets were predictable. But 20 years ago, researchers showed that there were slight fluctuations in their paths.
Now, the team has shown how in a small proportion of cases these fluctuations can grow until after several million years, the orbits of the inner planets begin to overlap.
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."
Professor Laskar's calculations also show that there is a possibility of Mercury crashing into Venus. But in that scenario, the Earth would not be significantly affected.
"If there is anyone around billions of years from now, they'd see a burst of light in the sky and the two planets would be merged," he said.
"The new planet would be a little bit bigger than Venus, and the Solar System would be a little more regular after the collision, but the Earth's orbit would not be affected."
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