Two Worlds Colliding

Could it really happen?

Still from the film Melancholia in which a larger planet destroys the earth

©2011 Zentropa Entertainments.

Lars von Trier’s Melancholia, which opened Friday, contemplates a scenario in which another planet crashes into Earth. What are the chances of another planet’s colliding with ours?

Zero, for at least the next 10 or 20 million years. That’s about how far astrophysicists can accurately predict orbital motion, but all the evidence suggests that our solar system has stabilized over the 4.6 billion years since it formed, set into a configuration that won’t be upended anytime soon. Young solar systems are more susceptible to collisions as planets jockey for position, and many astronomers believe that our own planet was struck by a Mars-sized object in its infancy. That collision spun off a piece of rock that’s now the moon. Today, the stable, round orbits of our planetary neighbors keep collisions at bay.


Though astronomers discovered a recent collision in a relatively mature star system some 300 light years away, this is the exception rather than the rule. That system consisted of two stars, giving its planets highly eccentric orbits. These orbits lead to greater interaction amongst the planets. In contrast, our solar system is especially sturdy due to its more circular orbits, and the ample space between planets.

Five billion years from now, the chances of a planetary collision in our solar system could increase from zero to 1 percent. A recent study suggests that Jupiter may eventually cause Mercury’s orbit to elongate and cross paths with that of Venus. The computer simulations used to make these predictions at scale of billions of years tend to be very imprecise, however.


In the meantime, the Earth faces a bigger threat from an icy, planet-sized object from the outer reaches of the solar system. Astronomers have a hard time predicting the paths of fast-moving objects, especially those on the periphery of the solar system, so it’s hard to know whether one of these might be on the horizon.

Even if a planet were to crash into the Earth billions of years in the future, its approach wouldn’t be as languid as the one depicted in Melancholia, which takes several days to arrive once it becomes visible from the ground. The damage would arrive swiftly and dramatically. Over the period of a single day we would see the oncoming planet go from looking smaller than the moon to filling the whole sky. We would also feel changes underneath our feet as gravitational distortion causes the Earth’s surface to break up and melt into lava.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Gregory Laughlin of UC-Santa Cruz, Mike Brown of CalTech, and Alycia Weinberger of the Carnegie Institution for Science.*

Correction Nov. 14, 2011: The original article incorrectly associated Alycia Weingerger with Cornell. (Return to senctence.)