Rick Robinson

Mar 16th 2020

Planet Collision in Fact and Fiction


If you merely want to wipe out most life on a planet, you can hit it with an asteroid, as happened to Earth some 80 million years ago, putting an end to the dinosaurs (and most other species at that time).

But if you want to really wreck a planet, wiping out all life, you need to hit it with something bigger, like another planet. And, paradoxically, a planet collision may also be just the thing to create life, which may have also happened to Earth, more than 4 billion years ago.

The possibility of a planet collision was recognized generations ago. Though there was not yet any known evidence for such a crackup, the sheer idea was spectacular enough to enter popular culture, namely the 1951 film “When Worlds Collide,” based on a 1932 science fiction novel of the same name. At the time, though, professional astronomers kept their distance from the subject of planet collision as direct evidence was lacking.

A Lunar Mystery

This began to change at the start of the Space Age, when the moon recieved new attention, including the mystery of its origin. As Astronomy describes, one theory, which has gained support to become the most likely explanation, is that the moon is the aftermath of a collision in the early days of the solar system between Gaia, or newborn Earth, and a roughly Mars-sized world that astronomers have named Theia.

The Earth-Theia collision, it is theorized, was not quite a straight head-on crash. Instead, it was a glancing blow. The impact was enough to smash Theia into fragments that scattered across the solar system. Earth, much bigger than Theia, remained mostly intact — but a divot was knocked out of it, so to speak. Some of the fragments from the divot went into orbit around the Earth, where they eventually coalesced to become the moon.

All of this is theory, albeit a robust theory. But as Science Alert reports, a few years ago astronomers discovered a star surrounded by an exceptionally dense cloud of dust. Such dust clouds are common around very young stars, and are believed to be the raw material of planets that have not yet formed. Mature stars, however, do not normally have such dense dust clouds around them.

The most likely explanation is that we are watching the aftermath of an exoplanet collision. The dust we detect is the cosmic equivalent of glass and metal fragments skidding across freeway lanes after a high-speed impact.

An Earth Collision With Planet X?

Planet collisions, it now appears, are not just theory anymore. And this raises the question of whether another such collision could possibly be in store for Earth. As Scientific American notes, mathematical models and exoplanet observations indicate that planetary systems can be chaotic, undergoing drastic changes of configuration that could include worlds in collision.

The good news is that any collision between the known solar system planets cannot take place for a very long time to come. Planets are constantly perturbing one another’s orbits, but the solar system planets are so widely spaced that putting them onto a collision course would be an extremely gradual process (at least in its early stages), taking millions of years.

Some astronomers, according to, theorize that a large undiscovered planet, dubbed Planet X, may still lurk in the far outer reaches of the solar system. A potential Earth collision with Planet X, however, would also take a long time to develop, because Planet X — if it exists — is in a very distant and, thus, very slow orbit.

But in 2017, NASA reported the discovery of an interstellar visitor passing through the solar system. This visitor was the size of a small asteroid, but there are also believed to be full-size “rogue” planets drifting around out there. If one is hurtling toward Earth, we might not discover it until it was only a few years — perhaps even months — from impact.

This was precisely the scenario of the “When Worlds Collide” and also of the 2011 film “Melancholia.” Is it a likely prospect? No. Earth has, after all, gone 4 1/2 billion years since its last planetary collision. But we can’t entirely rule it out.