Can Planets Collide with Each Other?

In George Pal’s 1951 movie “When Worlds Collide”, the rogue star Bellus passes through the solar system, destroying Earth after a spectacular sequence of earthquakes, lava flows, tidal waves, and forest fires caused by the gravity of its planet, Zyra. A group of daring scientists and engineers build a rocket to carry a few people to a new life on Zyra as the Earth vanishes in flames. This kind of event can’t happen today, but great collisions did shape the early solar system, and scientists keep watch for asteroids that may someday cross our path.

In the early period of our solar system, around 4.6 billion years ago, collisions between proto-planets as they built up from dust and gas in a spinning disk around the early Sun had enormous effects on the worlds we now see. Within that disk, small dense areas drew in more material with their gravitational pull, and “protoplanets” or “planetesimals” emerged to sweep up more dust and gas as they orbited the Sun. The gravitational pull of the larger planets nudged many of these out of stable orbits, to either fall toward the Sun, leave the system, or collide with another protoplanet.

Our Moon was formed when a Mars-sized world collided with Earth, throwing part of the crust and mantle into an orbit where it came together as a small world of its own. We know from the samples returned by Apollo astronauts that the Moon has many of the same rocks found here on Earth, but gases and water were driven away in the hot collision or lost to the weaker gravity of the Moon. Probably due to the glancing impact of a large proto-planet—Uranus is tilted so much that the north and south poles can point toward the Sun, leaving the other hemisphere in darkness for over 20 years at a time. It is also possible that the very slow spin of Venus, which rotates only once every 243 days in a direction opposite to that of the other inner worlds, is due to an ancient collision with another body. In time, collisions between large objects became less frequent as the remaining planets settled into stable orbits that could survive the shifting pulls of the other worlds. There was a last violent period as asteroids and comets in unstable orbits rained down on the surfaces of the young planets to form basins and craters. One of the largest craters in the solar system is the vast South-Pole Aitken (SP-A) basin on the Moon, which lies almost completely on the side not seen from Earth. The material excavated by SP-A covered the entire lunar surface, so all other geologic features post-date this colossal impact.

Objects from the size of dust grains to a few kilometers across still wander within the solar system. Sometimes two bodies are drawn together by their weak gravity so slowly that they form “contact binaries” like icy Arrokoth, which lies in the Kuiper Belt beyond the orbit of Pluto. Powerful radar and optical telescopes search for asteroids that might someday collide with Earth, and new space missions like the Double Asteroid Redirection Test (DART) look at ways to deflect an object that may pose a risk. In 2025, an Asteroid Terrestrial-Impact Last Alert System (ATLAS) telescope detected a comet that originated outside of our solar system. This fascinating visitor, a relic of planetary formation around another star, was observed by ground-based telescopes and several spacecraft during its passage.