The most famous planetary collision in our solar system likely created Earth's Moon. According to the Giant Impact Hypothesis, about 4.5 billion years ago, a Mars-sized body called Theia collided with the proto-Earth. The impact ejected material that eventually coalesced to form the Moon.
Uranus rotates on its side, with an axial tilt of about 98 degrees. Scientists believe this unusual orientation was caused by a collision with an Earth-sized object early in the solar system's history. The impact was powerful enough to dramatically change Uranus's rotation but not enough to disrupt the planet completely.
Mercury has an unusually large iron core relative to its size. One explanation is that Mercury was once a larger planet that suffered a giant impact, stripping away much of its outer silicate layers and leaving behind a planet dominated by its core.
The asteroid belt between Mars and Jupiter may be the remnants of a planet that never fully formed due to Jupiter's gravitational influence, or possibly the debris from a catastrophic collision between planetesimals early in the solar system's history.
The outcome of a planetary collision depends on several key factors: the relative sizes of the bodies, their impact velocity, the angle of impact, and their compositions. Direct head-on collisions tend to be more destructive, while glancing impacts can lead to partial disruption or even capture scenarios.
Planetary collisions release enormous amounts of energy, primarily as heat. This energy can melt or vaporize rock and metal, creating magma oceans on the resulting bodies. The Earth was likely completely molten after the Moon-forming impact, taking millions of years to cool and solidify.
Material ejected during a collision can follow several paths: some falls back to the larger body, some escapes the system entirely, and some may form a disk that can eventually coalesce into satellites. The size distribution of debris follows power laws that can be observed in asteroid families today.
Astronomers have observed unusual debris disks around some stars that may be the aftermath of recent planetary collisions. For example, the star HD 172555 shows evidence of a massive collision that occurred within the last few thousand years, with silica dust and gas suggesting high-velocity impacts between rocky bodies.
Some exoplanets have properties that might be explained by giant impacts. These include planets with extreme densities, unusual compositions, or highly eccentric orbits. For instance, the "super-Mercury" exoplanet K2-229b has an unusually high iron content that might result from a collision that stripped away its outer layers.