Planetary collisions are the most powerful and transformative events in a solar system’s history, fundamentally altering planetary evolution. While rare today, these catastrophic impacts were commonplace during the chaotic period of planetary formation known as the Late Heavy Bombardment. These cosmic crashes convert enormous amounts of kinetic energy into destructive and creative forces, capable of shattering worlds or forging entirely new ones. The outcome is a complex interplay of physics, where the speed, angle, and mass of the colliding bodies determine whether the result is a gentle merging, complete annihilation, or the birth of a new celestial structure.
Categorizing Planetary Impacts
The ultimate fate of two colliding planets depends almost entirely on the geometry and relative velocity of the impact. The spectrum of outcomes can be broadly divided into three main categories based on how the bodies interact upon contact.
One common outcome is the grazing impact, often called a “hit-and-run” collision, where two planetary bodies glance off each other at an oblique angle. These impacts strip away the outer layers and atmosphere of the colliding bodies while largely preserving their dense, iron-rich cores. This type of impact can still eject significant material into space.
At the opposite end of the spectrum is the disruptive, or catastrophic, impact, which occurs when bodies collide at very high speeds. These impacts deliver enough energy to shatter one or both planets entirely, scattering fragments and causing a significant loss of both mantle and core material. The third type, the accretional impact, or merger, involves slower, more head-on collisions where the material from both bodies combines to form a single, larger, and internally-restructured planet.
The Immediate Cataclysm: Energy Release and Material Transformation
The moment of impact is a hyper-energetic event where the kinetic energy of the colliding worlds is instantaneously converted into internal energy, heat, and immense shockwaves. Between 70 and 91 percent of the kinetic energy is transformed into this internal energy, driving the physical and chemical changes that follow. This energy release generates pressures exceeding one million atmospheres, creating shockwaves that propagate through the planetary interiors.
The intense shock heating causes widespread melting of the planetary crust and mantle, turning rock and ice into supercritical fluids and gas. For the densest materials, the shock causes massive vaporization, turning iron core material into plasma at pressures around 507 GigaPascals. This vaporized material expands rapidly, accelerating away from the impact site and sometimes dispersing the impactor’s iron core throughout the mantle of the larger body.
If a planet possesses an atmosphere, a high-speed, head-on impact can cause a dramatic stripping effect, crushing the atmosphere and even parts of the mantle beneath it. Simulations suggest that the grazing impact responsible for forming Earth’s Moon resulted in the loss of anywhere from 10 to 60 percent of Earth’s atmosphere. This atmospheric erosion significantly influences the long-term habitability and evolution of the remaining planet.
The Aftermath: Formation of New Structures
Following the initial collision, the ejected, vaporized, and molten material reorganizes under the influence of gravity. The massive cloud of superheated rock vapor and mantle debris forms a vast, hot, orbiting ring around the remaining planetary core. This structure is known as an extreme debris disk, which can be observed around young stars in distant systems.
Gravity quickly begins the process of re-accretion, pulling the orbiting material back together. This process can lead to the formation of one or more large moons, as seen with Earth’s Moon. New simulations suggest that, contrary to the traditional view of a gradual process, a large satellite could condense and form from the debris disk within hours after the impact.
The fate of the primary planetary bodies is equally transformative, with the remnant cores either merging or surviving as dramatically altered worlds. In a merger-type event, the two cores combine to form a single, larger planet that is often rotationally and compositionally restructured. Conversely, a catastrophic impact can strip away the lighter, silicate mantle of a planet, leaving behind an unusually dense, iron-rich remnant core.
Scientific Evidence of Planetary Collisions
The most well-known example of a planetary collision is the Giant Impact Hypothesis, which explains the formation of Earth’s Moon. This theory posits that a Mars-sized body, named Theia, struck the proto-Earth roughly 4.5 billion years ago, with the resulting debris forming our satellite. Evidence supporting this includes the identical isotopic ratios of oxygen and other elements found in both Earth rock and lunar samples, pointing to a common origin.
Within our Solar System, other planetary anomalies are thought to be the result of similar large-scale impacts. The planet Mercury, for instance, has an unusually large and dense iron core, which scientists believe resulted from an ancient, mantle-stripping collision. The highly tilted rotation axis of Uranus is also attributed to a massive impact event that knocked the planet onto its side early in its history.
Astronomers also find evidence of these events in exoplanetary systems orbiting distant stars. The exoplanet Kepler-107c is twice as dense as its neighbor, suggesting it lost its lighter, outer mantle in a catastrophic collision, leaving behind an exceptionally dense core. Furthermore, the observation of sudden, massive clouds of warm dust and debris around stars, such as in the BD +20 307 system, provides direct evidence of two mature planets having recently smashed into each other.