The terrestrial planets—Mercury, Venus, Earth, and Mars—are characterized by their dense, rocky, and metallic compositions. These bodies formed over tens of millions of years from the same initial cloud of gas and dust that created the Sun 4.6 billion years ago. This entire process is known as accretion, a gradual buildup of mass through collisions and gravitational capture. Accretion transformed a thin, rotating disk of cosmic debris into the solid planetary structures observed today.
Setting the Stage: The Solar Nebula and Protoplanetary Disk
Planet formation began with the collapse of an immense cloud of gas and dust known as the solar nebula. Under gravity, this cloud contracted, spinning faster and flattening into a vast, pancake-like structure called the protoplanetary disk. The proto-Sun formed at the center, and the disk contained all the raw material for the planets.
A significant temperature gradient existed across this disk, with the region closest to the proto-Sun being extremely hot. This established the “frost line,” located roughly between the orbits of Mars and Jupiter. Inside this line, temperatures were too high for volatile compounds, such as water, methane, and ammonia, to condense into solid ice grains.
Consequently, the inner solar system was left with only refractory materials—substances with high melting points like silicates (rock) and iron-nickel alloys (metal)—to form planets. Because these rocky and metallic components were less abundant than the volatile ices, the terrestrial planets remained relatively small, dense, and rocky. The cooler region beyond the frost line had a much greater supply of solid material, enabling the formation of the massive, ice-rich cores of the giant planets.
Initial Growth: From Dust Grains to Planetesimals
The initial stage involved the coagulation of microscopic dust particles suspended in the disk’s gas. These particles, originally micrometers in size, began to stick together through non-gravitational forces like van der Waals and electromagnetic attraction. This gentle collision and sticking led to the formation of aggregates up to centimeter and meter scales.
As these dust aggregates grew into pebble-sized objects, they became decoupled from the surrounding gas, causing them to drift inward toward the Sun. This was a challenging phase for growth, as collisions at higher velocities could lead to fragmentation rather than accretion. However, mechanisms such as turbulent concentration and gravitational instability allowed some of these bodies to bypass the fragmentation barrier and grow quickly.
Through continued accretion and the settling of materials toward the disk’s midplane, these objects reached kilometer sizes and are classified as planetesimals. At this scale, their mass was sufficient for their own gravity to become the dominant force in further growth. This shift from chemical adhesion to gravitational attraction marked the end of the initial growth phase and provided the building blocks for the next stage of planetary assembly.
Assembly: The Creation of Planetary Embryos
Once planetesimals reached a few kilometers in size, the growth process transitioned from simple sticking to being overwhelmingly driven by gravity. This next phase began with a period known as “runaway growth,” where the largest planetesimals in a region began to grow disproportionately faster than their neighbors. Their slightly larger mass gave them a stronger gravitational pull, or “gravitational focusing,” allowing them to sweep up material more efficiently.
Runaway growth did not continue indefinitely, however, as the massive bodies began to dynamically excite the orbits of the smaller planetesimals, increasing their collision velocities and reducing the efficiency of accretion. This led to the next stage, termed “oligarchic growth,” where a small number of dominant bodies, called planetary embryos, emerged. These embryos were substantial, reaching sizes ranging from that of the Moon to Mars, and they were spaced far enough apart to clear out their own orbital zones.
The inner solar system was populated by perhaps a hundred of these planetary embryos and a large number of smaller planetesimals. The final formation of the terrestrial planets was completed by violent, high-energy collisions between these Moon-to-Mars-sized bodies. These massive impacts, occurring over a period of tens of millions of years, eventually merged the planetary embryos into the four full-sized terrestrial planets, defining their final mass and orbital characteristics.
Planet Finalization: Differentiation and Clearing the Debris
The tremendous energy released during large-scale accretion events and the decay of short-lived radioactive isotopes caused the newly formed planets to become intensely hot. This internal heating was sufficient to melt the interior of these planetary embryos, initiating differentiation. During this process, the densest material, primarily molten iron and nickel, sank toward the center of the planet to form the metallic core.
At the same time, lighter silicate materials floated upward to form the mantle and crust, creating the distinct layered structure observed in all terrestrial planets today. This period of intense internal activity also included the largest collision event in Earth’s history, the hypothesized impact with a Mars-sized body that resulted in the formation of the Moon. Such impacts were the final moments that shaped the planets’ internal structure and rotation.
After the major assembly was complete, the inner solar system experienced a final intense surge of impacts known as the Late Heavy Bombardment (LHB), occurring roughly 4.1 to 3.8 billion years ago. This event was triggered by the gravitational destabilization of the outer solar system’s planetesimal belt due to the migration of the giant planets. The LHB was the final phase of clearing out the residual debris, delivering the last significant influx of material, and leaving behind the heavily cratered surfaces visible on the Moon and Mercury.