A planetesimal is a small, solid celestial body that represents a foundational stage in the formation of planetary systems. These objects emerged from the vast, swirling cloud of gas and dust known as the protoplanetary disk surrounding a young star. The name itself reflects this origin, as the term literally means “tiny planet.” Planetesimals are the precursors that accumulated to build planets, moons, and other large bodies in a solar system.
From Dust to Planetesimal: The Accretion Process
The first step in forming a planetesimal is the gentle sticking of microscopic dust and ice grains within the protoplanetary disk, a process called coagulation. These tiny particles, held together by electrostatic forces, collide at low speeds. This aggregation allows the solid material to grow progressively larger, from dust motes to pebbles and then to centimeter-sized objects.
A significant challenge in this growth path is the “meter-size barrier,” where objects approximately one meter in diameter face two competing problems. Aerodynamic drag from the surrounding gas causes these bodies to rapidly lose orbital energy, making them spiral quickly toward the central star where they would be destroyed. Simultaneously, collisions at these larger sizes become more destructive than constructive, often fragmenting the growing bodies.
To bypass this problem, the leading theory involves a process called the streaming instability. This mechanism is an interplay between the solid particles and the gas disk, where gas drag causes a local concentration of solids. The particles spontaneously clump together into massive filaments that become dense enough to collapse under their own self-gravity. This rapid gravitational collapse allows the solid material to jump directly from pebble-sized aggregates to kilometer-sized planetesimals, overcoming the growth barrier.
Defining Size and Composition
Once formed, a planetesimal is defined as a solid body large enough that its internal structure is dominated by its own gravity, rather than surface forces or gas drag. Scientists typically place this minimum size at approximately one kilometer in diameter. At this size, the body’s gravity is sufficient to pull in surrounding material, marking the transition from a passive aggregate to an actively growing seed.
The composition of a planetesimal was determined by its distance from the young star, creating a compositional gradient across the disk. Planetesimals that formed closer to the star, in the inner, warmer regions, were composed primarily of silicates and metals. These rocky and metallic bodies would eventually form the terrestrial planets.
Farther out in the disk, beyond the snow line where temperatures dropped low enough for volatile compounds to freeze, planetesimals incorporated large amounts of water ice, methane, and ammonia. These icy bodies, composed of rock, metal, and ice, would become the building blocks for the giant planets and the numerous icy objects found in the system’s outer reaches.
The Building Blocks of Planets
Planetesimals were the direct ingredients for constructing planetary embryos and, ultimately, the planets themselves. The first stage of growth beyond the planetesimal size is termed runaway growth. During this phase, the largest planetesimals rapidly grow faster than their peers because their greater mass allows them to gravitationally focus and attract more distant neighbors.
This period of accelerated accretion transitions into a slower phase known as oligarchic growth. In this stage, the few largest bodies, now called protoplanets, begin to gravitationally perturb the orbits of the remaining planetesimals. This action excites the planetesimals’ velocities, which slows down the protoplanets’ growth rate.
The protoplanets continue their growth by dominating the accretion in their local zone, sweeping up the smaller planetesimals until the local supply is exhausted. In the inner solar system, these massive, rocky embryos eventually collided to form the four terrestrial planets.
In the outer solar system, the icy planetesimals created cores massive enough (estimated at around ten Earth masses) to gravitationally capture vast envelopes of hydrogen and helium gas from the surrounding nebula, leading to the formation of the gas and ice giants.
Planetesimals Today: Surviving Remnants
Not all planetesimals were incorporated into the larger planets; a significant population survived the tumultuous process of planet formation. These surviving bodies continue to orbit the Sun, providing astronomers with unique time capsules of the solar system’s initial composition. Their locations depend on where they formed and the gravitational influence of the giant planets.
The Asteroid Belt, located between the orbits of Mars and Jupiter, is the primary reservoir of the rocky and metallic planetesimals that never fully coalesced into a larger planet. Farther out, beyond Neptune, the Kuiper Belt and the more distant Oort Cloud hold billions of the icy planetesimals. These icy remnants are the origin of many comets that occasionally travel into the inner solar system.
By studying the composition, orbital dynamics, and structure of these modern-day asteroids and comets, scientists can effectively sample the primordial conditions of the protoplanetary disk. These survivors offer direct evidence of the materials and processes present at the solar system’s beginning, approximately 4.5 billion years ago.