Planetesimali

Planetesimali, or planetesimals, are solid bodies that form in protoplanetary disks around young stars and serve as the building blocks of planets. They typically range from kilometers to hundreds of kilometers in diameter and arise from the coagulation of dust and ice in the disk.

Formation occurs as microscopic grains collide and stick, growing from sub-micron particles to larger aggregates. Over time, growth can proceed through hierarchical collisions and gravitational focusing, while processes such as streaming instabilities can rapidly concentrate material into larger bodies. In many models, a complementary pathway known as pebble accretion helps explain the fast growth of planetary cores by ingesting smaller, centimeter- to meter-sized particles.

Planetesimali are essential to planet formation. Through successive collisions and mergers, they give rise to planetary embryos and eventually full-fledged planets. Some remnants persist as asteroids and comets, preserving information about the early Solar System. Populations of planetesimals also populate the asteroid belt and the Kuiper belt, as well as other planetary systems.

Evidence and observations come from multiple sources. Meteorites contain ancient clues about early planetesimal material, while studies of protoplanetary disks with telescopes such as ALMA reveal ringed structures and clumps that are interpreted as signs of planetesimal formation. Directly observing individual planetesimals in distant disks remains challenging, but their existence is inferred from disk dynamics and the properties of mature planetary systems.

Composition of planetesimali varies with location in the disk; rocky, volatile-poor bodies form inside the snow line, while icy, volatile-rich bodies form beyond it, influencing the eventual makeup of forming planets.