What does planetesimal mean?
A planetesimal is a small solid body that forms through the gradual accumulation of dust and other material in a protoplanetary disk, the region of gas and dust surrounding a young star. These bodies are regarded as the building blocks of planets: gravitational attraction and collisions brought planetesimals together, producing progressively larger objects that could eventually form planetary embryos and mature planets. The term is used in scientific discussions of the early Solar System, including studies of planetary growth, the formation of asteroids and comets, and the chronology of events around the young Sun. As an adjective, planetesimal describes processes, bodies, or models involving these precursor objects. The word comes from the same Greek and Latin tradition as planet and therefore literally evokes a small or lesser planetary body. Although technically distinct from a protoplanet, it is often informally associated with that term because both refer to stages in planetary formation. Its principal value is that it names the intermediate objects that helped scientists explain how planets assembled rather than appearing fully formed.
A small solid body formed by accretion in a protoplanetary disk, regarded as a precursor or building block of a planet.
"A giant impact may have dispersed many planetesimals from the inner Solar System."
The singular can denote an individual body, while the plural planetesimals is common when referring collectively to objects in a forming planetary system.
Of, relating to, or characteristic of a planetesimal or planetesimals.
"The team compared numerical models of planetesimal accretion with observations of the asteroid belt."
This use is chiefly technical and appears especially in planetary-science contexts.
The regular plural adds -s and is the standard form in both singular and collective references.
"The abundance of planetesimals in the early Solar System supplied the raw material for the planets."
Before a planet, there were planetesimals—the small building blocks from which worlds were assembled.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does planetesimal come from?
The word comes through New Latin planetesimalis, based on Latin planetes, “wanderer,” which was borrowed from Greek planētēs. A diminutive-style formation conveys the idea of a small or lesser planetary body. The term became prominent in scientific accounts of planet formation during the early twentieth century.
Related word forms
How planetesimal is actually used
This is primarily a specialist term in astronomy, planetary science, and related technical writing. It is especially common in discussions of the formation and evolution of the Solar System.
Easily confused with planetesimal
A planetoid is a small planet-like body more broadly, whereas a planetesimal specifically denotes a precursor body formed by accretion within a protoplanetary disk.