planetesimal

noun

plan·​e·​tes·​i·​mal ˌpla-nə-ˈte-sə-məl 

-zə-məl

: any of numerous small celestial bodies that may have existed at an early stage of the development of the solar system

Examples of planetesimal in a Sentence

Recent Examples on the Web

Early last year — in a study described by its authors as the most precise measure of iron-60 in the early solar system to date — Fang and her colleagues reported low levels of iron-60 (measured via its stable decay product nickel-60) in a planetesimal formed just after the collapse of the cloud. —James Dinneen, Quanta Magazine, 2 Mar. 2026

Beyond the orbit of Neptune lie icy building blocks from the dawn of the solar system known as planetesimals. —Charles Q. Choi, Space.com, 26 Feb. 2026

The vast region contains trillions of comets, large objects made up of dust and ice, and thousands of planetesimals, which are considered small planets made up of dust and ice, according to NASA. —Sarah Moore, Freep.com, 26 Feb. 2026

Out there, the planetesimals are too sparse and move too slowly to find one another often, and therefore most have never agglomerated into planets. —Joseph Howlett, Scientific American, 19 Feb. 2026

Word History

Etymology

planet + -esimal (as in infinitesimal)

First Known Use

1903, in the meaning defined above

Phrases Containing planetesimal

• planetesimal hypothesis

Rhymes for planetesimal

• hexadecimal
• decimal
• infinitesimal

Kids Definition

planetesimal

noun

plan·​e·​tes·​i·​mal ˌplan-ə-ˈtes-ə-məl 

-tez-

: one of numerous small solid heavenly bodies which may have existed at an early stage of the development of the solar system and from which the planets may have been formed