[plan-i-tes-uh-muh l]Astronomy


one of the small celestial bodies that, according to one theory (planetesimal hypothesis), were fused together to form the planets of the solar system.


of or relating to a planetesimal or planetesimals.

Origin of planetesimal

First recorded in 1900–05; planet + (infinit)esimal Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for planetesimal

Historical Examples of planetesimal

  • Such, then, is the mechanism of the first phase in the history of a spiral nebula according to the Planetesimal Hypothesis.

    Curiosities of the Sky

    Garrett Serviss

  • In deference to the opinion of a number of geologists we must glance once more at the alternative view of the planetesimal school.

  • But the planetesimal hypothesis has no room for this enormous percentage of carbon-dioxide in the primitive atmosphere.

  • The Planetesimal Theory suggests that these thickened knots are embryo planets and the central portion of the nebul an embryo sun.

    The Meaning of Evolution

    Samuel Christian Schmucker

  • Let us now inquire briefly into the bearing of this planetesimal hypothesis upon the early geological history of the earth.


    William J. Miller

planetesimal in Science



Any of innumerable small bodies of accreted gas and dust thought to have orbited the Sun during the formation of the planets.♦ The theory that explains the formation of the solar system in terms of the aggregation of such bodies is known as the planetesimal hypothesis. According to this theory, first proposed in 1900, the planetesimals formed within a spiral disk of dust and gas surrounding a central nucleus. Their gravitational attraction eventually caused the planetesimals to coalesce into protoplanetary disks from which larger objects such as planets, asteroids, and satellites were formed, while the nucleus coalesced into the Sun.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.