Planetesimal, one of a class of bodies that are theorized to have coalesced to form Earth and the other planets after condensing from concentrations of diffuse matter early in the history of the solar system. According to the nebular hypothesis, part of an interstellar cloud of dust and gas.

Planetesimals are small rocky or icy objects that formed in the early Solar system and may have become planets or moons. Learn how planetesimals differ from planets, protoplanets, and asteroids, and see some famous examples of planetesimals.

Learn about the discredited theory that planets formed from small bits of matter (planetesimals) pulled out by a near-collision with a passing star. Find out why this theory was debunked and what are the alternative explanations for the origin of the solar system.

Planetesimals are small celestial bodies formed from dust, rock, and other materials during the creation of planets. Learn about the planetesimal theory, how they became planets, moons, comets, and more, and how they can reveal the history of our Solar System.

Learn how planets, including the ones in our solar system, likely start off as grains of dust smaller than the width of a human hair. See how dust particles combine to form pebbles, rocks, and planetesimals that orbit their star and clear material from their path.

The outermost ring is consistent with dynamical models of Solar System evolution proposing that the early Solar System had a primordial planetesimal disk beyond the current orbit of Uranus.

Arrokoth, a Kuiper Belt object, formed from a local cloud collapse of particles in the solar nebula, not by hierarchical accretion. Learn how New Horizons data and simulations support this conclusion and advance planetary science.

Planetesimal theory describes how the terrestrial planets and the Jovian planets formed. From the collision and accumulation of planetesimals in the solar nebula larger bodies formed and eventually clumped together to form planets. This theory was first proposed by Viktor Safronov.

The ultimate outcome of the planetesimal formation stage is the production of planetesimals of sufficient sizes to obtain gravitational cross sections much larger than their physical cross sections, hence accelerating the growth toward fully fledged planets.