In cosmogony, planetesimals are objects thought to exist within solar nebulae. Planetesimals are thought to form from the coalescing (due to collisional sticking and gravity) of particles, orbiting within the accretion disc of the solar nebula; an accumulation of planetesimals leads to the formation of a protoplanetary disc, which in turn coalesce into protoplanets/planets. Some use the term to refer, in general, to objects such as asteroids and comets; other, such as Comins use the term to refer, specifically, to objects with diameters of ~10km. This article needs to be cleaned up to conform to a higher standard of quality. ... In cosmogony, the solar nebula is the gaseous cloud (or accretion disc) from which our solar system is believed to have formed. ... Gravity is the force of attraction between massive particles. ... A particle is Look up Particle in Wiktionary, the free dictionary In particle physics, a basic unit of matter or energy. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Artists conception of a binary star system with one black hole and one main sequence star // Overview An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... In cosmogony, a protoplanet is a quasi-planetoid which is slightly larger than a planetesimal and orbits within a solar nebulas protoplanetary discs. ... A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. ... An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... Comet Hale-Bopp A comet is a small body in the solar system that orbits the sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both due primarily to the effects of solar radiation upon the comets nucleus, which itself is a minor planet... For the geometric term, see diameter. ...

It is generally argued that by about 3.8 billion years ago most of the planetesimals, within the solar system had either left orbit or coalesced into larger objects. Most of the remaining planetesimals orbit within the asteroid belt. Presentation of the solar system (not to scale) The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. ... Image of the main asteroid belt between the orbits of Mars and Jupiter. ...

The giant impact theory proposes that Earth's Moon formed from a colossal impact of a planetesimal named Theia with Earth early in the solar system's history. The Big Splash The giant impact theory (or Big Splash or Big Whack; cf. ... Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ... Theia (THAY-uh) is the hypothetical planet that, according to the giant impact theory of the Moons formation, collided with Earth over four billion years ago. ...

Reference

• Discovering the Essential Universe by Neil F. Comins (2001)