Editing Protoplanet (section)

==The planetesimal hypothesis==
A [[planetesimal]] is an object formed from dust, rock, and other materials, measuring from meters to hundreds of kilometers in size. 
According to the [[Chamberlin–Moulton planetesimal hypothesis]] and the theories of [[Viktor Safronov]], a protoplanetary disk of materials such as gas and dust would orbit a star early in the formation of a planetary system.  The action of [[gravity]] on such  materials form larger and larger chunks until some reach the size of planetesimals.<ref name="Cessna">{{cite web |last1=Cessna |first1=Abby |title=Planetesimals |url=https://www.universetoday.com/35974/planetesimals/ |website=Universe Today |access-date=5 April 2022 |date=26 July 2009}}</ref><ref name="Ahrens">{{cite journal |last1=Ahrens |first1=T J |title=Impact Erosion of Terrestrial Planetary Atmospheres |journal=Annual Review of Earth and Planetary Sciences |date=1 May 1993 |volume=21 |issue=1 |pages=525–555 |doi=10.1146/annurev.ea.21.050193.002521 |bibcode=1993AREPS..21..525A |url=https://doi.org/10.1146/annurev.ea.21.050193.002521 |access-date=5 April 2022 |issn=0084-6597|hdl=2060/19920021677 |hdl-access=free }}</ref>

It is thought that the collisions of planetesimals created a few hundred larger planetary embryos. Over the course of hundreds of millions of years, they collided with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first "generation" of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually, only a handful of embryos were left, which collided to complete the assembly of the [[planet]]s proper.<ref>{{cite book|title=An Introduction to the Solar System|pages=56|first=Neil|last=McBride|author2=Iain Gilmour |author3=Philip A. Bland |author4=Elaine A. Moore |author5=Mike Widdowson |author6=Ian Wright |publisher=[[Cambridge University Press]]|location=Cambridge|year=2004|isbn=9780521837354}}</ref>

Early protoplanets had more [[radioactive]] elements,<ref>{{cite web|url=https://www.universetoday.com/37053/protoplanets/|title=Protoplanets
|first=Abby|last=Cessna|year=2009|publisher=Universe Today}}</ref> the quantity of which has been reduced over time due to [[radioactive decay]]. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier [[Chemical element|element]]s sank to the center, whereas lighter elements rose to the surface. Such a process is known as [[planetary differentiation]]. The composition of some [[meteorite]]s show that differentiation took place in some [[asteroid]]s.