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==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 |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>
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==Observed protoplanets==
In February 2013 astronomers made the first direct observation of a protoplanet forming in a disk of gas and dust around a distant star, [[HD 100546]].<ref>{{cite web|url=http://www.eso.org/public/news/eso1310/|title=The Birth of a Giant Planet?|publisher=[[European Southern Observatory]]|date=28 February 2013|access-date=2 March 2013}}</ref><ref name="quanz13">{{cite journal|author=Quanz, Sasch P.|display-authors=4|author2=Amara, Adam|author3=Meyer, Michael P.|author4=Kenworthy, Matthew P.|author5=Kasper, Markus|author6=Girard, Julien H.|date=2013|title=A young protoplanet candidate embedded in the circumstellar disk of HD 100546|journal= Astrophysical Journal|volume=766|issue=1|at=L1|doi=10.1088/2041-8205/766/1/l1|arxiv = 1302.7122 |bibcode = 2013ApJ...766L...1Q |s2cid=56140977}}</ref> Subsequent observations suggest that several protoplanets may be present in the gas disk.<ref>{{cite journal |last1=Garufi |first1=A. |last2=Quanz |first2=S. P. |last3=Schmid |first3=H. M. |last4=Mulders |first4=G. D. |last5=Avenhaus |first5=H. |last6=Boccaletti |first6=A. |last7=Ginski |first7=C. |last8=Langlois |first8=M. |last9=Stolker |first9=T. |last10=Augereau |first10=J.-C. |last11=Benisty |first11=M. |last12=Lopez |first12=B. |last13=Dominik |first13=C. |last14=Gratton |first14=R. |last15=Henning |first15=T. |last16=Janson |first16=M. |last17=Ménard |first17=F. |last18=Meyer |first18=M. R. |last19=Pinte |first19=C. |last20=Sissa |first20=E. |last21=Vigan |first21=A. |last22=Zurlo |first22=A. |last23=Bazzon |first23=A. |last24=Buenzli |first24=E. |last25=Bonnefoy |first25=M. |last26=Brandner |first26=W. |last27=Chauvin |first27=G. |last28=Cheetham |first28=A. |last29=Cudel |first29=M. |last30=Desidera |first30=S. |last31=Feldt |first31=M. |last32=Galicher |first32=R. |last33=Kasper |first33=M. |last34=Lagrange |first34=A.-M. |last35=Lannier |first35=J. |last36=Maire |first36=A. L. |last37=Mesa |first37=D. |last38=Mouillet |first38=D. |last39=Peretti |first39=S. |last40=Perrot |first40=C. |last41=Salter |first41=G. |last42=Wildi |first42=F. |title=The SPHERE view of the planet-forming disk around HD 100546 |journal=Astronomy & Astrophysics |date=April 2016 |volume=588 |pages=A8 |doi=10.1051/0004-6361/201527940 |url=https://www.aanda.org/articles/aa/full_html/2016/04/aa27940-15/aa27940-15.html |access-date=5 April 2022 |language=en |issn=0004-6361|doi-access=free }}</ref>
Another protoplanet, AB Aur b, may be in the earliest observed stage of formation for a gas giant. It is located in the gas disk of the star [[AB Aurigae]]. AB Aur b is among the largest exoplanets identified, and has a distant orbit, three times as far as Neptune is from the Earth's sun. Observations of AB Aur b may challenge conventional thinking about how planets are formed. It was viewed by the [[Subaru Telescope]] and the [[Hubble Space Telescope]].<ref name="CBC">{{cite news |title=Gigantic Jupiter-like alien planet observed still 'in the womb' |url=https://www.cbc.ca/news/science/planet-in-the-womb-1.6408539 |access-date=5 April 2022 |work=CBC News |date=April 5, 2022}}</ref>
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