SolarSystem,theauthorshaveopenedanewwindowontheleast-understoodaspectofplanetformation.ThisgivesushopethatwemayyetunderstandhowEarthanditscousinscametobe.■JohnChambersisintheDepartmentofTerrestrialMagnetism,CarnegieInstitutionforScience,5241BroadBranchRoadNW,Washington,DC20015,USA.e-mail:chambers@dtm.ciw.edubeltexistedinbodieslargerthan100kmindiameter,andthatsmallerbodieswererela-tivelyrare.Becausethedynamicdepletionofstage3wasindependentofsize,bodieslargerthan100kmindiameteralsodominatedtheasteroidbeltattheendofstage2.Morbidelliandcolleagues’contribution1istoprovideadetailedmodelofstage2(planet-esimalstoplanetaryembryos).Theyconsiderawidevarietyofpossibleplanetesimalsizedistributions,onthebasisthatwedon’tknowenoughtosayhowbigplanetesimalswerewhentheyformed.Combiningtheresultsofthismodelwiththesize-independentdeple-tionofstage3,theauthorsfindthatthecur-rentsizedistributionoftheasteroidbeltcanbereproducedonlyifmostplanetesimalswereatleast100kminsizeattheendofstage1,withasizedistributionextendingupto1,000km(thesizeofthebelt’slargestobject,Ceres),andpossiblybeyond.Thisispreciselythesizerangepredictedbythenewmodels5,6forplanetesi-malformation.Furthermore,thenewstudy1suggeststhatplanetesimalsformedwithasizedistributionthatisessentiallyidenticaltothatseenintheasteroidbelttodayfordiameterslargerthan100km.Therarityofplanetesimalssmallerthan100kmindiameterattheendofstage1seemstoruleoutthepossibilitythatdustaggre-gatessomehowmadeitacrossthemetre-sizebarrierbygraduallysweepingupmaterialfromtheirsurroundings.Instead,objectsmusthavegrownveryrapidlyfromsub-metre-sizedpebblesinto100-km-sizedbodies,possiblyinasingleleap.Morbidelliandcolleagues’modelisnotthelastwordonthesubjectofplanetformation.Inparticular,theauthorsconsideredtheevolu-tionoftheasteroidbeltinisolation,neglectingprocessesthatmayhaveexchangedmaterialbetweentheasteroidbeltandotherpartsoftheSolarSystem.How