Editing Nebular hypothesis (section)

=== Achievements ===
[[File:SPHERE images a zoo of dusty discs around young stars.jpg|thumb|Dusty disks surrounding nearby young stars in greater detail.<ref>{{cite web|title=SPHERE Reveals Fascinating Zoo of Discs Around Young Stars|url=https://www.eso.org/public/news/eso1811/|website=www.eso.org|access-date=11 April 2018}}</ref>]]

The star formation process naturally results in the appearance of [[accretion disk]]s around young stellar objects.<ref name=Andre1994 /> At the age of about 1&nbsp;million years, 100% of stars may have such disks.<ref name=Haisch2001 /> This conclusion is supported by the discovery of the gaseous and dusty disks around [[protostar]]s and [[T Tauri star]]s as well as by theoretical considerations.<ref name=Padgett1999 /> Observations of these disks show that the [[dust]] grains inside them grow in size on short (thousand-year) time scales, producing 1&nbsp;centimeter sized particles.<ref name=Kessler-Silacci2006 />

The accretion process, by which 1&nbsp;km [[planetesimal]]s grow into 1,000&nbsp;km sized bodies, is well understood now.<ref name=Kokubo2002 /> This process develops inside any disk where the number density of planetesimals is sufficiently high, and proceeds in a runaway manner. Growth later slows and continues as oligarchic accretion. The end result is formation of [[planetary embryo]]s of varying sizes, which depend on the distance from the star.<ref name=Kokubo2002 /> Various simulations have demonstrated that the merger of embryos in the inner part of the protoplanetary disk leads to the formation of a few Earth-sized bodies. Thus the origin of [[terrestrial planet]]s is now considered to be an almost solved problem.<ref name=Raymond2006 />