ete. This means that Uranus and Neptune probably formed closer to the Sun—near or even between Jupiter and Saturn—and later migrated outward (see Planetary migration below).[ Harold F. Levison, Alessandro Morbidelli, Crista Van Laerhoven et al. (2007). "Origin of the Structure of the Kuiper Belt during a Dynamical Instability in the Orbits of Uranus and Neptune". Icarus 196 (1): 258.] Motion in the planetesimal era was not all inward toward the Sun; the Stardust sample return from Comet Wild 2 has suggested that materials from the early formation of the Solar System migrated from the warmer inner Solar System to the region of the Kuiper belt.[ Emily Lakdawalla (2006). "Stardust Results in a Nutshell: The Solar Nebula was Like a Blender". The Planetary Society. Retrieved 2007-01-02.] Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth. According to the computer studies, this same process may also occur around other stars that acquire planets.[ Moskowitz, Clara (29 March 2012). "Life's Building Blocks May Have Formed in Dust Around Young Sun". Space.com. Retrieved 30 March 2012.] After between three and ten million years, the young Sun's solar wind would have cleared away all the gas and dust in the protoplanetary disc, blowing it into interstellar space, thus ending the growth of the planets.[ B. G. Elmegreen (1979). "On the disruption of a protoplanetary disc nebula by a T Tauri like solar wind". Astronomy & Astrophysics 80: 77.][ Heng Hao (24 November 2004). "Disc-Protoplanet interactions". Harvard University. Retrieved 2006-11-19.] TOPIC 中世纪商人行会 Merchant Guilds in the Middle Ages[ http://www.middle-ages.org.uk/merchant-guilds-in-the-middle-ages.htm] The guilds in the Middle Ages were an important part of Medieval life. A higher social status could be achieved through membership to Merchant guilds. There were
