<P> The star formation process naturally results in the appearance of accretion disks around young stellar objects . At the age of about 1 million years, 100% of stars may have such disks . This conclusion is supported by the discovery of the gaseous and dusty disks around protostars and T Tauri stars as well as by theoretical considerations . Observations of these disks show that the dust grains inside them grow in size on short (thousand - year) time scales, producing 1 centimeter sized particles . </P> <P> The accretion process, by which 1 km planetesimals grow into 1,000 km sized bodies, is well understood now . 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 embryos of varying sizes, which depend on the distance from the star . 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 planets is now considered to be an almost solved problem . </P> <P> The physics of accretion disks encounters some problems . The most important one is how the material, which is accreted by the protostar, loses its angular momentum . One possible explanation suggested by Hannes Alfvén was that angular momentum was shed by the solar wind during its T Tauri star phase . The momentum is transported to the outer parts of the disk by viscous stresses . Viscosity is generated by macroscopic turbulence, but the precise mechanism that produces this turbulence is not well understood . Another possible process for shedding angular momentum is magnetic braking, where the spin of the star is transferred into the surrounding disk via that star's magnetic field . The main processes responsible for the disappearance of the gas in disks are viscous diffusion and photo - evaporation . </P> <P> The formation of planetesimals is the biggest unsolved problem in the nebular disk model . How 1 cm sized particles coalesce into 1 km planetesimals is a mystery . This mechanism appears to be the key to the question as to why some stars have planets, while others have nothing around them, not even dust belts . </P>

How and why did this theory have to be expanded after the discovery of extra solar planets