<P> The ultimate dissipation of protoplanetary disks is triggered by a number of different mechanisms . The inner part of the disk is either accreted by the star or ejected by the bipolar jets, whereas the outer part can evaporate under the star's powerful UV radiation during the T Tauri stage or by nearby stars . The gas in the central part can either be accreted or ejected by the growing planets, while the small dust particles are ejected by the radiation pressure of the central star . What is finally left is either a planetary system, a remnant disk of dust without planets, or nothing, if planetesimals failed to form . </P> <P> Because planetesimals are so numerous, and spread throughout the protoplanetary disk, some survive the formation of a planetary system . Asteroids are understood to be left - over planetesimals, gradually grinding each other down into smaller and smaller bits, while comets are typically planetesimals from the farther reaches of a planetary system . Meteorites are samples of planetesimals that reach a planetary surface, and provide a great deal of information about the formation of the Solar System . Primitive - type meteorites are chunks of shattered low - mass planetesimals, where no thermal differentiation took place, while processed - type meteorites are chunks from shattered massive planetesimals . </P> <P> According to the solar nebular disk model, rocky planets form in the inner part of the protoplanetary disk, within the frost line, where the temperature is high enough to prevent condensation of water ice and other substances into grains . This results in coagulation of purely rocky grains and later in the formation of rocky planetesimals . Such conditions are thought to exist in the inner 3--4 AU part of the disk of a Sun - like star . </P> <P> After small planetesimals--about 1 km in diameter--have formed by one way or another, runaway accretion begins . It is called runaway because the mass growth rate is proportional to R ~ M, where R and M are the radius and mass of the growing body, respectively . It is obvious that the specific (divided by mass) growth accelerates as the mass increases . This leads to the preferential growth of larger bodies at the expense of smaller ones . The runaway accretion lasts between 10,000 and 100,000 years and ends when the largest bodies exceed approximately 1,000 km in diameter . Slowing of the accretion is caused by gravitational perturbations by large bodies on the remaining planetesimals . In addition, the influence of larger bodies stops further growth of smaller bodies . </P>

Who suggested the earth origin from gas and dust particles