Reading Practice 29

[1] The nebular hypothesis suggests that our solar system was formed from nebulous material or clouds of gas and dust. In 1755, Immanuel Kant proposed this theory in his book, Universal Natural History and Theory of the Heaven. This planetary system formation is now considered to be at work throughout the cosmos. The nebular hypothesis is also called the solar nebular disk model or simply the solar nebular model. The theory suggests that stars form in huge, dense clouds of gas. The gas is comprised of molecular hydrogen. Within these clouds, matter or substance forms into small, rotating clumps that develop into stars. A ring of gas encircles these stars. This process may cause the formation of planets too.

 

[2] Generally, it takes about 1 million years for a star to form. Within 10 to 100 million years, a planetary system will evolve out of the protoplanetary disk. The protoplanetary disk feeds the central star. It is very hot when first formed and then cools to develop the T Tauri star stage; at this point, small dust grains made of rocks and ice may form. These grains may transform into planetesimals, smaller planets, and moons. This stage takes 100 million to a billion years.

 

[3] Giant planets form in a more complicated way. It is believed to happen beyond the frost line where planetary embryos consist of different sorts of ice. Consequently, these formations are much larger than in the inner part of the protoplanetary disk. Some embryos seem to continue to form. The accumulation of gas continues for several million years although planets like Jupiter- and Saturn are considered to have accumulated their mass over only 10,000 years.

 

[4] The formation of stars is a consequence of accretion disks around younger stellar objects. A study of these disks indicates that the dust grains inside grow in size on short time scales, producing 1-centimeter-sized particles. This accretion process, by which kilometer-sized planetesimals grow into much larger bodies, is better understood now. This process forms inside any disk where the number density of planetesimals is high enough and continues. Development slows down and the result is the development of planetary embryos of different sizes. Simulations have shown that the merger of these embryos in the inner part of the protoplanetary disk leads to the creation of small Earth-sized planets. Therefore, we now understand the development of these planets.

 

[5] Stars are believed to be formed inside giant clouds of cold molecular hydrogen, 300,000 times the mass of the Sun. Over millions of years, these giant molecular clouds collapse and separate. These cloud fragments develop small, dense cores, which then collapse into stars. The cores vary in size and are called protostellar nebulae.

 

[6] As indicated by the solar nebular disk model, rocky planets are created in the inner part of the protoplanetary disk, within the frost line, where the temperature is high enough to stop condensation of water ice, and other substances into grains. This results in a transformation of purely rocky grains and then in the creation of rocky planetesimals.