What is Protoplanetary Disk?

About Protoplanetary Disk:

• A protoplanetary disk is a disk of gas (99% by mass) and dust (1%), orbiting a newly formed star, from which planets may eventually form or be in the process of forming. 
• Disks are common by-products of star formation, and range in different mass and size.
• How are they formed?
  • Stars form from cold interstellar molecular clouds. A molecular cloud is an interstellar cloud of gas and dust in which molecules can form, the most common of which is hydrogen (H2). 
  • As they collapse into protostars under the force of gravity, the remaining matter forms a spinning disk.
  • Eventually, the star stops accreting matter, leaving the disk in orbit around it.
  • The leftover gas and dust inside that protoplanetary disk become the ingredients for planet formation.
  • Organic molecules present in the original molecular cloud become part of the protoplanetary disk and the planets that form from it.
• The chemical composition of the protoplanetary disk determines the composition of the eventual planets that form from it. 
• Protoplanetary disks typically disperse after 2–3 million years through the coalescence of their matter into planets and photoevaporation by the stellar radiation.