Protoplanetary disks are the nurseries where planet formation starts. These disks are located around young stars and have been observed in various stages of their evolution, from the initial stages of accretion to the later stages of planet formation.
The millimetre to centimetre-sized dust particles, with the influence of the gas, coagulates into larger particles, ultimately forming the planets, which can be several kilometres in diameter.
The types of planets, their atmospheres, masses, distribution, etc., depend very much on the properties of the disk that preceded them. For example, one can easily infer that if we have a planet-forming very close to the host star, it will most likely be a terrestrial planet, due to the availability of huge amounts of dust. On the other hand, giant gaseous planets are expected to form farther from the centre, where even though there is less dust, we still have abundant gas (given the fact that the dust-to-gas mass ratio is about 0.01). Understanding the physical and chemical processes that occur in these disks is crucial to our understanding of the formation and evolution of planets.
In recent years, there has been significant progress in our understanding of the physics and chemistry of protoplanetary disks, with the advent of new observational techniques and theoretical models. Observations of protoplanetary disks have been carried out using various instruments, such as the Atacama Large Millimeter Sub-millimetre, the Very Large Telescope (VLT), and the Hubble Space Telescope (HST).
These observations have provided insights into the properties of the disks, such as their mass, size, and temperature, as well as the distribution of gas and dust within them. Theoretical models of protoplanetary disks have also made significant progress in recent years, with the development of sophisticated numerical simulations that can capture the complex physical and chemical processes occurring within the disks. These simulations are based on a range of physical models, such as hydrodynamics, radiative transfer, and astrochemistry, and can provide detailed insights into the evolution of the disks and the formation of planets.