Protoplanetary disk

HL Tau protoplanetary disk

The evolutionary sequence of protoplanetary disks with substructures

Mamajek09 diskfraction

Protoplanetary Disk Simulated Spiral Arm vs Observational Data

Soot-line1

Disk comet nebula

Protoplanetary disk refers to a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, known as a T Tauri star in the case of low-mass stars, or a Herbig Ae/Be star for stars of intermediate mass. These disks are thought to be the progenitors of planetary systems. The process of planet formation is believed to occur in these disks, with the dust particles colliding and sticking together to form planetesimals, which in turn may combine to form planets.

Formation

Protoplanetary disks are formed from the collapse of a molecular cloud, a process that also leads to the formation of the star at the disk's center. This collapse is initiated by some disturbance, such as the shock wave from a nearby supernova. As the cloud collapses under its own gravity, it begins to spin faster due to the conservation of angular momentum, flattening into a disk. This disk material accretes onto the central protostar, while some of it condenses to form planets.

Structure

The structure of a protoplanetary disk can be divided into several regions: the inner disk, which is often characterized by high temperatures and where silicate dust can form; the outer disk, which is cooler and where icy materials can exist; and the midplane, which is the densest part of the disk and where most of the mass is concentrated. The disk's temperature and density decrease with distance from the protostar.

Observations

Observations of protoplanetary disks are challenging due to their small size and the fact that they are often obscured by dust. However, advances in technology, particularly in the field of radio astronomy with facilities like the Atacama Large Millimeter/submillimeter Array (ALMA), have allowed astronomers to study these objects in unprecedented detail. Observations can reveal the disk's structure, composition, and the processes occurring within it, such as the formation of gaps and rings that may indicate the presence of forming planets.

Importance in Planet Formation

Protoplanetary disks are crucial for understanding the process of planet formation. Theories of planet formation, including the core accretion model and the disk instability model, rely on the conditions within these disks to explain how planets can form from the dust and gas surrounding a young star. Observations of protoplanetary disks, therefore, provide empirical evidence to support these theories and help refine our understanding of the origins of planetary systems, including our own Solar System.

Evolution and Dissipation

The lifetime of a protoplanetary disk is thought to be relatively short in astronomical terms, typically a few million years. During this time, the material in the disk either accretes onto the central star, is blown away by stellar winds, or forms into planetary bodies. The dissipation of the disk marks the end of the active planet formation phase, although the processes leading to the formation of planetary atmospheres and the further evolution of planets may continue thereafter.