Protoplanetary disk

A protoplanetary disk is a rotating circumstellar disk composed of dense gas and dust. It surrounds a newly formed young star, such as a T Tauri star or a Herbig Ae/Be star. These disks are essential because they serve as the nurseries where planetary systems are born. While they are similar to accretion disks, they are different in key ways. An accretion disk is much hotter and spins at a much higher speed. Furthermore, accretion disks are typically found around black holes rather than stars.

The formation of these disks begins within massive molecular clouds made mostly of molecular hydrogen. When a specific section of such a cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. This collapsing cloud is known as a solar nebula. As the nebula shrinks, the random motions of the gas begin to average out. This happens in the direction of the nebula's net angular momentum. Due to the conservation of angular momentum, the rotation speed increases as the nebula's radius decreases.

This rapid rotation causes the cloud to flatten into a disk shape. You might imagine a chef spinning pizza dough to make it flat. This flattening occurs because centripetal acceleration from the orbital motion resists gravity in the radial direction. However, the cloud remains free to collapse in the axial direction. The final result is a thin disk that is supported by gas pressure in that axial direction. This initial collapse phase takes approximately 100,000 years. Once finished, the star reaches a surface temperature similar to a main sequence star and becomes a visible T Tauri star.

Protoplanetary disks have specific physical structures and layers. They are generally thin, meaning their vertical height is much smaller than their radius. Their mass is also much smaller than the mass of the central star. Most of the disk's mass consists of gas, but dust grains are vital for its evolution. These grains shield the mid-plane of the disk from energetic radiation from space. This creates a "dead zone," which is a region of quiescent gas where magnetorotational instability, or MRI, no longer operates. Surrounding this is an active zone consisting of a turbulent envelope of plasma.

The lifespan of a disk is a race between several cosmic forces. The disk facilitates the accretion of gas onto the star for about 10 million years. Eventually, the disk disappears, perhaps because the young star's stellar wind blows the gas away. It might also disappear because it stops emitting radiation after accretion ends. Scientists have observed the oldest protoplanetary disk to be 25 million years old. These disks can be massive, with radii reaching up to 1000 AU. Only the innermost parts of the disk reach temperatures higher than 1000 K.

Inside these disks, the building blocks of planets begin to form. Through electrostatic and gravitational interactions, dust and ice grains accrete into planetesimals. These planetesimals are the primary building blocks for both terrestrial and giant planets. This growth process must compete against the stellar wind and the pull of gravity toward the star. It also competes against internal stresses known as viscosity. This process explains why planets are arranged in a flat ecliptic plane. In our own history, the inner solar system likely held dozens of Mars-sized bodies before they consolidated into terrestrial planets.

As a system ages, it may transition into a debris disk. Debris disks are gas-poor disks of circumstellar dust found around older stars. These can exist around stars aged 10 million years to billions of years old. Because small dust grains are destroyed quickly by radiation pressure or collisions, this dust must be new. It is likely created by collisions between remaining planetesimals, such as asteroids or comets. These serve as extrasolar analogs to our own asteroid belt and Kuiper belt. Finally, these disks may even be the source of life. Computer models suggest that complex organic molecules might form within the dust grains of a protoplanetary disk before planets like Earth even exist.