protoplanetary disk

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A protoplanetary disk consists of gas (mostly hydrogen) and dust, heated by the star's radiation. The inner part is hotter, there ices evaporate; the outer part is colder, where icy planets and comets form. Disks show gaps — traces of forming planets. Disk lifetime is several million years.

History

The idea arose in the 18th century (Laplace's hypothesis). Nowadays, telescopes like Hubble observe disks.

How it works

The star's gravity attracts matter, and rotation makes it flatten into a pancake. In this pancake, dust collides, sticks together, and gradually grows into larger bodies.

💡 Astronomers discovered a disk around the star HL Tauri with a clear ring structure, revealing planetary embryos.

A protoplanetary disk is an accretion disk around a young star in which planet formation occurs. It forms from the remnants of a molecular cloud that collapses under gravity. Due to angular momentum conservation, the matter does not fall directly onto the star but forms a rotating disk. Temperature and chemical composition depend on distance from the star: inner regions are rich in metals and silicates, outer regions in ices. Disk dynamics include accretion onto the star, turbulence, radial matter transport, and particle growth from submicron dust grains to planetesimals. Processes: dust sedimentation, coagulation, instabilities. Observed in infrared and (sub)millimeter wavelengths (e.g., ALMA).

History

The concept of the protoplanetary disk goes back to the nebular hypothesis of I. Kant (1755) and P. Laplace (1796). In the 20th century, the theory was developed by V. S. Safronov, who mathematically described planet accumulation. Observational confirmations came with space telescopes: IRAS (1980s) detected an excess of infrared radiation from young stars, and Hubble and ALMA directly imaged disks.

How it works

Disk formation is a consequence of angular momentum conservation during the collapse of a rotating cloud. Gas and dust falling to the center increase their rotation speed, and centrifugal force in the equatorial plane balances gravity, forming a disk. Further evolution: dust settles to the midplane, grows through sticking. When planetesimals reach ~km size, gravitational focusing kicks in. In the gas phase, planetary migration is possible. Chemical transformations occur in the disk (beyond the ice line). The disk phase ends via photoevaporation or accretion, leaving a planetary system.

v_K = \sqrt{\frac{G M_*}{r}}

v_K — orbital velocity (m/s), G — gravitational constant (≈6.67×10^{-11} m^3 kg^{-1} s^{-2}), M_* — central star mass (kg), r — distance from star (m).

H = \frac{c_s}{\Omega_K}

H — vertical half-thickness of the disk (m), c_s — sound speed in the gas (m/s), Ω_K — Keplerian angular velocity (rad/s, Ω_K = v_K/r).

Open problems & fun facts

Mechanism for overcoming the meter-size barrier in planetesimal growth; Influence of magnetic fields and turbulence on disk evolution and accretion; Causes of short disk lifetimes (~3–10 Myr) and their dispersal

💡 In the protoplanetary disk of the star HD 100546, water snow was detected, indicating the existence of a broad 'ice line' — the boundary where water freezes.

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protoplanetary disk

History

How it works

History

How it works

History

How it works

Open problems & fun facts

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