Disk instability

Disk instability is a planet-formation model in which a region of a young circumstellar disk collapses directly under its own gravity, potentially forming a giant planet rapidly.

Toomre stability criterion: A criterion estimating whether a rotating, differentially supported disk will fragment under its own gravity. It identifies when disk self-gravity can overcome pressure and rotational support.
Core accretion: A planet-formation model in which solid material builds a core that later captures surrounding gas. It forms giant planets through staged growth rather than direct gravitational collapse of disk gas.
Hot Jupiter: A gas giant planet on a close-in orbit around its star. Explaining how a rapidly formed distant fragment might become a close-in giant requires later migration.
Opacity limit for fragmentation: The minimum fragment mass imposed when a collapsing gas cloud becomes opaque and cannot cool efficiently. It constrains whether disk-instability fragments begin too massive to explain ordinary giant planets.
Jeans instability: Gravitational collapse that begins when a gas region exceeds the mass or size needed to resist pressure. Disk fragments collapse through the same competition between gravity and gas pressure.
Pebble accretion: The growth of planetary cores by capturing millimeter- to centimeter-scale solid particles drifting through a protoplanetary disk. It offers a rapid solid-growth route that can precede gas capture without disk fragmentation.
Direct imaging of exoplanets: The detection of exoplanets by separating their light from the much brighter light of their host stars. Imaging distant massive planets tests whether disk instability can form companions at wide separations.
Metallicity (astronomy): The abundance of elements heavier than helium in an astronomical object. Observed planet occurrence trends with stellar metallicity can distinguish formation pathways.
Radiative cooling: The loss of thermal energy by matter through emitted electromagnetic radiation. A disk must shed heat quickly enough for a gravitationally compressed region to remain bound.
Gravitational instability (astrophysics): The growth of density perturbations when gravity overcomes forces that otherwise support matter. Disk instability is one setting-specific form of gravitational instability.

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