Planetary science

Planetary science studies planets, moons, small bodies, planetary systems, and the processes that form and evolve them.

Planetary system dynamics studies the orbital evolution of planets, planetesimals, and dust under gravity, radiation, collisions, and dissipative forces.

A planetesimal is a solid body in a young or evolved planetary system whose collisions and gravitational interactions help build or erode larger bodies.

For a small body with orbital elements $a,e,I$ relative to a planet on a circular orbit of semi-major axis $a_p$, the Tisserand parameter is approximatelyIt is derived from the Jacobi constant and is approximately conserved across separated weak encounters in the circular restricted three-body problem.

A debris disk is an optically thin circumstellar population of dust and larger solid bodies maintained mainly by collisions.

The fractional luminosity is $f=L_{\mathrm{disk}}/L_{\ast }$; for blackbody grains in an optically thin belt at radius $r$, it equals their total geometric cross-section divided by $4\pi r^2$.

A collisional cascade transfers mass from large bodies to progressively smaller fragments. A steady cascade has a size distribution whose mass flux through logarithmic size bins is independent of size.

A differential power-law size distribution has $n(D)dD=k{D}^{-\alpha }dD$ objects with sizes between $D$ and $D+dD$.

The catastrophic disruption threshold is the impact energy per unit target mass required both to shatter a body and to disperse enough fragments against self-gravity.

A rubblising collision shatters a body but does not supply enough energy to disperse its fragments against self-gravity, leaving a gravitationally bound rubble pile.

Interplanetary dust consists of small solid particles orbiting within a planetary system and responding to gravity, radiation, plasma, and collisions.

The radiation-pressure coefficient is the ratio of outward stellar radiation pressure to inward stellar gravity on a dust grain. The grain therefore feels effective gravitational parameter $(1-\beta )G{M}_{\ast }$.

Poynting–Robertson drag is the tangential component of stellar radiation force caused by aberration in an orbiting dust grain's frame. It removes orbital energy and angular momentum and drives the grain inward.

During inward migration, dust can be captured into an exterior mean-motion resonance with a planet. A phase-shifted conjunction then lets planetary torque replace angular momentum lost to Poynting–Robertson drag.