# Gravitational scattering

Gravitational scattering refers to the process by which two or more celestial objects interact through their gravitational fields, causing their trajectories to alter. This phenomenon is fundamental in astrophysics and the study of dynamic systems. When objects like stars, planets, or black holes pass close enough to influence each other’s motions, their paths can shift dramatically. These interactions typically result in either bound systems, like binary star systems, or unbound systems, where the objects continue moving apart after the interaction. An example of a body ejected from a planetary system by this process would be Kuiper belt bodies pushed from the Solar System by Jupiter.

## Observing gravitational scattering

Gravitational scattering events are usually studied using simulations and mathematical models of the gravitational field interactions between bodies. One significant feature of gravitational scattering is the effect of energy exchange. For instance, a high-velocity object may transfer some of its kinetic energy to a slower-moving object, resulting in a slingshot effect. This principle is utilized in space exploration for gravitational assists, where spacecraft gain momentum by passing close to a planet.
Observing gravitational scattering has provided insight into many astrophysical phenomena. In dense regions like star clusters or galactic cores, gravitational scattering plays a role in star formation and the distribution of stellar populations. For instance, hypervelocity stars, which are ejected from their galaxies, are often a result of gravitational scattering involving massive objects like black holes. In more extreme cases, close interactions between compact objects, such as black holes, can lead to the emission of gravitational waves, detectable by instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO).
Gravitational scattering is analyzed through both Newtonian mechanics and general relativity, with the latter being necessary for systems involving high mass or velocity.

## Gravitational scattering impacts

Gravitational scattering can cause orbits to change or even cause celestial bodies to depart their native planetary systems. A possible mechanism that may move planets over large orbital radii is gravitational scattering by larger planets or, in a protoplanetary disk, gravitational scattering by over-densities in the fluid of the disk. In the case of the Solar System, Uranus and Neptune may have been gravitationally scattered onto larger orbits by close encounters with Jupiter and/or Saturn. Systems of exoplanets can undergo similar dynamical instabilities following the dissipation of the gas disk that alter their orbits and in some cases result in planets being ejected or colliding with the star.
Planets scattered gravitationally can end on highly eccentric orbits with perihelia close to the star, enabling their orbits to be altered by the gravitational tides they raise on the star. The eccentricities and inclinations of these planets are also excited during these encounters, providing one possible explanation for the observed eccentricity distribution of the closely orbiting exoplanets. The resulting systems are often near the limits of stability. As in the Nice model, systems of exoplanets with an outer disk of planetesimals can also undergo dynamical instabilities following resonance crossings during planetesimal-driven migration. The eccentricities and inclinations of the planets on distant orbits can be damped by dynamical friction with the planetesimals with the final values depending on the relative masses of the disk and the planets that had gravitational encounters.