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Why is the Oort cloud thought to have a spherical shape?
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Small body
Asteroid Ida and its satellite, Dactyl, photographed by the Galileo spacecraft on August 28, 1993, from a distance of about 10,870 km (6,750 miles). Ida is about 56 km (35 miles) long and shows the irregular shape and impact craters characteristic of many asteroids. The Galileo image revealed that Ida is accompanied by a tiny companion about 1.5 km (1 mile) wide, the first proof that some asteroids have natural satellites.
The Oort cloud is considered to have a spherical shape because it consists of icy small bodies that approach the planetary region from all directions, exhibiting totally random orbital inclinations and orientations.12
This spherical distribution is a result of the cloud's formation and ongoing dynamical processes:
- Origin: The Oort cloud was formed from icy planetesimals that were gravitationally scattered far away from the Sun by the giant planets during the early stages of the solar system's development.123
- Gravitational Perturbations: At the extreme distances where the Oort cloud residesāstretching to interstellar distancesāthe orbits of these icy bodies are continuously influenced by random passing stars and tidal forces from the Milky Way Galaxyās disk. These external gravitational forces act on the objects from all directions, effectively randomizing their orbits and maintaining the cloud's spherical structure.12
Because of this, long-period comets originating from the Oort cloud are often referred to as "nearly isotropic comets," as they do not favor any specific plane of approach to the Sun.2