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How are small bodies different from planets, dwarf planets, and moons?
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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.
In astronomy, the term small body is defined by exclusion: it refers to any natural object in the solar system that is not the Sun, a planet, a dwarf planet, or a moon.12
The primary differences between these categories are based on classification standards set by the International Astronomical Union and the physical characteristics of the objects:32
- Planets and Dwarf Planets: These are larger bodies that have been explicitly classified based on their size, orbital characteristics, and ability to clear their orbital paths. For example, Pluto was reclassified from a planet to a dwarf planet in 2006.12
- Moons: While many moons are similar in size to asteroids or smaller, they are excluded from the small body classification primarily for historical reasons, as they are satellites of other planets or dwarf planets.13
- Small Bodies: This category encompasses a vast range of objects, including asteroids (minor planets), comets, and meteoroids. They are considered remnants of the planet-building process that occurred during the formation of the solar system.12