Small Solar System body

• 1. Introduction

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  Small Solar System bodies got their official name on the same day Pluto lost its planet status. In 2006, the International Astronomical Union passed a resolution that formally defined the word "planet." The same document created a companion category for everything else orbiting the Sun that did not qualify. "All other objects, except satellites, orbiting the Sun shall be referred to collectively as 'Small Solar System Bodies,'" the resolution stated. At one end of the spectrum, the category reaches the largest asteroids and distant icy worlds. At the other end, its lower boundary has never been fixed. Three named objects are explicitly ruled out of the category not because of their size, but because of where they came from.

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• 2. Members and Exceptions

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  Ceres sits in the asteroid belt but is not a small Solar System body. That large world qualifies as a dwarf planet, which places it above the category's threshold. The other named dwarf planets are also excluded: Pluto, Haumea, Makemake, Quaoar, Orcus, Sedna, Gonggong, and Eris. Some additional bodies may yet join that list, pending study into whether they have reached hydrostatic equilibrium. Comets, classical asteroids, trojans, centaurs, and trans-Neptunian objects not already designated as dwarf planets all fall within the category. The IAU definition also explicitly excludes objects that travel through the solar system from outside it.

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• 3. Interstellar Exclusions

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  1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS are those excluded objects, named directly in the IAU resolution. The resolution calls them "interstellar interlopers," objects traveling through the solar system rather than orbiting the Sun. Belonging to the small Solar System body category requires a permanent solar orbit. These three objects travel through the solar system without holding one. Natural satellites share an exclusion from the category too, but the reason is entirely different.

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• 4. Orbits, Not Size

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  Natural satellites, the IAU definition states, "differ from small Solar System bodies not in size, but in their orbits." A small Solar System body orbits the Sun directly. A natural satellite orbits another Solar System object: a planet, a dwarf planet, or even another small Solar System body. Even the largest moons, those that have reached hydrostatic equilibrium, are defined by this same orbital criterion. The population of solar-orbiting bodies that do qualify is not distributed evenly; most gather in concentrated zones shaped by gravitational forces.

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• 5. Two Great Belts

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  The asteroid belt and the Kuiper belt together hold the vast majority of all known small Solar System bodies. Jupiter's gravity shapes the internal structure of the asteroid belt. Neptune plays the equivalent organizing role in the Kuiper belt. The boundaries of both belts are loosely defined rather than precise. Smaller concentrations of small bodies exist in other parts of the solar system as well. Near-Earth asteroids, centaurs, comets, and scattered disc objects populate regions outside the two main belts. One question the belts do not answer is how far down the size scale the category extends.

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• 6. The Open Boundary

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  Meteoroids are the smallest macroscopic bodies known to orbit the Sun. Whether they fall within the small Solar System body definition remains unsettled. No lower size bound has been formally established for the category. Below the macroscopic scale, the solar system holds even smaller material. Interplanetary dust, particles of solar wind, and free particles of hydrogen fill interplanetary space. None of that microscopic material would fall within the category even if the boundary were drawn at meteoroids. The IAU has not yet determined whether to set a formal lower boundary, leaving open how small a small Solar System body can be.

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