Small Solar System body
In August of 2006, the International Astronomical Union gathered in Prague to vote on a new definition for planets. The resolution passed by a narrow margin reclassified Pluto as a dwarf planet and created a new category called Small Solar System Bodies. This decision meant that all objects orbiting the Sun were now sorted into three groups: planets, dwarf planets, or small bodies. The official text stated that any object not fitting the first two categories would be referred to collectively as 'Small Solar System Bodies'. Before this moment, astronomers used terms like minor planets or asteroids without a unified framework. The vote changed how scientists described everything from Ceres to comets forever.
Most small Solar System bodies reside within the main asteroid belt between Mars and Jupiter. This region contains classical asteroids that are not large enough to become dwarf planets. Jupiter exerts strong gravitational perturbations that shape the structure of this belt over time. These forces prevent smaller particles from coalescing into a single massive planet. Some asteroids form distinct clusters known as trojans that share orbits with larger planets. The boundaries of the belt remain fairly loosely defined due to these complex interactions. Scientists study these dynamics to understand the early formation of our solar system.
Beyond Neptune lies another vast reservoir of small bodies known as the Kuiper belt. Trans-Neptunian objects in this zone include centaurs and scattered disc members. These distant worlds often follow highly elliptical paths that take them far from the Sun. Neptune's gravity influences their trajectories and keeps many of them in stable regions. Some of these objects may eventually be reclassified if they prove to be in hydrostatic equilibrium. The scattered disc extends even further outward than the main Kuiper belt area. Astronomers continue to map these remote populations using powerful telescopes on Earth.
Comets represent one primary category of Small Solar System Bodies orbiting the Sun. They differ from asteroids by containing significant amounts of ice and volatile materials. When comets approach the inner solar system, their ices sublimate to create glowing tails. This process distinguishes them visually from rocky minor planets. The IAU resolution explicitly includes all comets within the SSSB definition regardless of size. Many comets originate from the Oort cloud or Kuiper belt before entering the inner system. Their composition offers clues about the chemical history of the early solar nebula.
Objects traveling through the Solar System from outside our star are excluded from this classification. Examples such as 1I/`Oumuamua and 2I/Borisov do not orbit the Sun permanently. These interstellar visitors follow hyperbolic trajectories that allow them to escape the solar gravitational field. The IAU definition specifically limits Small Solar System Bodies to objects bound to our Sun. Even if an interstellar object passes close enough for detailed study it remains unclassified under this term. Scientists track these rare encounters to understand the broader galactic environment beyond our neighborhood.
Astronomers debate whether a lower size bound will be established for small bodies in future definitions. Some larger members of this group may eventually be reclassified as dwarf planets. This change depends on determining whether they have achieved hydrostatic equilibrium. Current lists include potential candidates like Quaoar and Gonggong that await further examination. Hydrostatic equilibrium means gravity has pulled the body into a nearly round shape. If more large objects meet this criteria, the number of SSSBs could decrease significantly. Ongoing research continues to refine these boundaries for future generations of scientists.
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Common questions
When did the International Astronomical Union create the Small Solar System Bodies category?
The International Astronomical Union created the Small Solar System Bodies category in August of 2006. This decision occurred when the organization gathered in Prague to vote on a new definition for planets.
Where do most small Solar System bodies reside within the solar system?
Most small Solar System bodies reside within the main asteroid belt between Mars and Jupiter. This region contains classical asteroids that are not large enough to become dwarf planets.
What is the difference between comets and asteroids as small Solar System bodies?
Comets differ from asteroids by containing significant amounts of ice and volatile materials. When comets approach the inner solar system, their ices sublimate to create glowing tails which distinguishes them visually from rocky minor planets.
Why are interstellar objects like 1I/Oumuamua excluded from the Small Solar System Bodies classification?
Interstellar objects like 1I/Oumuamua are excluded because they follow hyperbolic trajectories that allow them to escape the solar gravitational field. The IAU definition specifically limits Small Solar System Bodies to objects bound to our Sun.
How might future definitions change the number of small Solar System bodies?
Future definitions may establish a lower size bound based on whether objects have achieved hydrostatic equilibrium. If more large objects meet this criteria, the number of SSSBs could decrease significantly.