Having settled the question of what constitutes a dwarf planet (in theory at least) the International Astronomical Union (IAU) decided, also in 2006, to create a third category for all the rest, termed “Small Solar System Bodies.” This was embodied in the IAU’s Resolution B5: definition of a Planet in the Solar System.
Explaining the thinking behind its three classifications, Resolution B5 says:
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information. The IAU, therefore, resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories…
After defining a planet and a dwarf planet [Link to: Dwarf Planets] the IAU bundled the rest together: All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”. These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies
It has to be said that this three-way classification approach is bound to need amending in light of future discoveries. It seems to be part of human nature to name everything. And since nature is ever evolving, it is forever defying our attempts to categorise and label it. We are at the stage now where those disinterested in bird life see no point in identifying every flying creature, for example, lumping all the little birds into the catch-all “LBJs” — Little Brown Jobs. For now this is what the IAU has done with its third classification: if you can’t give it a name then it’s an SSSB.
Small Solar System Bodies
So Small Solar System Bodies are basically “all the rest” that are neither planets nor dwarf planets. They include asteroids, trans-Neptunian objects (TNOs), comets, meteors, and planetesimals, interplanetary dust and debris, loose protoplanetary and, of course, even tiny particles of dust and debris left over from past planetary formations and ongoing collisions. Out of collision, destruction and accretion of cosmic materials random formations of various gravitational bodies and planetesimals continually adjust into small solar system bodies that live on the edge of chaos: in the asteroid belt, Kuiper Belt and the Oort Cloud. Some bodies (from solid particles to ephemeral dust), when let loose from these areas, set off across the solar system as comets and meteors.
The science is fluid
The following table coves the main residential addresses of the Small Solar System Bodies. It has to be said that, as a subject matter, they lie at the forefront of planetary and astronomical science and perforce do not enjoy much scientific precision. Distances, dimensions and categorisations are somewhat fluid. As the scientific community continues to push the boundaries of astronomy still further, no doubt the subject will become more of a matter of fact and less of a matter for speculation.
| Small Bodies - distance from the sun | Kilometres* |
|---|---|
| Near Earth Objects | 194 million (nominal) |
| Asteroid Belt (Hungaria) | 266 - 299 million |
| Outer Belt (Cybele, Hilda, Thule) | 489 - 667 million |
| Jupiter (Trojans) | 778 million |
| Centaurs | 284 million - 5.5 billion |
| Kuiper Belt Objects | 4.49 billion - 14.96 billion |
| Scattered Disc Objects | 4.8 billion - 150 billion |
| Sednoids | 11.4 billion - 148 billion |
| Oort Cloud | 299 billion - 14 960 billion‡ |
| *These are all best estimates, summarised from numerous sources. ‡ All flat disc phenomena except the Oort Cloud, which is a sphere. |
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What’s the difference?
Asteroids, meteors and comets – what’s the difference? Nasa explains it well.
Asteroids
An asteroid is a small rocky body that orbits the Sun. Asteroids are smaller than a planet, but they are larger than the pebble-size objects we call meteoroids. Most asteroids in our solar system are found in the main asteroid belt, a region between Mars and Jupiter. But they can also hang out in other locations around the solar system. For example, some asteroids orbit the Sun in a path that takes them near Earth.
Meteoroids
Sometimes one asteroid can smash into another. This can cause small pieces of the asteroid to break off. These pieces are called meteoroids. Meteoroids can also come from comets.
Meteors
If a meteoroid comes close enough to Earth and enters the atmosphere, it vaporizes and turns into a meteor: a streak of light in the sky. Because of their appearance, these streaks of light are sometimes called “shooting stars.” But meteors are not actually stars. Because meteors leave streaks of light in the sky, they are sometimes confused with comets. However, these two things are very different.
Meteorites
Sometimes meteoroids don’t vaporize completely in the atmosphere but survive their trip through Earth’s atmosphere and land on the Earth’s surface. When they do this they are called meteorites
Comets
Comets orbit the Sun, like asteroids. But comets are made of ice and dust—not rock. As a comet’s orbit takes it toward the Sun, the ice and dust begin to vaporise. That vaporised ice and dust become the comet’s tail. You can see a comet even when it is very far from Earth. However, when you see a meteor, it’s in our atmosphere.
By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar
Last updated: Wednesday, 1st April 2020