- Also called:
- small solar system body (SSSB)
Small body
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What is a small body in astronomy?
In the solar system, a small body is any natural object that is not the Sun, a major planet, a dwarf planet, or a moon. These objects include asteroids, comets, meteoroids, and interplanetary dust particles.
- Asteroids Also called minor planets, asteroids are mostly rocky and orbit the Sun, primarily in the asteroid belt between Mars and Jupiter.
- Comets Comets are predominantly icy and reside mainly in the Oort cloud and Kuiper belt.
- Meteoroids These are fragments of asteroids or comets. When they enter Earth's atmosphere, they create a glowing trail called a meteor. If they survive the passage through the atmosphere and impact the ground, they are called meteorites.
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What is an asteroid?
An asteroid, also called a minor planet or planetoid, is any of a host of small bodies, about 1,000 km (600 miles) or less in diameter, that orbit the Sun primarily between the orbits of Mars and Jupiter in a nearly flat ring called the asteroid belt. Asteroids are made of rock and metal.
Because of their small size and large numbers relative to the major planets, asteroids are also called minor planets. The two designations have been used interchangeably, though the term asteroid is more widely recognized by the general public.
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How is a comet different from an asteroid?
Comets and asteroids are both small bodies that orbit the Sun, but they differ in composition, origin, and orbits.
| Feature | Comet | Asteroid |
| --- | --- | --- |
| Composition | Composed of volatile ices (water, carbon dioxide, ammonia) mixed with dust particles.| Primarily made of rock and metal.|
| Origin | Originate from the distant Oort cloud or the Kuiper belt.| Mostly found in the asteroid belt between Mars and Jupiter; leftover building blocks of the solar system’s rocky planets.|
| Orbit | Highly eccentric orbits that can take them from the outer solar system to close passes by the Sun.| Shorter, fairly circular orbits, mostly within the inner solar system.|
| Appearance | Develop a coma and tails when near the Sun due to sublimation of ices.| Lack volatile components and do not develop a coma or tail.|When a comet approaches the Sun, the ices sublimate, creating a coma and often a tail. The word comet comes from the Greek word meaning "long-haired," which refers to the appearance of the bright coma.
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What is a meteoroid and how does it relate to meteorites?
A meteoroid is a small rocky or metallic object orbiting the Sun. When a meteoroid enters Earth's atmosphere, it becomes a meteor, appearing as a bright streak of light (also known as a shooting star) due to friction with the atmosphere. If a meteoroid survives its passage through the atmosphere and reaches the ground, it is then called a meteorite. Meteoroids are considered small bodies in the solar system, often fragments of asteroids or comets.
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Where are small bodies usually found in our solar system?
Small bodies in our solar system are mainly found in the Kuiper Belt beyond Neptune's orbit and the Oort Cloud at vast distances from the Sun. The Kuiper Belt is a source of short-period comets, while the Oort Cloud is the origin of long-period comets. Additionally, asteroids and their fragments reside in the inner solar system. Trojan asteroids are located along Jupiter's orbit. Centaur objects can be found between the orbits of Jupiter and Neptune. These small bodies are remnants from the solar system's formation, consisting of rocky and icy materials that never fully incorporated into planets.
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Why is studying small bodies important for understanding the solar system?
Small bodies are leftover materials from the solar system's formation. Studying them helps us understand the conditions and processes of the early solar nebula.
Asteroids, comets, and meteoroids provide clues about the building blocks of planets and the distribution of elements in the solar system. Meteorites, mostly asteroid fragments, offer insights into the chemical composition of the early solar system that we can't get any other way.
Icy comets are representative of the planetesimals that formed in the outer solar system. Analyzing these small bodies helps us piece together the history and evolution of our solar system.
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Small body, any natural solar system object other than the Sun and the major planets and dwarf planets and their satellites (moons). The small bodies populate the solar system in vast numbers and include the mostly rocky asteroids, or minor planets, the predominantly icy comets, and the fragments of such bodies—commonly called meteoroids—over a continuum of sizes down to microscopic grains known as interplanetary dust particles or micrometeoroids. The term small body is sometimes restricted to objects that can be observed telescopically while still outside Earth’s atmosphere. In practice, that results in a lower limit of a few metres on the diameters of small bodies.
The use of meteoroid has expanded to describe any small chunk of matter in interplanetary space, especially one less than a few tens of metres in size. The term formerly was limited to small bodies that were near enough to Earth to eventually enter its atmosphere. In that narrower meaning, meteoroid is usually linked with the terms meteor, meteorite, or both. As a meteoroid travels through the atmosphere, friction heats it up, creating a glowing trail of hot ionized gases (plasma) called a meteor. If the object survives both its passage through the atmosphere and its subsequent impact on the ground, it is called a meteorite. Over 99 percent of meteorites are fragments of asteroids. A small group is known to be of lunar origin, and a second group is generally acknowledged to have come from Mars (see Mars: Meteorites from Mars). There is also reason to believe that some are fragments of the rocky remnants of comets, although that remains to be firmly established.
Populations
The largest known small bodies, in the conventional sense, are several icy Kuiper belt objects found orbiting the Sun beyond the orbit of Neptune. Ceres—which is the largest main-belt asteroid and is now considered a dwarf planet—is roughly 950 km (590 miles) in diameter. The distant Kuiper belt object Eris, roughly the same size as Pluto, which has a diameter of about 2,350 km (1,460 miles), is also designated a dwarf planet rather than a small body, as are Haumea and Makemake, both of which have diameters of about 1,450 km (900 miles). These four dwarf planets are known as plutoids. At least four other Kuiper belt objects are also believed to be larger than Ceres. In the 1990s astronomers recognized that Pluto, which has a diameter of about 2,300 km (1,400 miles), is a giant icy leftover from the time when the planets formed. Pluto had been classified as a planet from its discovery in 1930 but was reclassified as a dwarf planet in 2006. Primarily for historical reasons, the moons of the planets are not considered small bodies. Nevertheless, many moons are asteroid-size or smaller, and some are probably captured asteroids or comet nuclei.
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Although small bodies are found throughout the solar system, most of those known are concentrated in several regions. Those regions in which they travel in fairly stable orbits include (1) the main asteroid belt, between the orbits of Mars and Jupiter, (2) the gravitationally stable points (Lagrangian points) along the orbital paths of Jupiter, Mars, Uranus, Neptune, and Earth where objects called Trojan asteroids are clustered (see celestial mechanics: The restricted three-body problem), (3) the Kuiper belt, a disk-shaped zone of icy bodies whose members are also called trans-Neptunian objects because they all have orbits beyond that of Neptune, and (4) the spherical Oort cloud of icy bodies inferred to be orbiting the Sun at distances typically more than 1,000 times that of Neptune or Pluto. In none of the above categories is Sedna, an object discovered in 2003 in the outer reaches of the solar system that is perhaps as large as 40 percent the size of Pluto. Sedna has a highly elliptical orbit in which its closest approach to the Sun is more than twice that of Neptune—i.e., billions of kilometres beyond the recognized extent of the Kuiper belt; it may be a member of a population of objects lying between the Kuiper belt and the Oort cloud.
Other small bodies travel in unstable orbits that cross the paths of one or more of the planets. Those include (1) most observed comets, (2) near-Earth asteroids, most with orbits that cross either Earth’s orbit or Mars’s orbit and some with orbits that lie mostly or entirely inside Earth’s orbit and cross the orbits of Venus or of both Venus and Mercury while closely approaching Earth’s, (3) Centaur objects, icy bodies that are thought to have been gravitationally perturbed out of the Kuiper belt and now travel mainly between the orbits of Jupiter and Neptune, and (4) individual idiosyncratic planet-crossing objects such as the asteroid Hidalgo, which travels between the inner edge of the asteroid belt and a point just beyond the orbit of Saturn. All objects in planet-crossing orbits eventually collide with the Sun or a planet or are permanently ejected from the solar system, although some survive for long periods (up to hundreds of millions of years) because of stabilizing resonances.
