For much of history, the solar system was thought to consist of large objects: Earth, the Moon, the Sun, and other planets. However, within the last two centuries astronomers have discovered a menagerie of small objects throughout the solar system, including asteroids, comets, Kuiper belt objects, and many small moons orbiting planets. Studies of these objects, from both telescopes and spacecraft, have provided new insights into the formation of the solar system and its true nature.
There is no good definition for what constitutes a "small body" in the solar system. Nevertheless, one definition used by some planetary scientists considers objects less than 400 kilometers (250 miles) in diameter to be small bodies. When objects are larger than 400 kilometers, they have enough mass so that their gravity is powerful enough to shape the object into a roughly spherical form. For smaller objects, their gravity is not powerful enough to accomplish this, and as a result many have irregular shapes. Even this, though, is not a precise definition. In reality, there is a continuum of objects from very large to very small, with no distinct, obvious differences between "large" and "small" bodies.
The Discovery of Small Bodies
The first objects that would qualify as small bodies were asteroids, discovered in the early nineteenth century. While the largest asteroids, such as Ceres (the first asteroid to be discovered), may be somewhat larger than the above definition for a small body*, within a few decades a number of small bodies were discovered. In 1877 American astronomer Asaph Hall discovered Phobos and Deimos, the two moons of Mars. Both moons are very small objects, each less than 30 kilometers (18.5 miles) in diameter and irregularly shaped. In 1892 Amalthea, a moon less than 250 kilometers (155 miles) in diameter, was found orbiting Jupiter. In the late nineteenth and early twentieth centuries, a number of small moons were found orbiting Jupiter and Saturn. In addition, during this time the rate of asteroid discoveries increased.
The era of spacecraft exploration brought many more discoveries of small bodies. The Voyager 1 and Voyager 2 spacecraft discovered many small moons orbiting Jupiter, Saturn, Uranus, and Neptune. Telescopes on the ground, as well as the Hubble Space Telescope, have also discovered small moons around these planets. The rate of asteroid and comet discoveries increased dramatically starting in the 1990s. By October 2001, more than 30,000 asteroids had been discovered, compared to 20,000 just at the beginning of that year. Dozens of comets are discovered each year as well, many by automated telescopes and spacecraft.
Difficulties in Classification
These discoveries, and follow-up observations, have shown how difficult it can be to classify small bodies. Astronomers in the past attempted to fit these bodies into one of three classes: asteroids, comets, and moons. Now, however, there is evidence that many asteroids may be extinct comets, having exhausted their supply of ice that generates a tail when approaching the Sun. Some small moons orbiting Jupiter, as well as Phobos and Deimos, may have originally been asteroids captured into orbit by the gravity of Mars and Jupiter. In the outer solar system astronomers discovered in the early 1990s a family of icy bodies called Kuiper belt objects, some of which are larger than even the largest asteroids and are far larger than an ordinary comet. Spacecraft and ground-based telescopes have discovered several asteroids that have their own small moons.
Some planetary scientists have elected to classify objects in a different way, based on their composition and likely location in the solar system where they formed. Bodies that formed from the Sun out to a distance of about 2.5 astronomical units (AU) are primarily rocky and metallic. That close to the Sun, temperatures are too high for anything else to condense out of the protoplanetary nebula from which the solar system formed. This explains the composition of much of the asteroid belt as well as the inner planets. At around 2.5 to 2.7 AU is what some call the "soot line." At this distance temperatures are low enough for carbon-rich compounds, such as soot, to form. Asteroids in the outer portion of the belt, beyond the soot line, tend to be rich in these materials. At around 3 to 4 AU is the "frost line," beyond which water ice can form. Objects that formed beyond this distance tend to be rich in water ice and often in carbon dioxide, methane, and other ices.
This scheme allows scientists to understand where an object originated, regardless of where it is today. With this information, scientists can then try to understand how the object evolved over the history of the solar system from the location where it formed to where it is now located. The problem with this approach is that there is only limited information about the composition of many small bodies, including many of the moons of the giant planets. Even Phobos and Deimos, the two moons of nearby Mars, have not been examined enough to know their compositions well.
Sorting out the true nature of small bodies in the solar system will take many more years of research and observations by telescopes and spacecraft. A number of missions by the National Aeronautics and Space Administration (NASA) and the European Space Agency will study asteroids and comets in detail in the early twenty-first century. In addition, NASA's Cassini spacecraft will arrive at Saturn in 2004 and spend several years studying the planet and its moons, which may uncover key clues about the origin of Saturn's small moons. Through these observations, it should be possible to learn not only about the origins of the small bodies in the solar system but also how the solar system itself formed.
see also Asteroids (volume 2); Comets (volume 2); Exploration Programs (volume 2); Jupiter (volume 2); Kuiper Belt (volume 2); Mars (volume 2); Meteorites (volume 2); Neptune (volume 2); Oort Cloud (volume 2); Planetesimals (volume 2); Saturn (volume 2); Uranus (volume 2).
Hartmann, William K. "Small Worlds: Patterns and Relationships." In The New Solar System, 4th ed., ed. J. Kelly Beatty, Carolyn Collins Petersen, and Andrew Chaikin. Cambridge, MA: Sky Publishing Corp., 1999.
Arnett, Bill. "Small Bodies." <http://www.nineplanets.org/smallbodies.html>. Small Bodies Node. University of Maryland. <http://pdssbn.astro.umd.edu/outreach/index.html>.
*Ceres was initially considered to be a planet until other asteroids with similar orbits were found.