Minor Planets

The first asteroid was discovered serendipitously by the Italian astronomer Giuseppe Piazzi (1746–1826) on the night of January 1, 1801. This asteroid, subsequently called Ceres after the Roman goddess of corn and harvests, has a diameter of 584 mi (940 km) and is the largest asteroid in our solar system. The next three largest asteroids, Pallas, Juno, and Vesta, were discovered in 1802, 1804, and 1807.

The number of cataloged asteroids has grown dramatically since 1801, and it is estimated that more than 100,000 minor planets larger than 0.62 mi (1 km) in diameter exist within our solar system. Astronomers have also found that the number of asteroids increases dramatically with decreasing diameter; that is, there is just one known asteroid larger than 560 mi (900 km) across, namely Ceres, but there are three larger than 280 mi (450 km), 22 larger than 140 mi (225 km), and so on. Despite their large numbers, however, the combined mass of all the asteroids is estimated to be just 0.04% the mass of Earth.

The low collective mass of the asteroids within our solar system suggests that, rather than being the remnants of a disrupted planet (as was once theorized), they are in fact left-over "building blocks" from the formation of the planets. Asteroid-like objects were, theory holds, among the first structures to form in our solar system Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group.

some 4.6 billion years ago, and some astronomers have suggested that the planets were formed through the collision and accretion of these primordial chunks of rock (planetesimals). Astronomers have also suggested that a planet did not form in the region presently occupied by the main-belt asteroids because of the disruptive gravitational influence of Jupiter.