The Asteroid Belt
Sen–Beyond Mars lies a circular band of material about 150 million kilometres wide, containing billions of lumps of rock ranging from hundreds of kilometres in size down to a few millimetres. Yet all this material collected together would form a body just four per cent the mass of our own Moon. This is the Asteroid Belt—a sparse region containing leftover debris from the formation of our Solar System, some 4.5 billion years ago.
Unlike with the other planets, Asteroid Belt material never coalesced to form a single world. This is because it's in a bad region for planet formation. As it moves along in its orbit around the Sun, the gravitational influence of Jupiter—the largest planet in the Solar System—is a constant disruption. Whereas other belts of material gently clumped together over millions of years to form the planets we know today—Mercury, Venus, Earth, Mars, etc—the young Jupiter destabilised any-such process in a region starting 329,115,316 kilometres from the Sun. The Asteroid Belt is thus sometimes known as the planet that never was.
NASA's Dawn spacecraft's view of Vesta, one of the four largest bodies in the Asteroid Belt. Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA.
Although that is in a way, true, it doesn't do justice to the largest asteroids in the Belt: Ceres, Vesta, Pallas and Hygiea. With average diameters of 952km, 544km, 525km and 431km respectively, half the Belt's total mass—about three million, million, million tonnes—is contained in just these four bodies. Ceres is even classed as a 'dwarf planet', putting it in the same category as Pluto, which itself was declassified from 'major planet' status in 2006.
Ceres was the first asteroid discovered, on New Year's Day 1801 by Giuseppe Piazzi of the University of Palermo, Sicily. Spurred on by British-German astronomer William Herschel's discovery of Uranus in 1781, astronomers made a concerted effort to look for a supposed 'missing planet' between Mars and Jupiter. Why there? Two astronomers, Johan Titius of Wittenberg and Johan Bode of Hamburg believed they had found a mathematical law governing the locations of planets within the Solar System, which Uranus matched exactly. Their law stated that there had to be a world between Mars and Jupiter.
Although today the Titius-Bode law's considered to be a mere coincidence by scientists (for example, Neptune doesn't fit the law), Ceres may be considered that 'missing' world. But try as they might, the 18th century astronomers couldn't magnify Ceres or other such bodies—considered planets at the time—into planet-like discs in their telescopes. Due to their small size they remained firmly point-like. It was this that prompted Herschel to consider a new category for these objects: 'asteroeides', from the Greek for 'star-like'.