In the beginning in a cold gas cloud near where the solar system lies today, there was a competition between a physical process, gravitational collapse, and a chemical process, the formation of dust and rocks. When stars formed the strong UV light emitted by the stars evaporated the gas and stopped the formation of the dust, rocks and ultimately the planets. Such a competition is taking place today in the Trifid nebula (left) as imaged by the Hubble Space Telescope.
Inside the early solar nebula, the sun shone dimly in visible light as the dust and protoplanets formed as illustrated in a painting by W. Hartmann (right). In the region now called the asteroid belt, this process stopped when Jupiter formed by a gravitational instability. Again physical and chemical processes were in competition. Jupiter orbited the sun at a different rate than the protoplanets, closer to the sun, and its periodic gravitational pull stirred up the bodies in the "asteroid belt" shutting off the planetary formation process leaving baby planets a fraction of the size of Earth but very much indicative of the protoplanets that led to the formation of the terrestrial planets.
The figure below shows Vesta and Ceres as seen by the Hubble Space Telescope and the Near-Earth asteroid Eros that was recently explored by the NEAR mission. These two bodies are much more massive than any body yet visited in this region of space and are truly small planets.
Ceres, the largest asteroid and the first to be discovered, is named after the Roman goddess of agriculture. It was discovered by Giuseppe Piazzi of the Palermo Observatory on Jan. 1, 1801. Additional observations by Piazzi were cut short due to illness. Carl Friedrich Gauss, at the age of 24, was able to solve a system of 17 linear equations to determine Ceres' orbit and to allow it to be rediscovered, a remarkable feat for this time. As a result within one year of its initial discovery, both Heinrich Olbers and Franz von Zach were able to relocate Ceres. It revolves around the Sun in 4.6 terrestrial years and has a diameter estimated at about 960 km (600 miles).
Vesta, the brightest asteroid, is named for the ancient Roman goddess of the hearth and is the only asteroid ever visible with the naked eye. Found on March 29, 1807, by Heinrich Olbers, it was the fourth minor planet to be discovered. It is the second most massive and the third largest asteroid. It revolves around the Sun in 3.6 terrestrial years and has an average diameter of about 520 km(320 miles). Its surface composition is basaltic.
- Background Part 3: The Search for a Planet Between Mars and Jupiter: the First International Scientific Program