How were they discovered? Somehow thanks to the Titius-Bode Law establishing the existence of a mathematical ratio between the distances of each planet respect to the Sun. Put another way, relates the distance of a planet to the Sun with the order number of the planet by a simple rule. Mathematically it is a succession that facilitates calculate the distance of a given planet to the Sun by its order number. The law itself says that the distance of the planetary orbits is = (n +4) / 10, where n is the progression 0, 3, 6, 12, 24, 48, 96... Now, following the formula instructions, adding 4 to each term of the succession we obtained the follwing: 4, 7, 10, 16, 28, 52, 100, ... Once again, according to the formula, we have to divide by 10 obtaining this new succession: 0.4-0.7-1.0-1.6-2.8 - 5.2 - 10.0 ... And here is the key, as can be easily seen, on the result of this formula, a planet is far from the Sun almost twice the previous one.
But there was a problem to be accordin with this scheme. That there should have been a planet located between the orbits of Mars and Jupiter, nearly twice the distance from Mars to the Sun. But there was nothing there, at least apparently. At the beginning it was thought about a planet whose surface were very dark, but later began to forge the idea that there should be a planet that was too small to see it. For this reason, everybody jump for joy when the astronomer and Sicilian monk Giuseppe Piazzi, founding director of the Palermo Astronomical Observatory, discovered in the cold early hours of January 1, 1801 this object so long sought. It was a tiny planet of 975 by 909 km that was called Ceres (many years later, in 2006, it would be considered a dwarf planet instead of an asteroid).
A year later, in 1802, another German astronomer named Olbers, who had worked with Piazzi in the quest for unknown planet, discovered a second body at a nearly the same distance from the Sun that Ceres. In comming years were discover many of them, continuing this discovering fever until today. It is estimated that there may be millions of them in this orbit. There are asteroids several hundred km in diameter (Ceres, Pallas, Vesta, Juno, Hebe) that can be seen with simple binoculars in ideal conditions, there are many others wiht the size of mountains and finally there are millions that are mere pebbles or rocks.
Most asteroids that are in our Solar System, have fairly stable orbits between Mars and Jupiter (the predicted by the Titius-Bode Law), but some are diverted to cross the orbits of the greatest planets.
How can be explain that in the place where there should be a planet it is in reality a swarm of asteroids and dwarfs planets? It is thought that perhaps are the foundation of a planet that failed to become a coherent mass to form a planet due to the Jupiter's gravity disruptions. In fact, many of the asteroids of this belt have been captured by the jovian gravitational field and circulate in the same orbit that the asteroids called Trojans.
For a better understanding we're going to see the Titius-Bode table:
|Comparison of distances of the planets to the Sun with the predicted by the Titius-Bode Law|
|Object||Distance from the Sun (A. U.)||Distance from the Sun as predicted by Titius-Bode|
Elsewhere in the Solar System are asteroids but less extend, coliding one to each other and often falling on heaviest bodies, and, by this reason, therefore making greater for gravitational attraction. Mercury, the Moon, Mars and even Earth show scars by the fall of these bodies on its surface. The Earth, possessing the atmosphere is better defended, and while many asteroids are disintegrate when passing through this protective layer, it is true that others that are greater contacts with the surface causing catastrophics disasters over the history of our planet, as was the extinction of the dinosaurs 65 million years ago.