Free Astronomy Magazine November-December 2017

NOVEMBER-DECEMBER 2017 A n imaginary scene in which a large planetesimal is growing by sweeping in smaller objects that cross its orbit. less than a thousandth of Earth’s mass), whose orbits are typically between those of Mars and Jupiter. In the nineteenth cen- tury, when astronomers realised that the first of these objects discovered were part of a real population (about 460 were dis- covered before 1900), the first hypotheses were presented to explain their existence. There were essentially two currents of thought on this: the asteroids (so called because they looked like stars, from the Greek asteroeid ē s) could be either the remainders of a destroyed planet, or the building blocks of a planet that never formed. The first theory was advanced by Heinrich Wilhelm Olbers after he discovered the sec- ond (Pallas, in 1802) and fourth (Vesta, in 1807) objects in the main belt. Olbers predicted the existence of many other frag- ments of the hypothetical de- stroyed planet (called Phaeton). When the discoveries of new as- teroids multiplied, his theory was reinforced, and it remained the lead- I n this pie representing the total mass of the asteroids in the main belt, one can evalu- ate the importance of the 12 largest asteroids compared to all the rest. ing theory for several decades. As time went by, however, nobody was able to find a valid reason why the supposed planet would have wound up in pieces. Astronomers thus started giving more cre- dence to the theory of an aborted planet, whose formation would have been pre-