BOUNDARIES OF OUR SOLAR WORLD 309 



about the same as Ceres. Here was a mystery. Two planets, 

 evidently true planets, had been found ; but they were more 

 like fragments than the respectable bodies they should have 

 been. The fact that their orbits were so close to each other 

 roughly speaking, identical heightened the impression. It was 

 surmised by Olbers that they were in reality bits of a larger 

 planet, and he set about a systematic search for the missing 

 pieces. 



Seven years of patient observation brought from the depths 

 a third. Again the observations were turned over to Gauss, to 

 map its path in space. It closely resembled the others. To 

 the second he had given the name of Pallas ; the third he 

 called Vesta. It was as absurdly small as the others. It is just 

 visible to the unaided eye. Olbers kept up his hunt for other 

 fragments, as he deemed them, for thirty years more : he had 

 no further reward. Soon after he died a fourth was found ; 

 then a fifth ; then they came tumbling in so fast that it takes 

 a long catalogue to number them. Up to the present time 

 near five hundred have been found. 



The largest was the third in order of discovery Vesta. We 

 now know that it is about five hundred miles in diameter. They 

 range from that down to ten or twenty. But still they remain 

 a puzzle. Even five hundred of them tossed together into a 

 mould would not make a planet very much larger than the 

 moon. Leverrier calculated from the observed disturbances in 

 the orbital course of Mars that their combined pull was not 

 equal to that of a planet one-quarter as large as the earth : 

 it is known now that it is a great deal less. What is the 

 explanation ? 



We do not know, for we have not as yet a theory of planet- 

 ary evolution sufficiently complete to give a satisfactory account 

 of this flying swarm. The idea of a " shattered planet " is 

 attractive ; it was the most obvious, and it was the first that 

 was suggested. It is conceivable that a planet might go to 

 pieces in a terrific explosion. The craters of volcanoes long 

 ago revealed to man the fact that the earth has a molten in- 

 terior. Doubtless this is true of all planets. Veritable ex- 

 plosions like that of Krakatoa a few years ago, are sufficient 

 indication of the violence with which such a cataclysm might 

 take place. If it ever did, the disrupted fragments would tend 

 to go on circling the sun in something of the same sort of orbit. 



