290 The Origin and Evolution of the Solar System [CH. xn 



attraction of the passing star, they would not fall back into the sun but 

 would describe elliptic orbits, passing fairly near to the sun's surface at their 

 closest approach. As they passed relatively near to the sun, the same pro- 

 cess as resulted in the formation of planets out of the sun, may have resulted 

 in the formation of satellites out of the planets. It is not difficult to account 

 for the systems of Jupiter and Saturn in this way, but the satellites of 

 Neptune, Uranus, Mars and the earth are less easy to explain. The system 

 which interests us most nearly, namely our earth-moon system, is just the 

 one about which it is most difficult to come to any definite conclusion. For 

 the earth-moon system is exceptional in the system of the planets, just as 

 the solar system to which it belongs appears to be exceptional, and for ought 

 we know may be unique, in the system of the stars. 



