FORMATION OF PLANETARY SYSTEMS 177 



By elimination of alternatives it appears that a con- 

 figuration resembling the solar system would only be 

 formed if at a certain stage of condensation an unusual 

 accident had occurred. According to Jeans the accident 

 was the close approach of another star casually pursuing 

 its way through space. This star must have passed 

 within a distance not far outside the orbit of Neptune; 

 it must not have passed too rapidly, but have slowly 

 overtaken or been overtaken by the sun. By tidal dis- 

 tortion it raised big protuberances on the sun, and caused 

 it to spurt out filaments of matter which have condensed 

 to form the planets. That was more than a thousand 

 million years ago. The intruding star has since gone on 

 its way and mingled with the others; its legacy of a 

 system of planets remains, including a globe habitable 

 by man. 



Even in the long life of a star encounters of this kind 

 must be extremely rare. The density of distribution of 

 stars in space has been compared to that of twenty 

 tennis-balls roaming the whole interior of the earth. 

 The accident that gave birth to the solar system may be 

 compared to the casual approach of two of these balls 

 within a few yards of one another. The data are too 

 vague to give any definite estimate of the odds against 

 this occurence, but I should judge that perhaps not one 

 in a hundred millions of stars can have undergone this 

 experience in the right stage and conditions to result in 

 the formation of a system of planets. 



However doubtful this conclusion as to the rarity of 

 solar systems may be, it is a useful corrective to the view 

 too facilely adopted which looks upon every star as a 

 likely minister 'to life. We know the prodigality of 

 Nature. How many acorns are scattered for one that 

 grows to an oak? And need she be more careful of her 



