PLANETARY EVOLUTION 



Mercury to be the smallest, but Neptune ought 

 to be the largest. The facts, however, do not 

 accord with this view. The four outer planets 

 are indeed much larger than the four inner ones. 

 But of the inner group the largest is not Mars, 

 but the earth ; while in the outer group we find 

 Jupiter three and a half times as large as Saturn, 

 which in turn is seven times larger than Uranus. 

 Now the key to these apparent anomalies must, 

 I think, be sought in the shapes of the rings 

 from which the planets were respectively formed. 

 Neptune and Uranus, formed from very thin 

 hoop-like rings, at a period when the solar 

 equator protruded but slightly, are indeed large 

 planets, but not so large as would be inferred 

 from the size of their orbits alone. But as the 

 solar nebula continued to contract, its increas- 

 ing equatorial velocity rendered it more and 

 more oblate in figure, so that the rings next de- 

 tached were quoit-shaped. Hence the result- 

 ing planets not only had their major diameters 

 but little inclined to their orbit-planes, but they 

 were also larger in size. The very broad quoits 

 which gave rise to Jupiter and Saturn may well 

 have contained more than fourteen times as 

 much planetary matter as the extensive but 

 slender hoops which formed the two oldest 

 planets. If, instead of looking at the sizes of 

 the resulting planets, we consider the thicknesses 

 of the genetic rings, as determined by compar- 

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