268 SEE— SOLAR SYSTEM BEYOND NEPTUNE. [April 2,, 



several trans-Neptunian planets ; but the recognition of these remote 

 bodies will be difficult, owing to their slow motion and the faintness 

 of the sun's light at that great distance. 



8. The notable expansion of our ideas of what constitutes a 

 nebula will thus be of great practical use in the progress of astron- 

 omy. The overthrow of tlie theory of Laplace is only a small part 

 of the service to science brought about by the discovery of the true 

 laws of the development of our system. As the comets recede to 

 distances amounting to thousands of times the earth's distance from 

 the sun, so also must embryo planets be imagined to bridge over the 

 gap heretofore separating the planets and comets. And we may 

 imagine planets to extend to at least ickd. perhaps 1,000 times the 

 earth's distance from the sun. Some of the comets may go 100 

 times further yet, but at such great distances we can never know 

 much about their motions in these remote regions of space. 



9. When we contemplate the vast extent of our primordial 

 nebula implied in the distances to which the comets recede, and 

 remember the large apparent areas covered by many other nebulas 

 in the sky, we see that our solar nebula evidently was of the ordinary 

 type, and that it certainly was not a gaseous mass in equilibrium 

 under hydrostatic pressure and extending only to the orbit of 

 Neptune. Of course all these old doctrines of Laplace are now 

 quite abandoncfl, but they long deceived us. and kept cosmogony 

 in a stationary condition for over a century. 



10. The origin of the primordial nuclei in the distance is a neces- 

 sary consequence of the working of planetary bodies towards the 

 dominant center of attraction — the sun. Hence the formation of a 

 system of planets is necessarily from without inward, just the 

 reverse of the traditions handed down by Laplace. This harmonizes 

 perfectly with the new theory of the spiral nebulae, which makes the 

 ring nebulas particular cases of the more general spiral tendency. 

 The formation in all cases is from the outside towards the center. 

 Planets form in all nebulae, and since small bodies approach the center 

 more rapidly than large ones, under the action of a resisting medium, 

 it follows that the planets thus capture systems of satellites such as 

 we observe attending the planets of the solar system. 



U. S. Naval Observatory, 

 March 7. 1911. 



