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



The general conception of the Tidal Theory ( 16) as applied to our solar 

 system is that a second mass has at some past period approached so close to 

 our sun. as to break it up by intense tidal forces into a number of detached 

 masses. As between the tidal and rotational theories, first appearances are 

 all in favour of the tidal theory. We have found that the rotational theory 

 applied to a mass comparable with that of our sun leads only to a binary star 

 ( 258) or perhaps ultimately to a triple or multiple system of a type which 

 is well known and has certainly no resemblance to our solar system ( 278). 

 The tidal theory on the other hand leads at once and naturally to the con- 

 ception of a number of separate masses becoming detached from the primary 

 mass and finally describing orbits about it. 



A further general feature which favours the tidal theory may be noticed. 

 A system in rotation, and consequently also a system which has broken up 

 by rotation, has an invariable plane, which is perpendicular to the original 

 axis of rotation. Such a system ought to remain symmetrical about this 

 plane, and the axis of rotation of the central mass ought to remain perpen- 

 dicular to this plane. 



In the solar system over 98 per cent, of the angular momentum is orbital, 

 the remainder arising almost entirely from the sun's rotation. Of the orbital 

 momentum over 99*9 per cent, belongs to the outer planets, whose orbits all 

 lie within 1J degrees of the invariable plane indeed the orbits of Jupiter, 

 Saturn and Neptune, contributing 94 - 3 per cent, of this momentum, lie within 

 45' of the invariable plane. The plane of the sun's rotation, on the other 

 hand, lies about 6 from this plane. The rotational theory fails to account 

 for this distance between the plane of the orbits and that of the sun's rota- 

 tion; the tidal theory explains it very naturally by supposing that the 

 present invariable plane records the plane of passage of the tide-generating 

 mass, while the present plane of the sun's rotation coincides approximately 

 with that of the rotation of the original mass. 



293. The details of tidal motion have been dynamically investigated for 

 two models for an incompressible mass of uniform density, and for Roche's 

 model, representing the limit of non-uniform density. In each case the mass 

 is found to break up into a number of separate masses, but the incom- 

 pressible mass breaks up into masses of comparable size, while the very 

 non-uniform mass breaks up only by the ejection of one or two streams of 

 matter, which will probably condense into masses small compared with the 

 central mass. Clearly these latter conditions give the closer approximation 

 to those observed in our solar system, so that if our system has broken up 

 tidally, it must have been far from homogeneous when the break-up occurred, 

 and Roche's model may be expected to give the better picture of the 

 process. 



