way in which our own system came into being, the white nebulæ assert their present constitution to be that from which we know our system sprang.
Another suggestive fact about the present members of our solar system which has something to say about a past collision is the densities of the different planets. The average density of the four inner planets, Mars, the Earth, Venus, and Mercury is nearly four times that of the four outer ones Neptune, Uranus, Saturn, and Jupiter.2 The discrepancy is striking and cannot be explained by size, as the smallest are the most massive, and if all were primally of like constitution, should be the least compressed. Nor can it be explained simply by greater heat tending to expand them, for Neptune and Uranus show no signs of being very hot. The minor differences between members of each group are probably explicable in part by these two factors, mass and heat, but the great gulf between the two groups cannot so be spanned. We are then driven to the supposition that the materials composing the outer ones were originally lighter. Now this is precisely what should happen had all eight been formed by disruption of a previous body. For its cuticle would be its least dense portion, and on disruption would travel farthest away, not because of being lighter, but because of being on the outside. Parts coming from deeper down would remain near, and be denser intrinsically.
