Smaller suns seem to be forming in them, each gathering
into its body the neighbouring material of the arm, and rising in
temperature as the mass is compressed into a globe. The
spectroscope shows that these knots are condensing masses of
white-hot liquid or solid matter. It therefore seems plain that
each planet will first become a liquid globe of fire, coursing
round the central sun, and will gradually, as its heat is
dissipated and the supply begins to fail, form a solid crust.
This familiar view is challenged by the new "planetesimal
hypothesis," which has been adopted by many distinguished
geologists (Chamberlin, Gregory, Coleman, etc.). In their view
the particles in the arms of the nebula are all moving in the
same direction round the sun. They therefore quietly overtake the
nucleus to which they are attracted, instead of violently
colliding with each other, and much less heat is generated at the
surface. In that case the planets would not pass through a
white-hot, or even red-hot, stage at all. They are formed by a
slow ingathering of the scattered particles, which are called
"planetesimals" round the larger or denser masses of stuff which
were discharged by the exploding sun.
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