"I follow you,"said Bearwarden,"and do not see how we could arrive at anything else.From Jupiter's low specific gravity,weighing but little more than an equal bulk of water,I should say the interior must be very hot,or else is composed of light material,for the crust's surface,or the part we see,is evidently about as dense as what we have on earth.These things have puzzled me a good deal,and I have been wondering if Jupiter may not have been formed before the earth and the smaller planets.""The discrepancies between even the best authorities,"replied Cortlandt,"show that as yet but little has been discovered from the earth concerning Jupiter's real condition.The two theories that try to account for its genesis are the ring theory and the nebulous.We know that the sun is constantly emitting vast volumes of heat and light,and that,with the exception of the heat resulting from the impact of falling meteors,it receives none from outside,the principal source being the tremendous friction and pressure between the cooling and shrinking strata within the great mass of the sun itself.A seeming paradox therefore comes in here,which must be considered:If the sun were composed entirely of gas,it would for a time continue to grow hotter;but the sun is incessantly radiating light and heat,and consequently becoming smaller.Therefore the farther back we go the hotter we find the sun,and also the larger,till,instead of having a diameter of eight hundred and eighty thousand miles,it filled the space now occupied by the entire solar system.
Here is where the two theories start.According to the first,the revolving nebulous mass threw off a ring that became the planet Neptune,afterwards another that contained the material for Uranus,and so on,the lightest substance in the sun being thrown off first,by which they accounted for the lightness of the four great planets,and finally Mars,the earth,and the small dense planets near the sun.The advocates of this theory pointed to Saturn's rings as an illustration of the birth of a planet,or,rather,in that case a satellite.According to this,the major planets have had a far longer separate existence than the minor,which would account for their being so advanced notwithstanding their size.This theory may again come into general acceptance,but for the present it has been discredited by the nebulous.According to this second theory,at the time the sun filled all the space inside of Neptune's,orbit,or extended even farther,several centres of condensation were formed within the nebulous,gaseous mass.The greatest centre became the sun,and the others,large and small,the planets,which--as a result of the spiral motion of the whole,such as is now going on before our eyes in the great nebulae of fifty-one M.Canuin venaticorum,and many others--began to revolve about the greatest central body of gas.As the separate masses cooled,they shrank,and their surfaces or extreme edges,which at first were contiguous,began to recede,which recession is still going on with some rapidity on the part of the sun,for we may be sure its diameter diminishes as its density increases.According to either theory,as I see it,the major planets,on account of their distance from the central mass,have had longer separate existences than the minor,and are therefore more advanced than they would be had all been formed at the same time.
"This theory explains the practical uniformity in the chemical composition of all members of this system by assuming that they were all once a part of the same body,and you may say brothers and sisters of the sun,instead of its offspring.It also makes size the only factor determining temperature and density,but of course modified by age,since otherwise Jupiter would have a far less developed crust than that with which we find it.I have always considered the period from the molten condition to that with a crust as comparatively short,which stands to reason,for radiation has then no check;and the period from the formation of the crust,which acts as a blanket,to the death of a planet,as very long.I have not found this view clearly set forth in any of the books I have read,but it seems to me the simplest and most natural explanation.Now,granted that the solar system was once a nebula,on which I think every one will agree--the same forces that changed it into a system of sun and planets must be at work on fifty-one M.Canum venaticorum,Andromeda,and ninety-nine M.Virginis,and must inevitably change them to suns,each with doubtless a system of planets.
