Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

Planetary Formation Update – July – August.

The question of how planets form continued to attract attention. Everyone agrees the accretion starting position is the disk of gas falling into the forming star. The gas also contains "dust", ranging in size from a colloidal dispersion to pieces a few millimetres in diameter. It is possible some pieces could be bigger, but we would not see them. The question then is, what happens next? The standard theory is that by some undefined mechanism, this accretes into planetesimals, which are about the size of asteroids, and the resultant distribution of these, which is smooth and continuous with regards to distance from the star, gravitationally collide. The asteroid belt is therefore likely to be the remnants of this process. In my opinion, that is wrong, and the first stages were driven by chemistry, and the distribution of growing bodies is highly enhanced in certain zones of temperature appropriate for the specific chemistry.
There was an interesting paper in Nature 511: 22-24  that surveyed problems with the standard theory of planetary formation, and ends with the question, "Why is our system so different from so many others?" Unfortunately, no answer was provided. As my theory shows, the reason is very simple: the admittedly limited evidence strongly suggests that our star cleaned out its accretion disk very quickly after formation, and this stopped accretion. Other systems kept going, which leads to more massive bodies and stronger gravitational interactions, and this results in what is effectively planetary billiards takes place. Unfortunately, once gravitational interactions get big enough, the resultant system becomes totally unpredictable.
Another interesting problem involved the question of rubble-pile asteroids. One major question is how rocky planets accrete, and the standard theory seems to assume that somehow moderate sized objects form, and gravity makes these come together, and as they get more rubble, they become bigger objects. Eventually they become big enough that they heat up, partly through radioactivity and partly through the loss of potential energy when bodies pile up, and the heated body starts to melt together. Asteroids are often believed to be piles of such rubble. However, two papers were published that make this proposition less likely. In this context, my theory requires rocky bodies accreted while the accretion disk is still present to be joined together chemically, and in the case of the asteroids, by cements similar to those used by the ancient Romans, and which also come from certain volcanoes such as Vesuvius. Such asteroids can still be piles of rubble, but cemented together where the surfaces meet. Effectively, they are very poorly compacted concretes. Also, non-cemented rubble piles would exist if the pieces came together following the disk clean-out.
The first (Nature 512: 174 – 176) involved asteroid (29075) 1950 DA, which has a density of 1.7 0.7. Since the solids are believed to be similar to enstatite chondrite, it should have a density of 3.55, hence it appears to have about 50% space inside it.  However, the rotational velocity is such that if it comprised rubble, the rubble should peel off. The authors argued that it must be held together with van der Waals forces from fine grains between the larger pieces. I have a problem with this. If the spaces are filled, then the density should be higher. Note that van der Waals forces are very weak at a very short range, and according to Feynman's calculations, they fall off inversely to the power of 6 with distance. The second paper (Icarus 241: 358 -372) analysed the size/frequency distribution of small asteroids and compared these with computed collision frequencies, and they found that the assumption of rubble-pile asteroids leads to a significant worse fit with observation than the assumption of monolithic bodies, hence they conclude that the majority of main-belt asteroids are monolithic.
Finally, there is the question of global magma oceans. The standard theory has rocky planets finally accreting through massive collisions, which lead to massive generation of heat, which in turn converts the rocky planet to magma. However, evidence has been presented (Earth Planet. Sci. Lett. 403: 225 – 235) that the geology of Mars is incompatible with this picture. My mechanism for planetary formation does not forbid a magma ocean, but unless there is a giant collision between two massive bodies, there will not be, and planets can form without one. In fact, they probably have to, because the energy of collision of massive bodies is generally such that size reduction occurs as material is shed to space.
Posted by Ian Miller on Aug 31, 2014 9:34 PM Europe/London

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