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 - August

Once again there were very few papers that came to my attention in August relating to my ebook on planetary formation. One of the few significant ones (Geochim Cosmochim Acta 120: 1-18) involved the determination of magnesium isotopes in lunar rocks, and these turned out to be identical with those of Earth and in chondrites, which lead to the conclusion that there was no significant magnesium isotopic separation throughout the accretion disk, nor during the Moon-forming event. There is a difference in magnesium isotope ratios between magnesium found in low and high titanium content basalts, but this is attributed to the actual crystallization processes of the basalts. This result is important because much is sometimes made of variation in iron isotope variations, and in variations for some other elements. The conclusion from this work is that apart from volatile elements, isotope variation is probably more due to subsequent processing than in planetary formation, and the disk was probably homogeneous.
Another point was that a planet has been found around the star GJ 504, at a distance of 43.5 A.U. from the star. Commentators have argued that such a planet is very difficult to accommodate within the standard theory. The problem is, if planets form by collision of planetesimals, and as these get bigger, collisions between embryos, the probability of collision, at least initially, is proportional to the square of the concentration of particles, and the concentration of particles depends to some power between 1 and 2, and usually taken as to the power 1.5, of the radial distance from the star. Now standard theory argues that it in our solar system, it was only around the Jupiter-Saturn distance that bodies could form reasonably quickly, and in the NICE theory, the most favoured computational route, Uranus and Neptune formed closer and had to migrate out through gravitational exchanges between them, Jupiter, Saturn, and the non-accreted planetesimals. For GJ 504, the number density of planetesimals would be such that collision probability would be about 60 times slower, so how did they form in time to form a planet four times the size of Jupiter, given that, in standard theory in our system, growth of Jupiter and Saturn was only just fast enough to get a giant?
In my opinion, the relative size compared with Jupiter is a red herring, because it also depends on when the gas disk is cleaned out by a stellar outflow. The reason is, in my model, bodies do not grow largely by collision of equally sized objects, but rather they grow by melt accretion of ices at a given temperature, and the rate of growth depends on the initial concentration of solids in the disk only, and of course, the gas inflow rate because that, together with the initial gas temperature and the position of the star within a cluster, determines the temperature, and the temperature determines the position of the planet. If GJ 504 formed under exactly the same conditions as Earth, this planet lies about midway between where we might expect Neptune and Uranus to lie, and which one it represents can only be determined by finding inner planets. In previous computations, the planet should, not form; in my theory, it is larger than would normally be expected but it is not unexpected, and there should be further planets within that orbit. Why is only one outer planet detected so far? The detection is by direct observation of a very young planet that is still glowing over red hot through gravitational energy release. The inner ones will be just as young, but the closer to the star, the harder it is to separate their light from that of the star, and, of course, some may appear very close to the star by being on certain orbital phases.
Posted by Ian Miller on Sep 1, 2013 8:58 PM Europe/London

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