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?

January Planetary Formation Update

January was mainly a good month for my theory of planetary formation because two papers were published that strongly supported two of the proposals in my ebook that are not generally accepted. There was a third paper that is also highly relevant.
 
The first relates to where the volatiles of the rocky planets came from. There are two propositions that are usually debated: the volatiles were delivered by comets, or by chondrites. A review written by Halliday (Geochim. Cosmochim. Acta, 105, 146-171) showed the ratios of H/C/N are inconsistent with any ratio of these. All my review could do was to say that only minor miracles would permit some combination, but now even minor miracles are ruled out. My argument is that the volatiles were accreted chemically along with everything else, and the different ratios of volatiles on these planets arose because the materials from which they were made originated in different temperature zones.
 
The second paper was even more important, although I am not quite sure the authors themselves appreciated the full impact of what they discovered. One problem for planetary formation is, when the planets start forming, they form in a disk full of gas, so the question is, why do not the planets head towards the star? They are, after all, orbiting in the equivalent of a stellar "atmosphere". There are a number of papers written on this, but they generally lead to all planets ending up close to the star. What I proposed was that because the gas is falling towards the star and the azimuthal velocity is less than the Keplerian velocity at that distance, a pressure wave builds up in front of the body. That would normally cause the orbit to decay, but because there are solids in the gas, and because the gas has definite radial velocity, the planet starts spinning up, as if rolling around its orbit as material with an inwards radial velocity is accreted on the leading face. All the giants do this, although Uranus provides a problem. The body then drags gas down across its leading face, and when it becomes more gravitationally significant, it holds onto some and drags it around to its starwards side, where it strikes gas coming the other way (in its sub-Keplerian orbit). Two gas streams flowing in opposite directions cancel their velocities, in which case the gas will stream towards the star, gradually merging into the more general flow. The original angular momentum lost must be conserved, and only the orbiting body can take it up. By gaining angular momentum, it tends to move away from the star, thus offsetting the expected orbit decay.
 
The paper that excited me was due to Casassus et al. (Nature 493: 191-194). What they did was to observe the star HD 142457, which is still in the late accretionary stage, and they found two filaments of gas streaming inwards with a radial velocity greater that the azimuthal velocity by approximately 10% or greater, and further, the flows in these filaments were approximately equal to the rate of stellar accretion. It gives a surprisingly pleasant feeling to find what you predicted, as much out of desperation as anything, actually turns up.
 
There have been a variety of estimates of how much gas is present in the late stage of stellar accretion. This is important because our planetary system requires about 1% of stellar mass, and previous estimates varied by about two orders of magnitude, with only the higher end having sufficient material. In my ebook, I assumed an average, hence I required initial planetary accretion to commence earlier.  The third paper (Bergin et al. Nature 493: 644-646) provides a new means of estimating the mass in the disk by using the emissions from hydrogen deuteride, and from observations on TW Hydrae, which has a disk between 3-10 My old, the disk contains 5% of the stellar mass, which the authors state is easily enough to form planets. As it happens, this system has been reported to have a planet of about 10 Jupiter masses rather close to the star, the star continues to accrete disk gas (For TW Hydrae, approximately 1% of gas per My), and after a period, the great bulk of this disk is blown away by a stellar outburst. Further, as Casassus showed, planetary accretion must be highly inefficient if planets throw gas inwards instead of accreting it. Accordingly, I think that the odds favour my concept that planetary formation must have started earlier. Either way, though, this evidence strongly suggests that planets are more likely to form around stars than not, so the probability that we are alone in the Universe has probably just got a lot lower.
Posted by Ian Miller on Feb 3, 2013 11:00 PM Europe/London

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