Every now and again, something happens that makes you feel both good and depressed at the same time. For me it was last week, when I looked up the then latest edition of Nature. There were two papers (Nature, vol 541 (Dauphas, pp 521 – 524; Fischer-Gödde and Kleine, pp 525 – 527) that falsified two of the most important propositions in the standard theory of planetary formation. What we actually know is that stars accrete from a disk of gas and dust, the disk lingers on for between a million years and 30 million years, depending on the star, then the star’s solar winds clear out the dust and gas. Somewhere in there, planets form. We can see evidence of gas giants growing, where the gas is falling into the giant planet, but the process by which smaller planets or the cores of giants form is unobservable because the bodies are too small, and the dust too opaque. Accordingly, we can only form theories to fill in the intermediate process. The standard theory, also called oligarchic growth, explains planetary formation in terms of dust accreting to planetesimals by some unknown mechanism, then these collide to form embryos, which in turn formed oligarchs or protoplanets (Mars sized objects) and these collided to form planets. If this happened, they would do a lot of bouncing around and everything would get well-mixed. Standard computer simulations argue that Earth would have formed from a distribution of matter from further out than Mars to inside Mercury’s orbit. Earth the gets its water from a “late veneer” from carbonaceous chondrites from the far side of the asteroid belt.
It is also well known that certain elements in bodies in the solar system have isotopes that vary their ratio depending on the distance from the star. Thus meteorites from Mars have different isotope ratios from meteorites from the asteroid belt, and again both are different from rocks from Earth and Moon. The cause of this isotope difference is unclear, but it is an established fact. This is where those two papers come in.
Dauphas showed that Earth accreted from a reasonably narrow zone throughout its entire accretion time. Furthermore, that zone was the same as that which formed enstatite chondrites, which appear to have originated from a region that was much hotter than the material that, say, formed Mars. Thus enstatite chondrites are reduced. What that means is that their chemistry was such that there was less oxygen. Mars has only a small iron core, and most of its iron is as iron oxide. Enstatite chondrites have free iron as iron, and, of course, Earth has a very large iron core. Enstatite chondrites also contain silicates with less magnesium, which will occur when the temperatures were too hot to crystallize out forsterite. (Forsterite melts at 1890 degrees C, but it will also dissolve to some extent in silica melts at lower temperatures.) Enstatite chondrites also are amongst the driest, so they did not provide Earth’s water.
Fischer-Gödde and Kleine showed that most of Earth’s water did not come from carbonaceous chondrites, the reason being, if it did, the non-water part would have added about 5% to the mass of Earth, and the last 5% is supposed to be from where the bulk of elements that dissolve in hot iron would have come from. The amounts arriving earlier would have dissolved in the iron and gone to the core. One of those elements is ruthenium, and the isotope ratios of Earth’s ruthenium rule out an origin from the asteroid belt.
Accordingly, this evidence rules out oligarchic growth. There used to be an alternative theory of planetary accretion called monarchic growth, but this was soon abandoned because it cannot explain first why we have the number of planets we have where they are, and second where our water came from. Calculations show it is possible to have three to five planets in stable orbit between Earth and Mars, assuming none are larger than Earth, and more out to the asteroid belt. But they are not there, so the question is, if planets only grow from a narrow zone, why are these zones empty?
This is where I felt good. A few years ago I published an ebook called “Planetary Formation and Biogenesis” and it required monarchic growth. It also required the planets in our solar system to be roughly where they are, at least until they get big enough to play gravitational billiards. The mechanism is that the planets accreted in zones where the chemistry of the matter permitted accretion, and that in turn was temperature dependent, so specific sorts of planets form in zones at specific distances from the star. Earth formed by accretion of rocks formed during the hot stage, and being in a zone near that which formed enstatite chondrites, the iron was present as a metal, which is why Earth has an iron core. The reason Earth has so much water is that accretion occurred from rocks that had been heat treated to about 1550 degrees Centigrade, in which case certain aluminosilicates phase separated out. These, when they take up water, form cement that binds other rocks to form a concrete. As far as I am aware, my theory is the only current one that requires these results.
So, why do I feel depressed? My ebook contained a review of over 600 references from journals until a few months before the ebook was published. The problem is, these references, if properly analysed, provided papers with plenty of evidence that these two standard theories were wrong, but each of the papers’ conclusions were ignored. In particular, there was a more convincing paper back in 2002 (Drake and Righter, Nature 416: 39-44) that came to exactly the same conclusions. As an example, to eliminate carbonaceous chondrites as the source of water, instead of ruthenium isotopes, it used osmium isotopes and other compositional data, but you see the point. So why was this earlier work ignored? I firmly believe that scientists prefer to ignore evidence that falsifies their cherished beliefs rather than change their minds. What I find worse is that neither of these papers cited the Drake and Righter paper. Either they did not want to admit they were confirming a previous conclusion, or they were not interested in looking thoroughly at past work other than that which supported their procedures.
So, I doubt these two papers will change much either. I might be wrong, but I am not holding my breath waiting for someone with enough prestige to come out and say enough to change the paradigm.